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Materials Around Us – CBSE NCERT Study Resources

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Materials Around Us

Study Materials

6th

6th - Science (EVS) Curiosity

Materials Around Us

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Overview of the Chapter: Materials Around Us

This chapter introduces students to the different types of materials that surround us in our daily lives. It explains the properties, uses, and classifications of these materials, helping students understand their significance in various applications.

Material: Any substance that is used to make objects or has a specific purpose is called a material.

Types of Materials

Materials can be broadly classified into the following categories:

  • Natural Materials: These are obtained from nature, such as wood, cotton, and metals.
  • Man-made Materials: These are created by humans through chemical processes, such as plastics and synthetic fibers.

Properties of Materials

Different materials have different properties, which determine their uses:

  • Hardness: The ability of a material to resist scratches or dents.
  • Flexibility: The ability to bend without breaking.
  • Transparency: The ability to allow light to pass through (e.g., glass).
  • Conductivity: The ability to conduct heat or electricity (e.g., metals).

Transparent Material: A material that allows light to pass through it clearly, like glass.

Uses of Materials

Materials are used in various ways depending on their properties:

  • Wood is used for making furniture due to its strength.
  • Plastic is used for containers because it is lightweight and durable.
  • Metals like iron are used in construction for their hardness.

Recycling of Materials

Recycling helps in reducing waste and conserving natural resources. Materials like paper, plastic, and metals can be recycled to make new products.

Recycling: The process of converting waste materials into reusable objects to prevent wastage.

Summary

This chapter helps students recognize the importance of materials in everyday life, their properties, and the need for sustainable practices like recycling.

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 the process of separating solid particles from a liquid called?
Answer:

Filtration
Our textbook shows filtering tea leaves.

Question 2:
Name a natural material used to make clothing.
Answer:

Cotton
We studied it grows on plants.

Question 3:
What property makes metals useful for electrical wires?
Answer:

Conductivity
Copper wires are common examples.

Question 4:
Which gas is released when vinegar reacts with baking soda?
Answer:

Carbon dioxide
It makes dough rise.

Question 5:
What is the state of matter of ice?
Answer:

Solid
Water freezes at 0°C.

Question 6:
Name a man-made material used for packaging.
Answer:

Plastic
Our textbook shows polythene bags.

Question 7:
What method separates salt from seawater?
Answer:

Evaporation
Sunlight dries the water.

Question 8:
Which fabric burns with a smell of hair?
Answer:

Wool
It comes from sheep.

Question 9:
What is the main component of glass?
Answer:

Sand
We studied silica in it.

Question 10:
Name a magnetic material.
Answer:

Iron
Used in fridge magnets.

Question 11:
What happens to sugar when dissolved in water?
Answer:

Dissolves
Makes sweet solution.

Question 12:
Which property makes rubber useful for tires?
Answer:

Elasticity
It bounces back.

Question 13:
What is the source of jute fibers?
Answer:

Jute plant
Used for sacks.

Question 14:
Name a liquid that mixes with water.
Answer:

Milk
Forms uniform mixture.

Question 15:
What is the process of changing water into water vapour called?
Answer:

The process of changing water into water vapour is called evaporation. It occurs when water is heated, and its molecules gain enough energy to escape into the air as vapour.

Question 16:
Name two materials that are soluble in water.
Answer:

Two materials that are soluble in water are sugar and salt. These dissolve completely in water to form a transparent solution.

Question 17:
What is the term used for materials that allow light to pass through them completely?
Answer:

Materials that allow light to pass through them completely are called transparent materials. Examples include glass and clear plastic.

Question 18:
Why is steel used to make cooking utensils?
Answer:

Steel is used to make cooking utensils because it is a good conductor of heat, durable, and does not react easily with food substances, making it safe for cooking.

Question 19:
What happens when a magnetic material is brought near a magnet?
Answer:

When a magnetic material (like iron or nickel) is brought near a magnet, it gets attracted to the magnet due to the magnetic force.

Question 20:
Define insoluble substances with an example.
Answer:

Insoluble substances are those that do not dissolve in a particular solvent (like water). An example is sand, which does not dissolve in water.

Question 21:
What is the difference between hard and soft materials?
Answer:

Hard materials (like diamond) cannot be scratched or compressed easily, while soft materials (like sponge) can be easily pressed or deformed.

