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Is Matter Around Us Pure? – CBSE NCERT Study Resources

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9th

9th - Science

Is Matter Around Us Pure?

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Overview of the Chapter

This chapter explores the concept of purity in matter and differentiates between pure substances and mixtures. It covers various types of mixtures, separation techniques, and the properties of solutions, suspensions, and colloids.

Pure Substance: A substance that consists of only one type of particle and has a fixed composition.

Types of Mixtures

Mixtures are classified into two categories:

  • Homogeneous Mixtures: Uniform composition throughout (e.g., salt solution).
  • Heterogeneous Mixtures: Non-uniform composition (e.g., sand and iron filings).

Solution: A homogeneous mixture of two or more substances, where the solute is uniformly distributed in the solvent.

Separation Techniques

Various methods are used to separate mixtures:

  • Filtration: Separates insoluble solids from liquids.
  • Evaporation: Used to recover solids dissolved in liquids.
  • Distillation: Separates liquids based on boiling points.
  • Chromatography: Used to separate components of a mixture based on solubility.

Properties of Solutions, Suspensions, and Colloids

Property Solution Suspension Colloid
Particle Size Less than 1 nm More than 100 nm 1 nm to 100 nm
Stability Stable Unstable Stable
Tyndall Effect No Yes Yes

Tyndall Effect: The scattering of light by colloidal particles, making the path of light visible.

Physical and Chemical Changes

Matter undergoes changes:

  • Physical Change: No new substance is formed (e.g., melting of ice).
  • Chemical Change: A new substance is formed (e.g., burning of paper).

All Question Types with Solutions – CBSE Exam Pattern

Explore a complete set of CBSE-style questions with detailed solutions, categorized by marks and question types. Ideal for exam preparation, revision and practice.

Very Short Answer (1 Mark) – with Solutions (CBSE Pattern)

These are 1-mark questions requiring direct, concise answers. Ideal for quick recall and concept clarity.

Question 1:
What is a pure substance?
Answer:

A substance with fixed composition and properties.

Question 2:
Give an example of a homogeneous mixture.
Answer:

Salt solution (NCERT example).

Question 3:
Name the method to separate oil and water.
Answer:

Using a separating funnel.

Question 4:
What is tyndall effect?
Answer:

Scattering of light by colloidal particles.

Question 5:
Give one real-world use of centrifugation.
Answer:

Separating cream from milk.

Question 6:
What is the solvent in air?
Answer:

Nitrogen (major component).

Question 7:
Name a sublimable substance.
Answer:

Ammonium chloride (NCERT example).

Question 8:
What is chromatography used for?
Answer:

Separating pigments in ink.

Question 9:
Give an example of a colloid.
Answer:

Milk (NCERT example).

Question 10:
What property allows fractional distillation?
Answer:

Different boiling points.

Question 11:
Name the lightest metal in NCERT.
Answer:

Lithium (NCERT example).

Question 12:
What is distilled water?
Answer:

Pure water without impurities.

Question 13:
Give a daily-life heterogeneous mixture.
Answer:

Soil (real-world example).

Question 14:
What separates in winnowing?
Answer:

Lighter husk from grains.

Question 15:
Give one example of a colloid.
Answer:

Milk.

Question 16:
Give an example of a suspension.
Answer:

Muddy water.

Question 17:
What is the boiling point of pure water?
Answer:

100°C at 1 atm.

Question 18:
Name the process to obtain salt from seawater.
Answer:

Evaporation.

Question 19:
What is a saturated solution?
Answer:

A solution with maximum solute at given temperature.

Question 20:
Give one example of a solid solution.
Answer:

Brass (copper + zinc).

Question 21:
What is distillation used for?
Answer:

Separating liquids with different boiling points.

Question 22:
What is sublimation?
Answer:

Direct change from solid to gas.

Question 23:
Give one example of a sublimable substance.
Answer:

Ammonium chloride.

Question 24:
Name a heterogeneous mixture from daily life.
Answer:

Soil.

Question 25:
What is the principle behind centrifugation?
Answer:

Denser particles settle at bottom.

Question 26:
Give an example of a colloidal solution.
Answer:

Milk.

Question 27:
Name a solute in tincture of iodine.
Answer:

Iodine.

Question 28:
How can we separate salt from seawater?
Answer:

By evaporation.

Question 29:
What is the solute in sugar solution?
Answer:

Sugar.

Question 30:
Name the process to obtain pure copper sulphate from its solution.
Answer:

Crystallization.

