Chapter 5 Changes Around Us: Physical and Chemical Extra Questions and Answers Class 7 Science

Extra Questions for Chapter 5 Changes Around Us: Physical and Chemical Class 7 Science Curiosity

Chapter 5 Changes Around Us: Physical and Chemical Important Questions Class 7 Science is provided by studyrankers. We have included all the important points from the chapter in this note. This chapter, Changes Around Us: Physical and Chemical is curated in such a way that it will help the students is understanding the concepts easily. Revision Notes for Chapter 5 Changes Around Us: Physical and Chemical is also give on this website which provide students with in depth knowledge of the chapter. We have also given Changes Around Us: Physical and Chemical NCERT Solutions which is very much needed to acquire knowledge about the chapter.

Class 7 Science Changes Around Us: Physical and Chemical Extra Questions

Very Short Answer Questions

Question 1. How can you say that ripening of a fruit is a chemical change?

Answer

Ripening of a fruit is a chemical change because after ripening, a new product with different properties is formed.

Question 2. What is the white substance formed when lime water reacts with carbon dioxide?

Calcium carbonate.

Question 3. What kind of change is chopping vegetables?

Irreversible physical change.

Question 4. What is bioluminescence, and where is it observed?

Answer

Bioluminescence is light produced by living organisms through a chemical change without heat. It is observed in fireflies, which glow in gardens at night.

Question 5. Explain chemical reaction along with examples.

Answer

A change in which one or more new substances are formed is called a chemical reaction. For example; rusting of iron.

Question 6. State the two important conditions for rusting.

Answer

Presence of water and presence of oxygen.

Question 7. Name the three requirements of the fire triangle.

Fuel, oxygen, and heat (ignition temperature).

Question 8. What is added to steel to make it stainless?

Answer

Stainless steel is made by mixing iron with carbon and metals like chromium, nickel, and manganese.

Question 9. What is weathering?

Answer

The breakdown of rocks into smaller pieces through physical or chemical processes.

Question 10. What is erosion?

Answer

The movement of rocks, soil, or sediments by wind or water.

Question 11. What is melting?

Answer

Melting is the process of turning a solid to a liquid.

Question 12. What do you understand by chemical change?

Answer

A change in which one or more new substances are formed is called a chemical change.

Question 13. Explain physical reaction along with examples.

Answer

A change in which a substance undergoes a change in its physical properties is called a physical change. For example; melting of ice.

Question 14. What is an irreversible change?

Answer

A change that cannot be undone, like burning wood.

Question 15. What is a chemical change?

Answer

A change, in which a new substance with different properties is formed, is known as a chemical change.

Question 16. Name the two methods by which rusting of iron can be prevented.

Answer

By painting or greasing and by galvanizing with Zinc.

Question 17. What is the fire triangle?

Answer

The three requirements for combustion: fuel, oxygen, and heat.

Question 18. What is the ignition temperature?

Answer

The minimum temperature at which a substance catches fire.

Question 19. What is bioluminescence?

Answer

Light produced by living organisms, like fireflies, through a chemical change.

Short Answer Questions

Question 1. What happens when magnesium ribbon is burnt in air?

Answer

When magnesium ribbon is burnt in air, it reacts with the oxygen present to form magnesium oxide. This process produces a bright flame and results in a white powder.

• The reaction can be summarised as:
  Magnesium (Mg) + Oxygen (O₂) → Magnesium oxide (MgO)
• Heat and light are also released during this reaction.

Question 2. Why formation of manure from leaves is a chemical change?

Answer

Formation of manure from leaves is a chemical change because:

• The composition of the resulting manure is different from that of the original leaves.
• New substances are created during the decomposition process.
• This transformation involves a series of chemical reactions

Question 3. Why tearing of paper into pieces is a physical change?

Answer

Tearing a piece of paper is a physical change because:

• Only the shape and size of the paper are altered.
• No new substance is created during this process.

Question 4. What is rusting?

Answer

If a piece of iron is left in the open for some time, it acquires a film of brownish substance. This substance is called rust the process is called rusting.