Question 22:
Why is cotton preferred for making clothes in summer?
Answer:

Cotton is preferred for summer clothes because it is breathable and absorbs sweat, keeping the body cool and comfortable.

Question 23:
Name a material that is lustrous but not a metal.
Answer:

Graphite is a lustrous material (shiny) but is not a metal. It is a form of carbon.

Question 24:
What property makes rubber suitable for making erasers?
Answer:

Rubber is suitable for making erasers because it is soft, flexible, and can easily remove pencil marks without damaging the paper.

Question 25:
Why are metals good conductors of electricity?
Answer:

Metals are good conductors of electricity because they have free electrons that can move easily, allowing electric current to flow through them.

Question 26:
What happens when wax is heated?
Answer:

When wax is heated, it melts and changes from a solid to a liquid state. This is a reversible physical change.

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 the difference between transparent and translucent materials?
Answer:

Transparent materials allow light to pass through them clearly, like glass. Translucent materials allow light but scatter it, making objects appear blurry, like frosted glass.

Question 2:
Why is iron used to make bridges and buildings?
Answer:

Iron is strong, durable, and can bear heavy loads, making it ideal for bridges and buildings. It is also malleable and can be shaped easily.

Question 3:
Give an example of a natural and a synthetic fibre.
Answer:

Cotton is a natural fibre, while nylon is a synthetic fibre.

Question 4:
Why are metals good conductors of electricity?
Answer:

Metals have free electrons that move easily, allowing electric current to flow through them efficiently.

Question 5:
What is the process of separating insoluble solids from liquids called?
Answer:

The process is called filtration. It uses a filter paper to separate the solid from the liquid.

Question 6:
Name two materials that are brittle.
Answer:

Glass and chalk are brittle materials as they break easily when force is applied.

Question 7:
Why is plastic considered a non-biodegradable material?
Answer:

Plastic does not decompose naturally and remains in the environment for hundreds of years, causing pollution.

Question 8:
What is the role of a condenser in the distillation process?
Answer:

The condenser cools the vapor back into liquid form during distillation, allowing pure liquid to be collected.

Question 9:
How does evaporation help in obtaining salt from seawater?
Answer:

Evaporation removes water, leaving behind salt crystals. Seawater is left in sunlight, and water evaporates, leaving salt.

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:
What is the difference between transparent, translucent, and opaque materials? Give one example of each.
Answer:

Transparent materials allow light to pass through them clearly, so objects can be seen distinctly. Example: glass.
Translucent materials allow some light to pass but scatter it, making objects appear blurry. Example: butter paper.
Opaque materials do not allow any light to pass, so objects behind them cannot be seen. Example: wood.

Question 2:
Why is water called a universal solvent? Explain with an example.
Answer:

Water is called a universal solvent because it can dissolve a wide variety of substances due to its polar nature.
Example: Sugar dissolves easily in water because water molecules break the bonds between sugar particles, forming a solution.

Question 3:
How can you separate a mixture of sand and salt? Describe the steps involved.
Answer:

  • Step 1: Add water to the mixture. Salt dissolves, but sand does not.
  • Step 2: Filter the mixture. Sand remains on the filter paper, while saltwater passes through.
  • Step 3: Evaporate the water to obtain pure salt.

Question 4:
What are conductors and insulators of electricity? Give two examples of each.
Answer:

Conductors allow electricity to flow through them easily. Examples: copper, aluminum.
Insulators do not allow electricity to pass. Examples: rubber, plastic.

Question 5:
Explain why metals are used for making cooking utensils.
Answer:

Metals are used for making cooking utensils because:

  • They are good conductors of heat, ensuring even cooking.
  • They are durable and can withstand high temperatures.
  • They are malleable, allowing them to be shaped easily.

Question 6:
What happens when you mix oil and water? Why does this occur?
Answer:

When oil and water are mixed, they form separate layers because oil is hydrophobic (repels water).
Oil is less dense than water, so it floats on top. This happens due to differences in their polarity.

Question 7:
What is the difference between transparent and translucent materials? Give one example of each.
Answer:

Transparent materials allow light to pass through them clearly, so objects can be seen distinctly. Example: glass.
Translucent materials allow some light to pass but scatter it, making objects appear blurry. Example: butter paper.