Very Short Answer (2 Marks) – with Solutions (CBSE Pattern)

These 2-mark questions test key concepts in a brief format. Answers are expected to be accurate and slightly descriptive.

Question 1:
What is a pure substance? Give one example.
Answer:

A pure substance consists of only one type of particle and has a fixed composition. It cannot be separated into other substances by physical methods.
Example: Gold (Au) is a pure substance as it contains only gold atoms.

Question 2:
Differentiate between homogeneous and heterogeneous mixtures with an example of each.
Answer:

Homogeneous mixture: Uniform composition throughout, components are not visible.
Example: Salt solution.
Heterogeneous mixture: Non-uniform composition, components are visible.
Example: Sand and iron filings.

Question 3:
Why is air considered a mixture and not a compound?
Answer:

Air is a mixture because its components (like nitrogen, oxygen, CO2) are not chemically bonded and can be separated by physical methods.
In a compound, elements combine chemically in fixed ratios.

Question 4:
Name the technique used to separate butter from curd. Explain the principle involved.
Answer:

Technique: Centrifugation.
Principle: Denser particles (curd) settle at the bottom, while lighter particles (butter) float when spun at high speed.

Question 5:
What is the Tyndall effect? Name a mixture that exhibits it.
Answer:

The Tyndall effect is the scattering of light by colloidal particles, making the beam visible.
Example: Milk (a colloid) shows this effect.

Question 6:
How can you separate a mixture of salt and ammonium chloride?
Answer:

Method: Sublimation.
Ammonium chloride sublimes on heating, leaving salt behind.

Question 7:
Define saturated solution. How can you make it unsaturated?
Answer:

A saturated solution cannot dissolve more solute at a given temperature.
Method to make it unsaturated: Add more solvent or increase temperature.

Question 8:
Why is water called a universal solvent?
Answer:

Water dissolves a wide range of substances due to its polar nature, forming hydrogen bonds with solutes.
This property makes it essential for biological and chemical processes.

Question 9:
What happens when a beam of light passes through a true solution? Why?
Answer:

The beam remains invisible because particles in a true solution are too small to scatter light (unlike colloids).

Question 10:
Give two differences between evaporation and distillation.
Answer:

Evaporation:
1. Leaves solid residue.
2. No condensation step.
Distillation:
1. Collects both solute and solvent.
2. Involves condensation of vapors.

Question 11:
Why is chromatography used to separate components of ink?
Answer:

Different ink components travel at varying speeds on the chromatography paper due to differences in solubility and adhesion, allowing separation.

Short Answer (3 Marks) – with Solutions (CBSE Pattern)

These 3-mark questions require brief explanations and help assess understanding and application of concepts.

Question 1:
Differentiate between homogeneous and heterogeneous mixtures with examples.
Answer:

A homogeneous mixture has a uniform composition throughout, and its components are not visually distinguishable. Example: Salt solution (salt dissolved in water).
A heterogeneous mixture has a non-uniform composition, and its components can be seen separately. Example: Sand and iron filings.

Question 2:
Explain the principle behind chromatography as a separation technique.
Answer:

Chromatography works on the principle of differential adsorption. Different components of a mixture are adsorbed at different rates on a stationary phase (like filter paper) when a mobile phase (solvent) moves over it.
Example: Separating ink dyes where each dye travels at a different speed, forming distinct bands.

Question 3:
How can you separate a mixture of salt and ammonium chloride? Explain the process.
Answer:

This mixture can be separated using sublimation.

Step 1: Heat the mixture in a china dish.
Step 2: Ammonium chloride sublimes (changes directly from solid to gas).
Step 3: The gas condenses on a cooler surface, leaving salt behind in the dish.

Question 4:
Why is air considered a mixture and not a compound? Give two reasons.
Answer:

Air is a mixture because:

  • Its composition varies (e.g., oxygen levels differ in different places).
  • Its components (like nitrogen, oxygen) retain their properties and can be separated physically.
Unlike compounds, air shows no fixed ratio of elements.

Question 5:
Describe how centrifugation helps in separating components of a mixture.
Answer:

Centrifugation uses centrifugal force to separate denser and lighter components.

Process:
1. The mixture is spun rapidly in a centrifuge.
2. Denser particles settle at the bottom, while lighter ones stay on top.
Example: Separating cream from milk.

Question 6:
What is a saturated solution? How can you make it unsaturated?
Answer:

A saturated solution is one where no more solute can dissolve at a given temperature.
To make it unsaturated:
1. Add more solvent (e.g., water) to dissolve extra solute.
2. Heat the solution to increase solubility, allowing more solute to dissolve.