The process of rusting can be represented by the following equation:

Iron (Fe) + Oxygen (O₂, from the air) + water (H₂O) → rust (iron oxide Fe₂O₃)

 

Question 5. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

Answer

Burning of wood: Burning is a non-reversible chemical change because when we burn wood new substances are formed as the carbon in the wood reacts with oxygen in the air to create ash and smoke, and energy in the form of light and heat.

Cutting of wood it into small pieces: Cutting of wood into small pieces are physical change as no new substance is formed. Only shape and size changes when wood is cut into small pieces.

Question 6. What is a physical change?

Answer

A physical change occurs when a substance alters its physical properties without forming a new substance. Key characteristics include:

• Changes can involve shape, size, colour, and state of the substance.
• These changes are generally reversible.
• No new substances are created during a physical change.

Question 7. Explain the changes that occur when a candle burns.

Answer

When a candle burns, both physical and chemical changes take place. The wax melts (physical change), evaporates, and then the vapour burns in air to form carbon dioxide and water (chemical change). Melting and re-solidifying wax are reversible, but the burning process is irreversible and forms new substances.

Question 8. Differentiate between physical and chemical reactions.

Answer 

Physical Reaction	Chemical Reaction
A change in the physical properties of a substance.	A change in which one or more new substances are formed.
A physical change is generally reversible.	A chemical change is generally irreversible.

Question 9. State the condition necessary for the occurrence of any reactant.

Answer

For the occurrence of any reaction, the following condition is necessary:

• The molecules or atoms of the reactants must collide with each other.
• This collision is essential to break old bonds and form new ones.

Question 10. Why new products are formed in a chemical reaction?

Answer

New products are formed in a chemical reaction due to the following reasons:

• The old bonds between atoms in the reactants are broken.
• New bonds are created, leading to the formation of different substances.
• The resulting products have different properties compared to the original reactants.

Question 11. Explain the process of rusting.

Answer

The process of rusting occurs when iron is exposed to damp air or water for a long time. This results in a reddish-brown flaky substance known as rust. The key steps in rusting are:

• Iron reacts with oxygen from the air.
• This reaction requires the presence of water or moisture.
• The outcome of this reaction is a compound called iron oxide.

Rusting is accelerated in humid conditions, making it a significant issue for iron objects.

Question 12. Setting of curd is regarded as a chemical change, explain why?

Answer

The setting of curd is considered a chemical change for the following reasons:

• The original substance, milk, cannot be recovered after curd is formed.
• Curd has distinct taste, smell, and chemical properties compared to milk.
• New substances are created during the process, indicating a transformation.

Question 13. What happens when baking soda is treated with vinegar?

Answer

When baking soda is mixed with vinegar, a reaction occurs that produces a hissing sound and bubbles. This is due to the formation of carbon dioxide gas.

The chemical equation for this reaction is:

• Vinegar (Acetic acid) + Baking soda (Sodium hydrogen carbonate) → Carbon dioxide + other substances.

The bubbles you see are the carbon dioxide gas being released during the reaction.

Question 14. What happens when a piece of iron metal is placed in copper sulphate solution?

Answer

When iron is placed in Copper sulphate solution, the iron is coated with a brown coloured substance "copper" and CuSO₄ solution changes from blue to light green. This is due to iron displacing copper as it is more reactive than copper.

Question 15. Name the three requirements for combustion.

Answer

The three requirements for combustion are 

1. fuel (a combustible substance) 
2. Oxygen (from the air)
3. Heat (to reach the ignition temperature).

Question 16. Why formation of manure from leaves is a chemical change?

Answer

The formation of manure from leaves is a chemical change because:

• It produces a new substance with a different composition.
• The original leaves undergo a transformation that alters their chemical structure.

Question 17. When we keep a piece of iron in the open area for few days, a brownish, flaky substance, called rust, is deposited on it.

(a) Is rust different from iron?
(b) Is formation of rust from iron a chemical change?
(c) Can you change rust back into iron by some simple method?
(d) Give some other examples of a similar type of changes.

Answer

(a) Yes. Rust is oxide of iron.

(b) Yes, it is a chemical change as properties of iron has changed.

(c) No.