Question 8:
Why are metals like iron and copper used for making electrical wires?
Answer:

Metals like iron and copper are used for electrical wires because:

  • They are good conductors of electricity.
  • They are ductile (can be drawn into thin wires).
  • They are strong and do not break easily.
Copper is preferred over iron as it conducts electricity better.

Question 9:
Explain why plastic is considered a non-biodegradable material.
Answer:

Plastic is non-biodegradable because:

  • It does not decompose naturally by microorganisms.
  • It remains in the environment for hundreds of years, causing pollution.
  • Unlike materials like paper or wood, it does not break down into harmless substances.

Question 10:
How can you separate a mixture of sand and salt? Explain the steps.
Answer:

Steps to separate sand and salt:

1. Dissolve the mixture in water. Salt dissolves, but sand does not.
2. Filter the solution. Sand remains on the filter paper.
3. Evaporate the water to obtain salt crystals.

Question 11:
What are soluble and insoluble substances? Give one example of each.
Answer:

Soluble substances dissolve completely in water. Example: sugar.
Insoluble substances do not dissolve in water. Example: chalk powder.

Solubility depends on the nature of the substance and the solvent.

Question 12:
Why is water called a universal solvent?
Answer:

Water is called a universal solvent because:

  • It dissolves a wide range of substances like salts, sugars, and gases.
  • Its polar nature helps break down many compounds.
  • It is essential for biological and chemical processes.

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 the properties of metals with examples from daily life and NCERT.
Answer:
Concept Overview

Metals are shiny, hard, and good conductors of heat and electricity. Our textbook shows examples like iron and copper.

Process Explanation
  • Metals can be hammered into sheets (malleability).
  • They can be drawn into wires (ductility).
Real-world Application

We use aluminum for foils and copper for electrical wires. Iron is used in construction.

Question 2:
Describe how soluble and insoluble substances are separated using filtration. Give NCERT and real-life examples.
Answer:
Concept Overview

Soluble substances dissolve in water, while insoluble ones do not. NCERT mentions salt and sand mixtures.

Process Explanation
  • Mix the substance in water.
  • Filter using a sieve or filter paper.
Real-world Application

We separate tea leaves from liquid using a strainer. Muddy water is filtered to remove soil.

Question 3:
What are natural and synthetic fibers? Compare their uses with NCERT and daily examples.
Answer:
Concept Overview

Natural fibers come from plants/animals (cotton, wool), while synthetic fibers are man-made (nylon). NCERT shows jute and polyester.

Process Explanation
  • Natural fibers are breathable.
  • Synthetic fibers are strong and waterproof.
Real-world Application

We wear cotton clothes in summer and use nylon ropes for durability.

Question 4:
Explain transparent, translucent, and opaque materials with NCERT and real-world examples.
Answer:
Concept Overview

Transparent materials allow full light (glass), translucent allow partial (butter paper), and opaque block light (wood). NCERT uses these terms.

Process Explanation
  • Transparent: Clear visibility.
  • Translucent: Blurry vision.
Real-world Application

We use glass for windows, butter paper for lampshades, and wood for doors.

Question 5:
How are mixtures and compounds different? Provide NCERT and practical examples.
Answer:
Concept Overview

Mixtures are physically combined (salt + water), while compounds are chemically bonded (H2O). NCERT explains air as a mixture.

Process Explanation
  • Mixtures can be separated easily.
  • Compounds need chemical methods.
Real-world Application

We make lemonade (mixture), but water (compound) cannot be separated easily.

Question 6:
What are natural and synthetic fibres? Compare their properties with NCERT and daily-use examples.
Answer:
Concept Overview

Natural fibres come from plants/animals (cotton, wool), while synthetic are man-made (nylon). NCERT shows jute as natural fibre.

Process Explanation
  • Natural fibres absorb water, synthetic repel.
  • Synthetic fibres are stronger.
Real-world Application

We wear cotton clothes in summer for comfort. Raincoats are made of synthetic polyester to repel water.

Question 7:
Explain transparent, translucent and opaque materials with NCERT and household examples.
Answer:
Concept Overview

Transparent materials allow full light (glass), translucent partial (butter paper), opaque none (wood). NCERT uses tracing paper example.