Question 7:
Differentiate between homogeneous and heterogeneous mixtures with one example of each.
Answer:

A homogeneous mixture has a uniform composition throughout, and its components are not visually distinguishable. Example: Salt solution (salt dissolved in water).
A heterogeneous mixture has a non-uniform composition, and its components can be seen separately. Example: Sand and iron filings.

Question 8:
Explain why colloidal solutions show the Tyndall effect while true solutions do not.
Answer:

Colloidal solutions contain larger particles (1-100 nm) that scatter light, making the beam visible (Tyndall effect).
In true solutions, particles are too small (<1 nm) to scatter light, so the beam remains invisible.

Question 9:
Describe the process of chromatography and its principle.
Answer:

Chromatography separates components of a mixture based on their solubility and adsorption.

  • A sample is applied on a stationary phase (e.g., filter paper).
  • A mobile phase (solvent) moves the components at different rates.
  • Components separate due to varying affinities.
Principle: Differential migration of substances.

Question 10:
How can you separate a mixture of ammonium chloride and sodium chloride? Explain the process.
Answer:

Use sublimation:

  • Heat the mixture in a china dish.
  • Ammonium chloride sublimes (turns to vapor).
  • Cool the vapor to collect pure ammonium chloride.
  • Sodium chloride remains as residue.

Question 11:
What is a saturated solution? How can you make it unsaturated?
Answer:

A saturated solution contains the maximum solute dissolved at a given temperature.
To make it unsaturated:

  • Add more solvent (e.g., water).
  • Increase temperature (if solubility is temperature-dependent).

Long Answer (5 Marks) – with Solutions (CBSE Pattern)

These 5-mark questions are descriptive and require detailed, structured answers with proper explanation and examples.

Question 1:
Explain chromatography with an example. How is it useful in real life?
Answer:
Concept Overview

Chromatography is a method to separate components of a mixture based on their solubility. We studied it in our textbook for separating dyes in ink.

Process Explanation
  • A solvent moves up a paper carrying mixture components at different rates.
  • More soluble components travel farther.
Real-world Application

It is used in forensic labs to analyze crime scene samples like ink or drugs.

[Diagram: Paper chromatography setup]
Question 2:
Describe distillation with a labeled diagram. Give one NCERT and one daily-life example.
Answer:
Concept Overview

Distillation separates liquids based on boiling points. Our textbook shows its use to purify seawater.

Process Explanation
  • Mixture is heated; vapor of lower boiling liquid forms first.
  • Vapor condenses back to liquid in a condenser.
Real-world Application

It is used to produce alcoholic beverages by fermenting and distilling grains.

[Diagram: Simple distillation apparatus]
Question 3:
What is a suspension? Compare its properties with a solution using NCERT examples.
Answer:
Concept Overview

A suspension is a heterogeneous mixture where particles settle, like chalk in water. Solutions are homogeneous, like sugar in water.

Process Explanation
  • Suspensions scatter light (Tyndall effect) and leave residue on filtering.
  • Solutions are clear and stable.
Real-world Application

Muddy water is a suspension, while lemonade is a solution.

Question 4:
How can you separate a mixture of salt and sand? Explain steps with a diagram.
Answer:
Concept Overview

We can use dissolution, filtration, and evaporation as per NCERT Activity 2.3.

Process Explanation
  • Dissolve salt in water; sand remains insoluble.
  • Filter to separate sand. Evaporate water to get salt.
Real-world Application

Similar methods are used in salt pans to extract sea salt.

[Diagram: Filtration setup]
Question 5:
Differentiate between physical and chemical changes with NCERT and daily examples.
Answer:
Concept Overview

Physical changes are reversible (e.g., melting ice), while chemical changes form new substances (e.g., burning paper).

Process Explanation
  • Physical: No new product, like dissolving sugar (NCERT).
  • Chemical: Irreversible, like rusting iron.
Real-world Application

Cooking food is a chemical change, while cutting vegetables is physical.

Question 6:
Explain sublimation with an example. How is this property useful in daily life?
Answer:
Concept Overview

Sublimation is the process where a solid changes directly into gas without becoming liquid. Our textbook shows ammonium chloride as an example.

Process Explanation
  • When heated, solid particles gain energy
  • They escape into the air as gas
Real-world Application

We use this in air fresheners where solid blocks slowly turn into vapor. [Diagram: Solid to gas transition]

Question 7:
Differentiate between homogeneous and heterogeneous mixtures using NCERT examples.
Answer:
Concept Overview

Homogeneous mixtures have uniform composition while heterogeneous mixtures don't. Our textbook compares salt solution (homogeneous) and soil (heterogeneous).