(d) (i) Setting of curd into milk (ii) Burning of magnesium ribbon in air (iii) Cooking of food.

Question 18. Why lime water turns milky on passing carbon dioxide gas into it?

Answer

Chemical formula of lime water is calcium hydroxide which is a colourless solution. When we pass carbon dioxide gas into lime water, it forms white coloured insoluble calcium carbonate.
Lime water + Carbon dioxide ➝ Calcium carbonate + Water

Question 19. Explain how painting of an iron gate prevents it from rusting.

Answer

For rusting, the presence of both oxygen and water (or water vapour) is essential. Painting of an iron gate prevents it from coming in contact with oxygen, or water, or both and thus prevents it from rusting.

Question 20. Why stretching of rubber band is a physical change?

Answer

When rubber band is stretched only its size changes and it comes back in its original shape and size, once it is released. Moreover, it does not cause any change in its chemical composition. Hence, stretching of rubber band is a physical change.

Question 21. What happens when magnesium ribbon is burnt in air?

Answer

When the magnesium ribbon burnt in air, it reacts with oxygen found in the air to form Magnesium Oxide. After it burns, it forms a white powder of the magnesium oxide.

The change can be represented by the following equation:

Magnesium (Mg) + Oxygen (O₂) → Magnesium oxide (MgO)

Question 22. Complete the following reaction

Ca (OH)₂ + CO₂ →

Answer

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

Ca(OH)₂: Calcium hydroxide

CO₂: Carbon dioxide

CaCO₃: Calcium carbonate

Question 23. Magnesium ribbon bums in air and changes to white substance, i.e. magnesium oxide. When magnesium oxide dissolves in water, what type of change take place? Give reason in support of your answer. Express the change in the form of equation.

Answer

Mixing of ash obtained by the burning of magnesium with water is a chemical change. When magnesium is burnt in air, it forms magnesium oxide in the form of white ash.

Magnesium (Mg) + Oxygen (O₂) → Magnesium oxide (MgO)

When magnesium oxide dissolves in water, it forms a new substance, magnesium hydroxide.

Magnesium oxide (MgO) + Water (H₂O) → Magnesium hydroxide Mg(OH)₂

So, it is a chemical change.

Question 24. Classify the following processes into physical or chemical changes.

(a) Beating of aluminium metal to make aluminium foil

(b) Digestion of food

(c) Cutting of a log of wood into pieces

(d) Burning of crackers

Answer

Physical changes: (a) Beating of aluminium metal to make aluminium foil and (c) Cutting of a log of wood into pieces.

Chemical changes: (b) Digestion of food and (d) Burning of crackers.

Question 25. Plants prepare their food by a process called photosynthesis. Can we call photosynthesis is a chemical change? Explain.

Answer

During photosynthesis, the plants intake carbon dioxide and water in the presence of chlorophyll and sunlight to form two new substances, i.e. glucose (food) and oxygen gas. So, photosynthesis is a chemical change.

Question 26. What happens when an iron blade of a knife is dipped in a copper sulphate solution? What kind of change takes place?

Answer

When an iron blade of a knife is dipped in a copper sulphate solution, then iron blade is coated with reddish brown deposits of copper. And the blue colour of copper sulphate solution changes to light green due to the formation of iron sulphate. So, it is a chemical change.

Question 27. Explain the changes occurring in burning of candle.

Answer

When a candle burns, it undergoes both physical and chemical changes:

• The wax melts as it heats up, which is a physical change because it can solidify again when cooled.
• As the candle burns, it produces light and gases, such as carbon dioxide, indicating a chemical change due to the combustion of the wick.

Question 28. Baking soda is mixed with lemon juice, bubbles are formed with the evolution of gas, explain the changes occurred here.

Answer

When baking soda is mixed with lemon juice, a chemical reaction occurs, resulting in the formation of bubbles. This happens because:

• A new substance, carbon dioxide, is produced during the reaction.
• The release of gas creates visible bubbles.
• This process is an example of a chemical change as new substances are formed.

Question 29. Name some of the process in which both chemical and physical changes take place.