Process Explanation
  • Depends on material's light interaction.
  • Can be tested with torch.
Real-world Application

We use glass windows for clear view. Bathroom windows have frosted glass for privacy. [Diagram: Light passing through different materials]

Question 8:
How does rusting of iron occur? Describe prevention methods with NCERT and real-life applications.
Answer:
Concept Overview

Rusting is iron reacting with oxygen and water. NCERT shows iron nail experiment turning brown.

Process Explanation
  • Forms hydrated iron oxide (rust).
  • Needs both air and moisture.
Real-world Application

We paint iron gates to prevent rust. Ships use zinc coating (galvanization) for protection.

Question 9:
Explain the classification of materials based on their properties with examples.
Answer:
Concept Overview

Materials are classified into groups like metals, non-metals, and polymers based on properties such as hardness, conductivity, and flexibility.

Process Explanation
  • Metals: Shiny, conductive (e.g., iron in NCERT).
  • Non-metals: Dull, insulators (e.g., sulfur).
  • Polymers: Flexible, lightweight (e.g., plastic bags).
Real-world Application

We use metals for wires (conductivity) and polymers for bottles (durability). Our textbook shows how rubber (non-metal) is used for tires.

Question 10:
Describe how recycling helps reduce waste with NCERT and daily-life examples.
Answer:
Concept Overview

Recycling converts waste into reusable materials, reducing landfill and resource use.

Process Explanation
  • Paper recycling saves trees (NCERT example).
  • Plastic bottles are melted to make new products.
Real-world Application

We separate waste at home for recycling. Our textbook shows how recycling aluminum saves energy compared to mining.

Question 11:
What are natural and synthetic materials? Compare them using NCERT examples.
Answer:
Concept Overview

Natural materials come from nature (e.g., cotton), while synthetic ones are man-made (e.g., polyester).

Process Explanation
  • Natural: Biodegradable (NCERT: wool from sheep).
  • Synthetic: Durable (e.g., nylon ropes).
Real-world Application

We wear cotton (natural) in summer and polyester (synthetic) for sportswear. Our textbook shows how jute bags replace plastic.

Question 12:
Explain conductors and insulators with examples from daily life and NCERT.
Answer:
Concept Overview

Conductors allow heat/electricity (e.g., copper), while insulators block them (e.g., wood).

Process Explanation
  • Conductors: Used in wires (NCERT: iron nails).
  • Insulators: Protect handles (e.g., plastic).
Real-world Application

We use ceramic (insulator) in kitchen gloves. Our textbook shows how silver conducts electricity best.

Question 13:
How do biodegradable and non-biodegradable materials impact the environment? Give examples.
Answer:
Concept Overview

Biodegradable materials decompose (e.g., paper), while non-biodegradable persist (e.g., plastic).

Process Explanation
  • Biodegradable: Enrich soil (NCERT: vegetable peels).
  • Non-biodegradable: Cause pollution (e.g., polythene).
Real-world Application

We compost food waste but recycle plastic. Our textbook shows how banana leaves decompose faster than plastic.

Question 14:
Compare transparent, translucent, and opaque materials with NCERT and daily examples.
Answer:
Concept Overview

Transparent materials allow full light (glass), translucent partial (butter paper), and opaque none (wood). NCERT explains this in Chapter 2.

Process Explanation
  • Transparent: Clear visibility
  • Translucent: Blurry shadows
Real-world Application

We use glass windows (transparent) and lampshades (translucent). Opaque objects like books block light completely.

Question 15:
Explain reversible and irreversible changes with NCERT and cooking examples.
Answer:
Concept Overview

Reversible changes can be undone (melting ice), while irreversible cannot (burning paper). Our textbook shows these in Chapter 6.

Process Explanation
  • Reversible: State changes
  • Irreversible: New substances form
Real-world Application

Making lemonade (reversible) vs baking cake (irreversible). [Diagram: Ice melting vs paper burning]

Question 16:
Describe magnetic and non-magnetic materials with NCERT and household examples.
Answer:
Concept Overview

Magnetic materials (iron) are attracted to magnets, while non-magnetic (plastic) aren't. NCERT demonstrates this in Chapter 10.