Process Explanation
  • Homogeneous: Particles not visible
  • Heterogeneous: Visible boundaries
Real-world Application

We see this in lemonade (homogeneous) vs salad (heterogeneous). [Diagram: Particle distribution comparison]

Question 8:
Describe how chromatography works to separate dye colors with a real-world use.
Answer:
Concept Overview

Chromatography separates dissolved substances through differential movement. NCERT shows separating ink components using filter paper.

Process Explanation
  • Solvent carries dyes at different speeds
  • Forms distinct colored bands
Real-world Application

Forensic labs use it to analyze crime scene samples. [Diagram: Paper chromatography setup]

Question 9:
Explain tyndall effect with an experiment from our textbook and its natural occurrence.
Answer:
Concept Overview

Tyndall effect is scattering of light by colloidal particles. We studied this using milk in water experiment.

Process Explanation
  • Beam becomes visible in colloid
  • Doesn't occur in true solutions
Real-world Application

We observe this in sunlight through fog. [Diagram: Light path through colloid]

Question 10:
Compare physical and chemical changes using NCERT's iron nail example and a kitchen example.
Answer:
Concept Overview

Physical changes are reversible (like bending nail) while chemical changes form new substances (rusting nail).

Process Explanation
  • Physical: State/shape changes
  • Chemical: Composition alters
Real-world Application

In kitchen, melting butter vs burning toast shows both changes. [Diagram: Iron nail before/after rusting]

Question 11:
Explain the process of centrifugation with a suitable example. How is it different from sedimentation?
Answer:

Centrifugation is a method used to separate components of a mixture based on their densities using centrifugal force. It is commonly used in laboratories and industries for separating fine suspended particles from a liquid.


Process:
1. The mixture is placed in a centrifuge tube.
2. The tube is spun at high speed in a centrifuge machine.
3. Due to centrifugal force, denser particles move outward and settle at the bottom, while lighter particles remain above.


Example: Separation of cream from milk. When milk is centrifuged, the lighter fat (cream) rises to the top, while the denser skimmed milk remains at the bottom.


Difference from Sedimentation:

  • Sedimentation relies on gravity to settle heavier particles, while centrifugation uses artificial centrifugal force for faster separation.
  • Centrifugation is much faster and efficient for fine particles that do not settle easily by sedimentation.

Question 12:
Describe chromatography as a technique to separate mixtures. Give a real-life application and explain its principle.
Answer:

Chromatography is a technique used to separate mixtures into their individual components based on their differential movement through a stationary phase (like filter paper) and a mobile phase (like a solvent).


Principle: Different substances in a mixture travel at different speeds due to variations in their solubility in the mobile phase and their affinity to the stationary phase. This causes them to separate into distinct bands or spots.


Procedure:
1. A spot of the mixture is placed on chromatography paper.
2. The paper is dipped in a solvent, which moves up by capillary action.
3. Components separate as the solvent carries them at different rates.


Real-life Application: Used in forensic science to separate dyes in ink. For example, investigators use chromatography to analyze ink samples from documents to detect forgery.


Key Term: Rf value (Retention Factor) helps identify substances based on how far they travel compared to the solvent front.

Question 13:
Explain the process of chromatography with a suitable example. How is it useful in separating components of a mixture?
Answer:

Chromatography is a technique used to separate the components of a mixture based on their different solubilities and adsorption properties. It involves two phases: a stationary phase (like filter paper) and a mobile phase (like a solvent).

Example: Separating ink into its constituent dyes. Here's how it works:

  • A small spot of ink is placed on filter paper (stationary phase).
  • The paper is dipped into a solvent (mobile phase), which rises up the paper by capillary action.
  • As the solvent moves, it carries the ink components at different rates based on their solubility and adhesion to the paper.
  • This results in distinct colored bands, showing the separation of dyes.

Usefulness: Chromatography is highly effective because:

  • It can separate even minute quantities of substances.
  • It works for complex mixtures like pigments, drugs, or amino acids.
  • It helps identify unknown substances by comparing their movement with known samples.

This method is widely used in laboratories, forensic science, and pharmaceutical industries for purity analysis and research.

Question 14:
Explain the process of centrifugation with a suitable example. How is it different from sedimentation and decantation?
Answer:

Centrifugation is a method used to separate components of a mixture based on their densities using centrifugal force. In this process, the mixture is spun at high speed in a centrifuge machine, causing the denser particles to settle at the bottom while the lighter particles remain on top.


