NCERT Solutions for Class 7 Science Chapter 5 Physical and Chemical Changes

1. Classify the changes involved in the following processes as  physical or chemical changes:

1. Photosynthesis

2. Dissolving sugar in water

3. Burning of coal

4. Melting of wax

5. Beating aluminium to make aluminium foil.

6. Digestion of food

Ans: Physical properties of a substance are shape, size, color and state if the substance undergoes a change in its physical properties, then it’s called a physical change. 

One or more new substances are formed in the chemical change. 

Physical change - Dissolving sugar in water, melting of wax, and beating aluminium to make aluminium foil.

Chemical change – Photosynthesis, Burning of coal, Digestion of food.

2. State whether the following statements are true or false. In case a statement is false, write the corrected statement in your notebook.

1. Cutting a log of wood into pieces is a chemical change.  

Ans: False. Cutting a log of wood into pieces is a physical change.

2. Formation of manure from leaves is a physical change.  

Ans: False. The formation of manure from leaves is a chemical change.

3. Iron pipes coated with zinc do not get rusted easily.

Ans: True

4. Iron and rust are the same substance.  

Ans: False. Iron and rust are different substances.

5. Condensation of steam is not a chemical change.

Ans: True

3. Fill in the blanks in the following statements:

1. When carbon dioxide is passed through lime water, it  turns milky due to formation of ----------.

Ans: When carbon dioxide is passed through lime water, it turns  milky due to  formation of calcium carbonate.

2. The chemical name of baking soda is ----------.

Ans: The chemical name of baking soda is sodium hydrogen  carbonate.

3. Two methods by which rusting of iron can be prevented  are ---------- and ----------.

Ans: Two methods by which rusting of iron can be prevented are painting and galvanizing.

4. Changes in which only ---------- properties of a substance change are called physical changes.

Ans: Changes in which only physical properties of a substance  change are called physical changes.

5. Changes in which new substances are formed are called ---------- changes.

Ans: Changes in which new substances are formed are called chemical changes.

4. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of a gas. What type of change is it?  Explain.

Ans: It’s a chemical change because a new substance carbon dioxide  gas is formed. When baking soda is mixed with lemon juice, then bubbles are formed with the evolution of carbon dioxide gas.

5. When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar  process in which both the chemical and physical changes take place.

Ans: First the wax of the candle melts, then vaporizes and burns. Here Melting of wax is a physical change as no new substance is formed and the melted wax can be converted back.

When the wax burns, two new substances are formed which are smoke and carbon dioxide. So, it’s a chemical change.

In LPG both the chemical and physical changes take place. As we know LPG is present as liquid form in the cylinder. 

Here the physical change occurs when it comes out of the cylinder. That time it converts into gaseous form. 

When that gas burns in air it undergoes chemical change.

6. How would you show that the setting of curd is a chemical change?

Ans: When some sour substance is added to milk and kept overnight, it turns into curd. Once the curd is formed, then there is no way curd can be converted back into milk. Both milk and curd are different and they have different properties So, formation of curd is a  chemical change.

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

Ans: Here chemical change is burning of wood  and cutting of wood is a physical change. Burning of wood produces ash and smoke that means two new substances are formed. Hence the properties of wood are changed. So, it’s a chemical change. When a log of wood is cut into small pieces, it’s a physical change. Each small piece  bears the properties of wood and there’s no new substance formed.

8. Describe how crystals of copper sulphate are prepared.

Ans: Take a water filled beaker and add a few drops of dilute Sulphuric acid into it. Then the water is heated, when it starts boiling slowly add copper sulphate powder during this process stir continuously. Add more copper sulphate till no more powder can be dissolved. The solution is now filtered and allowed to cool down. Crystal of copper sulphate slowly forms at the rock bottom of the beaker. By the method of crystallization Crystals of copper sulphate can be prepared.

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

Ans: Rusting  is the process in which a piece of iron  in the open air acquires a  film of brownish substance called rust. For rusting, iron should be in contact with both air and moisture. When an iron gate is painted then the layer of paint cuts the  contact between air, moisture and iron. Thus, it prevents rusting. So, when we paint an iron gate it prevents rusting.

10. Explain why rusting of iron objects is faster in coastal areas than in deserts.

Ans: Rusting  is the process in which a piece of iron  in the open air acquires a  film of brownish substance called rust. Both air and moisture are necessary for rusting of iron objects. Due to the presence of sea,  In coastal areas there is more moisture in the air. But, in the desert there’s a scarcity of water and hence air is nearly dry. This is the reason why rusting is faster in coastal areas than in deserts.

11. The gas we use in the kitchen is called liquefied petroleum gas (LPG). In the cylinder it exists as liquid. When it comes out from the cylinder it becomes a gas (change A) then it burns (change B). The following statements pertain to these changes. Choose the correct one.

(i) Process – A is a chemical change.

(ii) Process – B is chemical change.

(iii) Both processes A and B are chemical changes.

(iv) None of these processes is a chemical change.

Ans: (ii) Process – B is a chemical change.

In a chemical change one or more new substances are formed. 

12. Aerobic bacteria digests animal waste and produces biogas (change – A). The biogas is then burnt as fuel (change - B). The  following statements pertain to these changes. Choose the correct one.

(i) Process – A is a chemical change.

(ii) Process –B is a chemical change.

(iii) Both processes A and B are chemical changes.

(iv) None of these processes is a chemical change.

Ans: (iii) Both processes A and B are chemical changes.

In a chemical change one or more new substances 

Topics Covered in Science Class 7 Chapter 5 Physical and Chemical Changes

Benefits of NCERT Solutions for Class 7 Science Chapter 5- Physical and Chemical Changes

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Conclusion

NCERT Solutions for Science Chapter 5 class 7 physical and chemical changes provides students with a profound understanding of the fundamental concepts of chemistry and the transformations matter can undergo. This chapter explores the distinctions between physical and chemical changes, offering comprehensive explanations and practical examples. The class 7th science chapter 5 question answers empower students to appreciate the world of science by explaining everyday phenomena and reactions.

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