Process Explanation
  • Magnetic: Contain iron, nickel, cobalt
  • Non-magnetic: Other materials
Real-world Application

We use fridge magnets (magnetic) while plastic toys (non-magnetic) don't stick. [Diagram: Magnet attracting nails]

Question 17:
Explain the process of sedimentation and decantation with a real-life example. How are these methods useful in separating mixtures?
Answer:

Sedimentation is the process where heavier particles in a mixture settle down at the bottom when left undisturbed. Decantation is the method of pouring out the clear liquid without disturbing the settled particles.

For example, when muddy water is kept in a glass, the soil particles settle down after some time. The clean water above can then be carefully poured out, leaving the mud behind.

  • These methods are useful because:
  • They help separate insoluble solids from liquids.
  • They are simple and do not require complex equipment.
  • They are used in water purification plants to remove impurities.

This process is also used at home to separate rice from water after washing or to clear juices from fruit pulp.

Question 18:
Differentiate between transparent, translucent, and opaque materials with examples. Why is it important to classify materials based on their light transmission properties?
Answer:

Transparent materials allow almost all light to pass through them, making objects clearly visible. Example: glass.
Translucent materials allow some light to pass but scatter it, making objects appear blurry. Example: butter paper.
Opaque materials do not allow any light to pass, blocking visibility completely. Example: wood.

Classifying materials based on light transmission is important because:

  • It helps in choosing the right material for specific purposes (e.g., windows need transparent glass).
  • It ensures safety (e.g., opaque materials are used for privacy).
  • It aids in scientific studies of light and vision.
Understanding these properties helps us use materials effectively in daily life.
Question 19:
Describe the properties and uses of metals and non-metals with examples. How do their properties make them suitable for specific purposes?
Answer:

Metals (like iron, copper, and aluminum) have the following properties:

  • They are hard, lustrous, and good conductors of heat and electricity.
  • They can be hammered into sheets (malleability) or drawn into wires (ductility).

Uses: Iron is used in construction, copper in electrical wires, and aluminum in packaging.

Non-metals (like sulfur, carbon, and oxygen) have these properties:

  • They are usually brittle, dull, and poor conductors.
  • They exist in all states (solid, liquid, gas).

Uses: Carbon is used in pencils, oxygen for breathing, and sulfur in medicines.

Their properties determine their uses. For example, metals are used in wires due to conductivity, while non-metals like rubber are used as insulators.

Question 20:
Differentiate between transparent, translucent, and opaque materials with examples. Why is this classification important in daily life?
Answer:

Transparent materials allow almost all light to pass through them, making objects clearly visible (e.g., glass, clean water). Translucent materials allow partial light, blurring objects (e.g., frosted glass, butter paper). Opaque materials block all light, making objects behind them invisible (e.g., wood, metal).

This classification is important because:

  • It helps choose the right material for specific needs (e.g., windows use transparent glass for visibility).
  • Translucent materials provide privacy while allowing light (e.g., bathroom windows).
  • Opaque materials are used where light blockage is needed (e.g., curtains for darkness).

Understanding these properties ensures efficient use of materials in construction, packaging, and daily objects.

Question 21:
Describe the properties and uses of metals and non-metals with examples. How do their properties make them suitable for different purposes?
Answer:

Metals are shiny, malleable (can be hammered into sheets), ductile (can be drawn into wires), and good conductors of heat and electricity. Examples include iron (used in construction), aluminum (used in foil and cans), and copper (used in electrical wires).

Non-metals are usually dull, brittle, and poor conductors. Examples include sulfur (used in matches), oxygen (essential for breathing), and carbon (used in pencils as graphite).

  • Metals are used where strength and conductivity are needed, like in buildings or electronics.
  • Non-metals are used where insulation or lightness is required, like in plastic containers or rubber tires.

Their properties determine their uses. For example, gold is used in jewelry because it is shiny and does not rust, while chlorine (a non-metal) is used to purify water because it kills bacteria.

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:
Riya observed that some materials like iron and plastic behave differently when exposed to water. Case Summary: Iron rusts, but plastic remains unchanged.
Question: Why does this happen, and how can we prevent rusting?
Answer:
Case Summary
Iron reacts with water and oxygen to form rust, while plastic is water-resistant.
Scientific Principle
We studied that rusting is a chemical change (NCERT Ch-4).
Solution Approach
  • Apply paint or grease to iron surfaces.
  • Use stainless steel, which resists rust.