Example: Separation of cream from milk. When milk is centrifuged, the lighter cream rises to the top, while the denser skimmed milk settles at the bottom.


Differences:

  • Sedimentation is the natural process where heavier particles settle down due to gravity, whereas centrifugation uses artificial force to speed up the separation.
  • Decantation involves pouring out the liquid after sedimentation, but centrifugation does not require waiting for particles to settle naturally.

Centrifugation is faster and more efficient than sedimentation and decantation, making it useful in laboratories and industries.

Question 15:
Describe the chromatography technique with a labeled diagram. How is it useful in real-life applications?
Answer:

Chromatography is a technique used to separate mixtures into their individual components based on their solubility and adsorption properties.


Process:

  1. A sample is placed on a stationary phase (e.g., filter paper).
  2. A mobile phase (solvent) moves through the stationary phase, carrying components at different rates.
  3. Components separate into distinct bands or spots.

Diagram: (Imagine a vertical strip with dots at different heights representing separated dyes.)


Real-life Applications:

  • Forensics: Analyzing ink or drug samples.
  • Medicine: Detecting contaminants in blood.
  • Food Industry: Identifying food additives.

Key Advantage: Highly precise and works with tiny samples.

Question 16:
Explain the process of centrifugation with a suitable example. How is this method useful in separating mixtures?
Answer:

Centrifugation is a method used to separate components of a mixture based on their density differences by spinning the mixture at high speeds. Here’s how it works:


1. The mixture is placed in a centrifuge tube and spun rapidly in a centrifuge machine.
2. The denser particles move outward and settle at the bottom, while lighter particles remain at the top.
3. The separated components can then be easily collected.


Example: Separating cream from milk. When milk is centrifuged, the lighter cream rises to the top, while the denser skimmed milk settles at the bottom.


Usefulness: This method is highly efficient for separating colloidal suspensions or tiny solid particles from liquids, such as in medical labs for blood separation or in dairy industries.

Question 17:
Differentiate between homogeneous and heterogeneous mixtures with examples. Why is air considered a homogeneous mixture?
Answer:

Homogeneous mixtures have a uniform composition throughout, while heterogeneous mixtures have non-uniform composition with visible boundaries.


Examples:
1. Homogeneous: Saltwater (salt dissolves uniformly in water).
2. Heterogeneous: Sand and iron filings (components are visibly distinct).


Air as a Homogeneous Mixture: Air is a mixture of gases like nitrogen, oxygen, and carbon dioxide, which are uniformly distributed at a molecular level. No visible separation exists, making it homogeneous. Even though it contains different gases, they mix completely without forming distinct layers.

Question 18:
Describe the chromatography technique with a labeled diagram. How does it help in identifying substances in a mixture?
Answer:

Chromatography is a technique used to separate and identify components of a mixture based on their solubility and adsorption properties.


Process:
1. A strip of chromatography paper is taken, and a small spot of the mixture is placed near the bottom.
2. The paper is dipped into a solvent, which rises up the paper by capillary action.
3. Different components travel at different rates, separating into distinct spots.


Diagram: (Imagine a vertical paper strip with spots at different heights representing separated dyes.)


Identification: Each component has a unique retardation factor (Rf), calculated as:
Rf = Distance travelled by solute / Distance travelled by solvent
By comparing Rf values with known substances, components can be identified.


Example: Separating ink into its constituent dyes. Black ink often splits into blue, red, and yellow bands.


Application: Used in forensic science to analyze drug samples and in food industry to check for adulterants.

Question 19:
Describe the chromatography technique with a labeled diagram. How can it be used to separate ink components?
Answer:

Chromatography is a technique used to separate mixtures into their individual components based on their solubility and movement through a medium (like filter paper).


Process:

  • A spot of the mixture (e.g., ink) is placed on chromatography paper.
  • The paper is dipped in a solvent, which moves up by capillary action, carrying the components at different rates.
  • Components separate into distinct bands or spots based on their solubility.

Separation of Ink: Ink is a mixture of dyes. When the solvent moves up the paper, different dyes travel at different speeds, creating separate colored bands.


Diagram (Description): A labeled diagram would show:

  • Chromatography paper with a baseline and ink spot.
  • Solvent rising up the paper.
  • Separated dye bands at different heights.


This technique is widely used in forensic science, pharmaceuticals, and research for analyzing mixtures.

Case-based Questions (4 Marks) – with Solutions (CBSE Pattern)

These 4-mark case-based questions assess analytical skills through real-life scenarios. Answers must be based on the case study provided.