Answer

One example of a process that involves both chemical and physical changes is the burning of wood:

• The moisture in the wood turns to vapour when heated, which is a physical change.
• The actual burning produces carbon dioxide and other products, indicating a chemical change.

Question 30. Explain why burning of wood and cutting it into small pieces are considered as two different types of changes.

Answer

Burning of wood produces ash and smoke. Hence the properties of wood are changed and new substances are formed. So, it is a chemical reaction. When a log of wood is cut into small pieces, there is no new substance formed. Each small piece bears the properties of wood. So, its a physical change. Obviously, burning and cutting of wood are two different types of changes.

Question 31. Explain how painting of an iron rod prevents it from rusting.

Answer

To prevent rusting, it is essential to stop iron from contacting both air and moisture. Painting an iron gate achieves this by:

• Creating a protective barrier that blocks air and moisture.
• Reducing the chances of rust formation.
• Ensuring the iron remains dry and free from corrosive elements.
• Regular maintenance of the paint layer is important to keep the iron protected. 

Question 32. Burning of any substance is the chemical change. Discuss.

Answer

Burning of any substance is a chemical change. Burning involves a series of changes:

• Heat is always produced.
• The old bonds in the reactants are broken.
• New bonds are formed, leading to new products.
• The properties of these new products differ from the original substances.

Thus, burning is a clear example of a chemical change. 

Question 33. Why cutting of wood is a physical change?

Answer

Cutting of wood is classified as a physical change for the following reasons:

• The identity of the wood remains the same.
• Its composition does not change.
• No new substances are formed during the process.

Thus, cutting wood is a reversible change that affects its shape and size but not its fundamental properties. 

Question 34. What is rusting, and why is it considered a chemical change?

Answer

Rusting is a chemical reaction where iron reacts with air (oxygen) and moisture to form rust (iron oxide), a new substance. It is a chemical change because a new material with different properties is produced.

Long Answer Questions

Question 1. How can we prevent rusting?

Answer

To prevent rusting, it is essential to keep iron, water, and oxygen apart. Here are some effective methods:

• Apply a coat of paint or grease regularly to create a barrier.
• Use wax or oil coatings, especially for vehicles.
• Keep tools and equipment dry; wipe them down after use.
• Store items in a dry place to avoid moisture.

Question 2. State Four characteristics which are included in the physical properties of matter.

Answer

Physical properties of matter include the following characteristics:

• Shape: The form or outline of an object.
• Size: The dimensions or magnitude of an object.
• Colour: The visual perception of light reflected from an object.
• State: The physical form of matter, such as solid, liquid, or gas.

Question 3. Explain the process of rusting of iron, and measures to prevent the same.

Answer 

The process of rusting occurs when iron reacts with oxygen and water, forming a reddish-brown substance known as rust (iron oxide). This reaction is accelerated in humid conditions where moisture is abundant. To prevent rusting, consider the following methods:

• Galvanisation: This involves coating iron with a layer of zinc, which protects it from moisture and oxygen.
• Painting or Greasing: Applying a coat of paint or grease creates a barrier that prevents contact with air and water.
• Using Stainless Steel: Stainless steel, made by mixing iron with chromium and other metals, is resistant to rust.

Regular maintenance of protective coatings is essential to ensure long-lasting protection against rusting. 

Question 4. A part from new products, many other things accompany a chemical change, what are those things?

Answer

Part from new products, many other things accompany a chemical change, those things are:

• Heat, light or any other radiation may be given off or absorbed.
• Change in smell may take place.
• Sound may be produced.
• Change in colour may take place.
• A gas may be formed

Question 5. Why chemical changes are very important in our life?

Answer

A chemical change occurs when the composition of a substance is altered, resulting in the formation of one or more new substances. This process is essential in our daily lives for several reasons:

• New substances are created through chemical changes, such as during the digestion of food and the ripening of fruits.
• Medicines are produced as a result of complex chemical reactions.
• Many useful materials, including plastics and detergents, are formed through chemical processes.
• Every new material discovered is linked to the study of chemical changes.

Chemical changes are vital as they lead to the creation of essential substances and materials that support life and industry.

Question 6. Explain the physical and chemical processes along with examples.