Example: NCERT shows iron nails rusting in water. Real-world: Ships are painted to prevent rust.

Question 2:
Aarav tested transparency of materials. Case Summary: Glass allowed light to pass, but wood blocked it.
Question: Classify these materials and give one use of each based on their property.
Answer:
Case Summary
Glass is transparent, wood is opaque.
Scientific Principle
Our textbook shows transparency depends on material (NCERT Ch-2).
Solution Approach
  • Glass: Used in windows for visibility.
  • Wood: Used for doors for privacy.

Example: NCERT lists glass as transparent. Real-world: Sunglasses use tinted glass.

Question 3:
Priya noticed that cotton and synthetic cloth dry at different rates. Case Summary: Cotton dried slower.
Question: Explain why and suggest a material for rainy season.
Answer:
Case Summary
Cotton absorbs more water than synthetic fabric.
Scientific Principle
We studied absorption properties in NCERT (Ch-3).
Solution Approach
  • Cotton has tiny gaps holding water.
  • Use synthetic clothes in rain as they dry faster.

Example: NCERT compares cotton and polyester. Real-world: Raincoats use waterproof synthetics.

Question 4:
An experiment compared thermal conductivity of metals and non-metals. Case Summary: Metal spoon heated faster than plastic.
Question: What does this show? Give one safety use of this property.
Answer:
Case Summary
Metals conduct heat better than plastics.
Scientific Principle
NCERT explains conductivity in metals (Ch-4).
Solution Approach
  • Metals transfer heat quickly.
  • Plastic handles on pans prevent burns.

Example: NCERT shows metal rods heating. Real-world: Electric wires have plastic coating.

Question 5:
Riya observed that some materials like iron nails sink in water, while a plastic toy floats. Explain why this happens using properties of materials.
Answer:
Case Summary

Riya noticed that iron nails sink, but plastic toys float in water.

Scientific Principle
  • Objects sink if their density is higher than water.
  • Objects float if their density is lower.
Solution Approach

Our textbook shows iron is denser than water, so nails sink. Plastic is less dense, so toys float. This is also seen with ships (hollow iron) floating due to shape.

Question 6:
A shopkeeper wraps hot pakoras in newspaper instead of plastic. Why is this a better choice?
Answer:
Case Summary

Newspaper is used to wrap hot pakoras instead of plastic.

Scientific Principle
  • Plastic melts at high temperatures.
  • Newspaper is a heat insulator and doesn’t react.
Solution Approach

We studied that plastic can melt or release harmful chemicals when heated. Newspaper, like in NCERT’s ‘garbage’ chapter, is safer for hot food. Real-world example: Street vendors use paper for hot snacks.

Question 7:
During rain, Seema’s cotton clothes got wet but her friend’s raincoat didn’t. Compare the materials.
Answer:
Case Summary

Cotton clothes absorb water, while raincoats don’t.

Scientific Principle
  • Cotton has tiny gaps that absorb water (porous).
  • Raincoats are made of waterproof materials like plastic.
Solution Approach

Our textbook shows cotton absorbs sweat, but synthetic materials (like NCERT’s ‘fabrics’ chapter) repel water. Umbrellas also use waterproof fabric.

Question 8:
Mohan saw his mother store pickles in glass jars, not metal containers. Explain the reason.
Answer:
Case Summary

Pickles are stored in glass jars instead of metal containers.

Scientific Principle
  • Pickles contain acids that react with metals.
  • Glass is chemically inert and doesn’t react.
Solution Approach

We studied in NCERT that acids corrode metals (like vinegar cleaning coins). Glass, as shown in ‘materials’ chapter, is safe for acidic foods. Real-world example: Jams are also stored in glass.

Question 9:
Riya observed that some materials like iron nails sink in water, while others like wooden blocks float. Case Summary: She tested objects of different materials.
Question: Why do some materials float while others sink? Explain with NCERT examples.
Answer:
Case Summary
Riya tested objects like iron nails and wooden blocks.
Scientific Principle
Objects float if their density is less than water. Our textbook shows a cork floats due to low density.
Solution Approach
  • Iron nails sink as they are denser than water.
  • Wood floats like boats, as its density is lower.