Question 1:
Rahul observed that milk left outside forms a cream layer. He wonders if this is a pure substance or a mixture. Explain with NCERT examples.
Answer:
Case Summary

Rahul observes cream separation in milk, questioning its purity.

Scientific Principle
  • Milk is a colloidal mixture (NCERT Class 9).
  • Cream separates due to gravity.
Solution Approach

We studied that mixtures show physical changes. Like saltwater (NCERT), milk is a heterogeneous mixture. Real-world example: butter is churned from cream.

Question 2:
A student is given iron filings and sulfur powder. How can she confirm if mixing them forms a compound or mixture? Use NCERT methods.
Answer:
Case Summary

Testing if iron+sulfur is a compound/mixture.

Scientific Principle
  • Compounds show chemical changes (NCERT).
  • Mixtures retain properties.
Solution Approach

Our textbook shows using a magnet to separate iron (mixture). Heating forms iron sulfide (compound), like NCERT’s water example. Real-world: alloys are mixtures.

Question 3:
Priya finds iodine crystals sublime directly to gas. Is this a pure substance? Compare with NCERT’s ammonium chloride example.
Answer:
Case Summary

Priya observes iodine sublimation, questioning purity.

Scientific Principle
  • Sublimation indicates fixed composition (NCERT).
  • Pure substances have distinct properties.
Solution Approach

We studied ammonium chloride sublimation (NCERT) as pure. Iodine behaves similarly. Real-world: dry ice (CO₂) also sublimes.

Question 4:
An experiment shows copper sulfate solution turns blue on adding water. Is this a physical or chemical change? Justify with NCERT.
Answer:
Case Summary

Color change in copper sulfate solution.

Scientific Principle
  • Physical change reverses easily (NCERT).
  • No new substance forms.
Solution Approach

Our textbook shows dissolving salt is physical. Here, hydration changes color reversibly. Real-world: sugar syrup shows similar behavior.

Question 5:
Rahul observed that saltwater left in a dish evaporates, leaving behind white crystals. Sand mixed in water settles down. Explain the separation techniques involved and their scientific principles.
Answer:
Case Summary

Rahul observed two mixtures: saltwater (homogeneous) and sand-water (heterogeneous).

Scientific Principle
  • Evaporation: Used for solids dissolved in liquids (like saltwater).
  • Sedimentation: Used for insoluble solids (like sand).
Solution Approach

Our textbook shows evaporation separates salt from seawater. In real life, sedimentation is used in water treatment plants to remove impurities.

Question 6:
A student is given a mixture of iron filings and sulfur powder. Describe a method to separate them and justify the choice.
Answer:
Case Summary

Iron filings (magnetic) and sulfur powder (non-magnetic) form a heterogeneous mixture.

Scientific Principle
  • Magnetic separation: Works due to magnetic properties of iron.
Solution Approach

We studied that a magnet can attract iron filings, leaving sulfur behind. NCERT example: Separating iron from garbage. Real-world use: Recycling centers separate metals from waste.

Question 7:
Priya noticed that ink spreads in water but oil forms a layer. Classify these as solutions, suspensions, or colloids and explain.
Answer:
Case Summary

Ink dissolves (homogeneous), while oil forms a separate layer (heterogeneous).

Scientific Principle
  • Solution: Ink in water (particles < 1 nm).
  • Suspension: Oil in water (particles > 100 nm).
Solution Approach

Our textbook shows ink as a true solution (like sugar water). Oil-water is a suspension, similar to muddy water settling over time.

Question 8:
During a lab activity, students separated dyes in black ink using chromatography. Tabulate their observations and explain the principle.
Answer:
Case Summary

Black ink is a mixture of dyes separated by chromatography.

Scientific Principle
Dye ColorDistance Travelled (cm)
Blue5.2
Red3.8
Solution Approach

We studied that dyes travel at different speeds due to solubility and adsorption. NCERT example: Separating pigments in chlorophyll. Real-world use: Forensic labs analyze ink samples.

Question 9:
Riya observed that milk left outside forms a cream layer. Her teacher explained this as a colloidal solution.

(a) Why does milk show the Tyndall effect?
(b) How can Riya separate cream from milk?
Answer:
Case Summary

Milk is a colloid where fat particles are dispersed in water.


Scientific Principle
  • Tyndall effect occurs due to scattering of light by colloidal particles.
  • Centrifugation separates cream (lighter) from milk (denser).

Solution Approach

Our textbook shows colloids like milk scatter light. For separation, we use centrifugation, similar to separating butter from curd.

Question 10:
Amit mixed iron filings and sulfur powder.