Answer

A change in which a substance undergoes a change in its physical properties is called physical change.  Physical properties involves shape, size, colour and state of a substance. It is generally reversible. In physical change, no new substance is formed.

For example: evaporation, condensation etc.

A change in which one or more new substances are formed is called chemical change. Chemical change is irreversible. It is also called chemical reaction; in this type of change a new substance is formed.

Example: Burning of coal, photosynthesis.

The difference between physical change and chemical change is as follows:

Physical change

1. No new substance is formed. A substance undergoes a change in its physical properties.
2. Physical change is generally reversible.
   Example: Melting of ice, lighting of bulb

Chemical Change

1. One or more new substances are formed by chemical reaction.
2. Chemical change is irreversible.
   Example: burning of coal, photosynthesis

Question 7. Rusting of an iron object is faster is coastal areas than in desert area. Explain why?

Answer

Rusting of an iron object occurs more rapidly in coastal areas than in desert regions due to the following reasons:

• Coastal areas have higher levels of moisture in the air because of the nearby sea.
• Deserts, on the other hand, have dry air with little to no moisture.
• Both air and moisture are essential for rusting to take place.
• Thus, the increased humidity in coastal areas leads to faster rusting compared to the arid conditions of deserts.

Question 8. Explain how burning a candle involves both physical and chemical changes, providing specific examples of each.

Answer

Burning a candle involves:

• Physical Changes: 
  Melting wax into liquid as the flame heats it (no new substance forms). These are reversible, as wax can solidify again.
• Chemical Change: 
  The wax vapour burns with oxygen to form carbon dioxide, water, heat, and light, creating new substances. 
  This is irreversible, as the original wax cannot be restored.

Question 9. Why is oxygen essential for combustion, and how can this be demonstrated using a candle?

Answer

(a) Candle burning in presence of oxygen
(b) Candle not burning due to absence of oxygen

• During combustion, carbon in the fuel reacts with oxygen to form carbon dioxide gas. 
• Carbon dioxide can be detected by passing it through lime water, which turns milky due to the formation of calcium carbonate. 
• This confirms that oxygen supports burning and that combustion produces carbon dioxide.

Question 10. Discuss the environmental impact of chemical changes caused by human activities, such as burning fuels and drying paint. Explain how these changes contribute to pollution, and suggest two ways to reduce their environmental effects. Provide one example of a chemical change with both desirable and undesirable outcomes.

Answer

Environmental Impact of Chemical Changes: 

• Human activities, like burning fuels in vehicles or drying paint, release carbon dioxide and pollutants, increasing atmospheric pollution over time.
  These changes have long-term effects on the environment, like climate change.
• Some environmental changes involve Weathering of rocks, Soil Erosion etc.

Contribution to Pollution: 

• Carbon dioxide from burning fuels increases greenhouse gases, leading to global warming and climate change. 
• This causes smog and respiratory issues and also degrades air quality.

Ways to Reduce Environmental Effects:

• Use Cleaner Fuels: Switching to renewable energy sources like solar or wind power reduces carbon dioxide emissions, as these do not involve combustion of fossil fuels.
• Use Less Plastic Carry a reusable water bottle or cloth bag instead of buying plastic bottles or bags.
• Example of Chemical Change with Desirable and Undesirable Outcomes: Decomposition of food is a chemical change. It is undesirable when food rots in the kitchen, becoming inedible and wasteful. However, it is desirable when used to make compost, as it enriches soil for plant growth.

Question 11. Describe the role of the fire triangle in combustion, and explain how removing one element can stop a fire.

Answer

The fire triangle is a simple model that shows the three things a fire needs to burn:

• Fuel (something that can burn)
• Heat (to raise the fuel temperature)
• Oxygen (from the air to feed the fire)

Role of Fire Triangle

• It helps us understand why fires start and keep going: heat turns fuel into flammable gases, those gases mix with oxygen and burn, releasing more heat and the cycle continues.
• It shows us how to stop a fire safely: remove any one of the three sides no fuel, no heat or no oxygen and the fire will go out.