This matches NCERT's example of a plastic bottle floating.

Question 10:
Aarav noticed his steel lunchbox gets hot quickly, but his cloth napkin does not. Case Summary: He compared heat conduction.
Question: Why do some materials conduct heat better? Use NCERT examples.
Answer:
Case Summary
Aarav compared steel and cloth.
Scientific Principle
Conductors (like steel) transfer heat fast. NCERT shows copper as a good conductor.
Solution Approach
  • Steel has tightly packed atoms, transferring heat quickly.
  • Cloth traps air, acting as an insulator.

This is similar to NCERT's example of a wooden handle staying cool.

Question 11:
Priya saw a rubber band stretch but a glass bangle breaks when bent. Case Summary: She tested flexibility.
Question: Why are some materials flexible? Relate to NCERT examples.
Answer:
Case Summary
Priya tested rubber and glass.
Scientific Principle
Elasticity lets materials stretch. NCERT mentions rubber as elastic.
Solution Approach
  • Rubber has coiled polymer chains that stretch.
  • Glass is rigid due to its brittle structure.

This matches NCERT's example of clay being moldable.

Question 12:
Rohan found sugar dissolves in water but sand does not. Case Summary: He tested solubility.
Question: Why do some materials dissolve? Explain with NCERT examples.
Answer:
Case Summary
Rohan tested sugar and sand.
Scientific Principle
Soluble materials mix completely. NCERT shows salt dissolving in water.
Solution Approach
  • Sugar particles break down and mix uniformly.
  • Sand is insoluble as its particles don’t separate.

This aligns with NCERT's example of chalk not dissolving.

Question 13:
Rahul observed that his mother stores sugar in an airtight container while salt is kept in an open jar. He wondered why sugar needs special storage. Based on this case, answer:

1. Why is sugar stored in an airtight container?
2. What property of sugar makes this necessary?
Answer:

Sugar is stored in an airtight container because it is hygroscopic, meaning it absorbs moisture from the air.

1. If left open, sugar becomes sticky and forms lumps due to moisture absorption, making it difficult to use.
2. The hygroscopic nature of sugar is the key property requiring airtight storage. Unlike salt (which is less hygroscopic), sugar needs protection from humidity to maintain its free-flowing texture.

Question 14:
Priya noticed that her iron tiffin box developed reddish-brown patches after a few months, while her plastic lunchbox remained unchanged. Analyze this case and answer:

1. What causes the reddish-brown patches on the iron tiffin?
2. Why doesn’t this happen to the plastic lunchbox?
Answer:

The reddish-brown patches on the iron tiffin box are due to rusting, a chemical reaction between iron, oxygen, and moisture.

1. Rust (iron oxide) forms when iron reacts with water and air, weakening the metal.
2. Plastic is a non-reactive material and does not corrode or react with air/water, so it remains unchanged. Unlike iron, plastics are synthetic polymers resistant to environmental factors.

Question 15:
Rahul observed that his mother stores sugar in an airtight container while salt is kept in an open jar. He wondered why they are stored differently.

(a) Explain the scientific reason behind storing sugar in an airtight container.
(b) Why is salt not stored similarly?

Answer:

(a) Sugar is stored in an airtight container because it is hygroscopic, meaning it absorbs moisture from the air.
Exposure to air can make sugar sticky or lumpy due to moisture absorption, reducing its quality.

(b) Salt, on the other hand, is also hygroscopic but less so compared to sugar.
Common salt (sodium chloride) does not absorb moisture as quickly, so it can be stored in an open jar without significant changes.
However, in very humid conditions, even salt may clump, so storing it in a dry place is still advisable.

Question 16:
Priya noticed that iron nails left outside for a few days developed a reddish-brown coating, while aluminum foil remained unchanged.

(a) What is the reddish-brown coating called, and why does it form?
(b) Why does aluminum not show the same change?

Answer:

(a) The reddish-brown coating is called rust, which forms due to a chemical reaction called rusting.
When iron reacts with oxygen and water in the air, it forms iron oxide (rust).
Rust weakens the iron and makes it flaky.

(b) Aluminum does not rust because it forms a thin, protective layer of aluminum oxide when exposed to air.
This layer prevents further reaction with oxygen and moisture, keeping the metal underneath safe.
This property is called corrosion resistance.