(a) Is this a mixture or compound? Justify.
(b) Suggest a method to separate them.
Answer:
Case Summary

Iron + sulfur form a heterogeneous mixture as particles are visible.


Scientific Principle
  • Mixtures retain properties of components; compounds don’t.
  • Magnetic separation works as iron is magnetic.

Solution Approach

We studied that mixtures like this can be separated physically. Using a magnet, iron is attracted, similar to NCERT’s sand-iron example.

Question 11:
A solution of salt in water was heated until dryness.

(a) Name the process.
(b) Why can’t we use this method for alcohol-water mixtures?
Answer:
Case Summary

Saltwater is a true solution recoverable by evaporation.


Scientific Principle
  • Evaporation leaves non-volatile solute (salt).
  • Alcohol-water forms azeotrope, needing distillation.

Solution Approach

Our textbook shows evaporation for salt. For alcohol, fractional distillation is used, as both components are volatile.

Question 12:
A student tested two liquids: honey (dense) and kerosene (light).

(a) Which property determines their layering?
(b) How would a solution differ from this?
Answer:
Case Summary

Honey and kerosene form layers due to immiscibility and density difference.


Scientific Principle
  • Density and miscibility affect layering.
  • Solutions are homogeneous (e.g., saltwater).

Solution Approach

We studied that immiscible liquids separate, like oil-water in NCERT. Solutions like sugar-water don’t layer as particles dissolve uniformly.

Question 13:

Rahul observed that when he mixed salt in water, it disappeared, but the mixture still tasted salty. However, when he mixed sand in water, it settled at the bottom. Based on this:

  • Identify the type of mixtures formed in both cases.
  • Explain why salt disappears but sand does not.
  • Suggest a method to separate salt from water.
Answer:

1. Type of mixtures:
Salt in water forms a homogeneous mixture (solution) because it dissolves uniformly.
Sand in water forms a heterogeneous mixture (suspension) as it does not dissolve and settles down.

2. Reason for disappearance:
Salt (solute) breaks into tiny particles that disperse evenly in water (solvent) due to solvation.
Sand particles are larger and insoluble, so they remain visible and settle due to gravity.

3. Separation method:
Salt can be separated by evaporation:
Heat the solution until water evaporates, leaving salt crystals behind.

Note: This process is used in salt pans to obtain sea salt.

Question 14:

Priya noticed that her mother uses a sieve to separate wheat flour from impurities, while a magnet is used to remove iron filings from sand. Answer:

  • Name the separation techniques in both cases.
  • Justify why these methods are suitable.
  • Give one more example where each technique is used in daily life.
Answer:

1. Separation techniques:
Sieve: Sieving (based on particle size difference).
Magnet: Magnetic separation (based on magnetic properties).

2. Suitability:
Sieving works for flour as impurities are larger than flour particles.
Magnetic separation works because iron is magnetic while sand is not.

3. Daily life examples:
Sieving: Separating tea leaves from tea.
Magnetic separation: Removing iron nails from scrap at recycling plants.

Key concept: Separation methods depend on the physical properties of mixture components.

Question 15:
Rahul observed that when he mixed salt in water, it disappeared, but the water tasted salty. However, when he mixed sand in water, it settled down. Based on this observation, answer the following:

(a) Classify the mixtures formed by salt-water and sand-water.
(b) Explain the reason behind the difference in behavior of salt and sand in water.
Answer:

(a) The mixture of salt and water forms a homogeneous mixture (or solution), as the salt particles dissolve uniformly in water and cannot be seen separately.

The mixture of sand and water forms a heterogeneous mixture, as the sand particles do not dissolve and remain visibly suspended or settled at the bottom.

(b) The difference in behavior occurs due to the nature of the substances:

  • Salt (solute) is soluble in water (solvent), meaning its particles break down into tiny, invisible ions that disperse evenly throughout the water.
  • Sand is insoluble in water, so its particles remain as large, visible grains that do not mix uniformly and settle due to gravity.

This demonstrates the key difference between soluble and insoluble substances in forming mixtures.

Question 16:
Priya was given two samples: one was a solution of sugar in water, and the other was a suspension of chalk powder in water. She was asked to distinguish between them using two different methods. Suggest the methods and explain how they would help in identification.
Answer:

Method 1: Observation of Uniformity

The sugar solution will appear clear and uniform throughout because sugar dissolves completely in water, forming a homogeneous mixture.

The chalk powder suspension will appear cloudy or uneven, as chalk does not dissolve and particles remain suspended or settle over time, indicating a heterogeneous mixture.