If you take away any one corner of the triangle, the fire cannot continue:

• Remove Fuel: Smother or clear away the burning material (e.g., push wood logs apart or pour water on a small fire) so nothing left can burn.
• Remove Heat: Cool the fire below its ignition temperature (e.g., spray water to absorb heat).
• Remove Oxygen: Cover the fire so air cannot reach it (e.g., use a fire blanket, foam extinguisher or sand).

Question 12. When baking soda is mixed with vinegar, bubbles are formed with the evolution of a gas. Name the gas evolved. What happens when this gas is passed through lime water?

Answer

When baking soda (sodium hydrogen carbonate) and vinegar (acetic acid) are mixed together, then a chemical change takes place between sodium hydrogen carbonate and acetic acid to form three new substances.
The change in the test tube is as follows:

Sodium hydrogen + Acetic acid → Sodium acetate + Carbon dioxide + Water

Sodium hydrogen: Baking soda

Acetic acid: Vinegar

Chemical reaction between baking soda and vinegar

Carbon dioxide gas produced in the reaction passing through freshly prepared lime water as shown in figure.

Passing carbon dioxide gas through lime water

Lime water is calcium hydroxide solution. When carbon dioxide gas is passed through lime water, then calcium hydroxide combines with carbon dioxide to form a white solid substance, calcium carbonate which makes lime water milky.

This chemical change can be written in the form of word equation as follows:

Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

• Calcium Carbonate makes lime water milky.

The reaction between lime water and carbon dioxide gas is a chemical change because a new substance calcium carbonate is formed during this change. The turning of lime water into milky is a standard test of carbon dioxide.

When baking soda (NaHCO₃) reacts with vinegar which contains acetic acid carbon dioxide comes out, which turns lime water milky, therefore it is a chemical change. In all these activities, we saw that in each change, one or more new substances are formed. When magnesium ribbon was burnt, the ash was the new substance formed.

The reaction of copper sulphate with iron produced two new substances, i.e. iron sulphate and copper. Vinegar and baking soda together produced carbon dioxide which turned lime water milky. So, all those changes in which one or more new substances formed, are called chemical changes. These are permanent changes which can usually not be reversed to form the original substance.

Question 13. Explain the following.

(a) Lime water turns milky on passing carbon dioxide gas through it.

(b) Bubbles are produced when acetic acid is added to a solution of sodium hydrogen carbonate.

Answer

(a) Carbon dioxide gas produced in the reaction passing through freshly prepared lime water as shown in figure.

Passing carbon dioxide gas through lime water

Lime water is calcium hydroxide solution. When carbon dioxide gas is passed through lime water, then calcium hydroxide combines with carbon dioxide to form a white solid substance, calcium carbonate which makes lime water milky. This chemical change can be written in the form of word equation as follows:

The reaction between lime water and carbon dioxide gas is a chemical change because a new substance calcium carbonate is formed during this change. The turning of lime water into milky is a standard test of carbon dioxide.

(b) When baking soda (sodium hydrogen carbonate) and vinegar (acetic acid) are mixed together, then a chemical change takes place between sodium hydrogen carbonate and acetic acid to form three new substances.

The change in the test tube is as follows:

Sodium hydrogen + Acetic acid → Sodium + Carbon + Water

Chemical reaction between baking soda and vinegar

Question 14. Give an example of a chemical reaction for each of the following situations:

(a) A change in colour is observed.

(b) A gas is evolved.

(c) Sound is produced.

Answer

(a) Chemical reaction between copper sulphate solution and iron metal. In this reaction, blue colour of copper sulphate solution changes to light green colour due to the formation of iron sulphate.

Copper sulphate + Iron → Iron Sulphate + Copper

Copper sulphate: Blue
Iron: Grey

Iron Sulphate: Light Green

Copper: Brown

(b) When baking soda and vinegar are mixed together then a chemical change takes place and bubbles of carbon dioxide gas are formed along with some other substances.

Baking Soda + Vinegar → Sodium acetate + Carbon dioxide + Water

Baking Soda: Sodium hydrogen carbonate

Vinegar: Acetic acid

(c) Explosion of a firework produces heat, light, sound and unpleasant gases. Explosion of a firework is a chemical change.