Question 17:
Riya observed that her mother stores sugar in an airtight container while salt is kept in an open jar. She wondered why sugar needs extra care. Explain the scientific reason behind this observation and list two other substances that require similar storage conditions.
Answer:

Sugar is stored in an airtight container because it is hygroscopic, meaning it absorbs moisture from the air. This can cause sugar to become sticky or form lumps, making it difficult to use. Salt, on the other hand, is less hygroscopic and can be stored in an open jar without significant issues.


Two other substances that require similar storage conditions are:

  • Honey (absorbs moisture and can ferment)
  • Flour (can attract pests and absorb odors if not stored properly)

Proper storage helps maintain the quality and usability of these materials.

Question 18:
During a science experiment, Arjun noticed that a piece of iron nail left outside rusted, while a plastic spoon remained unchanged. Help Arjun understand why this happens by explaining the difference in properties of iron and plastic. Also, suggest two methods to prevent rusting of iron objects.
Answer:

Iron nails rust because iron reacts with oxygen and moisture in the air to form iron oxide (rust). This is a chemical change. Plastic, however, is a synthetic material that does not react easily with air or water, so it remains unchanged.


Two methods to prevent rusting are:

  • Galvanization: Coating iron with a layer of zinc to protect it from moisture.
  • Painting or Oiling: Creating a barrier between iron and air/moisture.

Understanding these properties helps us choose the right materials for different uses.

Question 19:
Riya observed that her mother stores sugar in an airtight container after use, while salt is kept in an open jar. She wondered why they are stored differently. Explain the scientific reason behind this observation, including the properties of the materials involved.
Answer:

Riya's mother stores sugar and salt differently due to their distinct properties. Sugar is hygroscopic, meaning it absorbs moisture from the air, which can cause it to clump or spoil. Storing it in an airtight container prevents exposure to humidity.

On the other hand, salt (like common table salt) is less hygroscopic and does not absorb moisture as easily. However, if left in humid conditions for long, it may also clump. In most households, salt is used frequently, so keeping it in an open jar is convenient without significant risk of spoilage.

Key terms: hygroscopic (ability to absorb moisture), airtight container (prevents air exposure).

Question 20:
During a school experiment, students were given three materials: a piece of wood, a metal spoon, and a plastic ruler. They were asked to identify which material is a good conductor of heat. Design an experiment to test this and explain the expected observations.
Answer:

To test which material is a good conductor of heat, follow these steps:

  • Take a cup of hot water and place the wooden piece, metal spoon, and plastic ruler in it.
  • Wait for 2 minutes.
  • Touch each material carefully to check how hot they feel.

Expected Observations:

  • The metal spoon will feel the hottest because metals are good conductors of heat.
  • The plastic ruler and wooden piece will feel less hot or even cool because they are poor conductors (insulators).

Key terms: conductor of heat (transfers heat efficiently), insulator (resists heat flow).

Question 21:
Priya noticed that her iron gate started developing reddish-brown patches over time, while the aluminum window frames remained shiny. Analyze the situation and answer:

1. What is the process causing the iron gate to change color?
2. Why does aluminum not show similar changes?
Answer:

The reddish-brown patches on the iron gate are due to rusting, a type of corrosion that occurs when iron reacts with oxygen and moisture in the air to form iron oxide.


Aluminum does not rust because it forms a thin, protective layer of aluminum oxide when exposed to air. This layer prevents further reaction with oxygen and moisture, keeping the aluminum shiny and durable.

Question 22:
Rahul observed that his mother stores sugar in an airtight container while salt is kept in an open jar. He wondered why sugar needs extra care. Based on this case, answer:

1. Why does sugar attract moisture from the air?
2. What property of salt makes it safe to store in an open jar?
Answer:

Sugar attracts moisture from the air because it is hygroscopic. This means it has the ability to absorb water molecules from its surroundings, which can cause it to clump or become sticky if not stored properly.


Salt, on the other hand, is also hygroscopic but to a much lesser extent compared to sugar. Common salt (sodium chloride) does not absorb moisture as readily under normal conditions, making it safe to store in an open jar. Additionally, salt has a crystalline structure that is less prone to clumping when exposed to air.

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