Method 2: Filtration Test

  • When the sugar solution is filtered, no residue is left on the filter paper because sugar particles are too small to be trapped.
  • When the chalk suspension is filtered, chalk particles are retained on the filter paper as residue, proving it is a suspension.

These methods highlight the differences in particle size and solubility between solutions and suspensions.

Question 17:
Rahul observed that when he mixed salt in water, it disappeared, but the taste was still present. He heated the solution, and only water evaporated, leaving salt behind. Based on this case, explain: (a) What type of mixture is saltwater? (b) Why did the salt reappear after heating?
Answer:

(a) Saltwater is a homogeneous mixture or solution because the salt particles are uniformly distributed in water and not visible to the naked eye.

(b) The salt reappeared after heating because the water evaporated, leaving behind the dissolved salt. This process is called evaporation, which separates the solute (salt) from the solvent (water).

Additional Insight: This method is used in salt pans to obtain salt from seawater, demonstrating a real-life application of separation techniques.

Question 18:
Priya noticed that her mother uses a sieve to separate wheat flour from impurities like husk. Later, she saw her mother using a magnet to remove iron filings from sand. Compare these two separation methods and explain the principle behind each.
Answer:

Comparison:

  • Sieving: Used to separate larger impurities (husk) from finer particles (wheat flour) based on particle size.
  • Magnetic Separation: Used to separate magnetic substances (iron filings) from non-magnetic substances (sand).

Principle:
Sieving works on the principle of size difference, while magnetic separation relies on the magnetic property of one component.

Application: Sieving is common in kitchens, while magnetic separation is used in recycling plants to separate metals.

Question 19:
Rahul observed that when he mixed salt in water, it disappeared, but the taste was still present. His teacher explained this as a true solution. Based on this, answer:
(a) Define a true solution and give two characteristics.
(b) Why does the salt not remain visible in water?
Answer:

(a) A true solution is a homogeneous mixture where solute particles (like salt) completely dissolve in the solvent (like water), forming a clear mixture.
Characteristics:
1. Particles are smaller than 1 nm and invisible even under a microscope.
2. The mixture is stable—no settling or scattering of light (Tyndall effect).

(b) Salt (solute) breaks into tiny particles (ions: Na⁺ and Cl⁻) that disperse uniformly in water (solvent). These particles are too small to scatter light or be seen, but their presence is detected by taste.

Question 20:
Priya noticed that her tea leaves settled at the bottom of her cup, while her friend’s juice had pulp floating uniformly.
(a) Classify both mixtures and justify.
(b) How can Priya separate the tea leaves from her drink? Explain the process.
Answer:

(a)
1. Tea leaves in water: Suspension (heterogeneous).
Reason: Large, insoluble particles settle down and can be filtered.
2. Pulp in juice: Colloidal solution.
Reason: Smaller particles remain suspended, scattering light (Tyndall effect) but not settling.

(b) Priya can use filtration:
1. Pour the tea through a filter paper or strainer.
2. The residue (tea leaves) stays on the paper, while the filtrate (clear tea) passes through.
Note: Filtration works because tea particles are large enough to be trapped.

Question 21:
Rahul observed that when he mixed salt in water, it disappeared, but the taste was still present. However, when he mixed sand in water, it settled down. Based on this, answer:
(a) Why does salt disappear in water but sand does not?
(b) What type of mixture is saltwater, and how can the salt be recovered?
Answer:

(a) Salt disappears in water because it is soluble, meaning its particles break down into tiny, invisible ions that mix uniformly with water molecules. This forms a homogeneous mixture. On the other hand, sand is insoluble in water, so its particles remain suspended or settle down, forming a heterogeneous mixture.


(b) Saltwater is a true solution (homogeneous mixture). To recover salt, we can use evaporation:
1. Pour the saltwater into a clean container.
2. Heat it gently until all water evaporates.
3. Salt crystals will remain as residue.

Question 22:
Priya noticed that tea leaves settle at the bottom of her cup, but milk mixes completely in tea. Explain:
(a) Why tea leaves form a suspension while milk forms a colloid?
(b) How can you separate tea leaves from the liquid tea?
Answer:

(a) Tea leaves form a suspension because their large, insoluble particles scatter light (Tyndall effect) and settle down due to gravity. Milk forms a colloid as its tiny fat droplets disperse uniformly in water, appearing homogeneous but actually scattering light.


(b) To separate tea leaves:
1. Use a strainer or filter paper.
2. Pour the tea through it.
3. The liquid (filtrate) passes through, while leaves (residue) are retained.

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