NCERT Solutions For Class 7 Science Chapter 6 : Physical And Chemical Changes

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

(a) Photosynthesis

(b) Dissolving sugar in water

(c) Burning of coal

(d) Melting of wax

(e) Beating aluminium to make aluminium foil.

(f) Digestion of food

Ans: Physical properties of a substance are shape, size, colour 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, 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.

(a) Cutting a log of wood into pieces is a chemical change. 

Ans: False

Cutting a log of wood into pieces is a physical change.

(b) The formation of manure from leaves is a physical change. 

Ans: False

The formation of manure from leaves is a chemical change.

(c) Iron pipes coated with zinc do not get rusted easily. 

Ans: True

(d) Iron and rust area same substance. 

Ans: False

Iron and rust are different substances.

(e) Condensation of steam is not a chemical change.

Ans: True

3. Fill in the blanks in the following statements:

(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of ----------.

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

(b) The chemical name of baking soda is ----------.

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

(c) 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.

(d) 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.

(e) 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 a 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 the air it undergoes a 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, the formation of curd is a  chemical change.

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

Ans: Here chemical change is the burning of wood and cutting of wood is a physical change. The 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 the 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 a 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 a brownish substance called rust. Both air and moisture are necessary for rusting of iron objects. Due to the presence of the sea,  In coastal areas, there is more moisture in the air. But, in the desert, there’s a scarcity of water and hence the 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 a 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 a chemical change.

(iii) Both processes A and B is a chemical change.

(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 digest animal waste and produce 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 are formed.

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Chapter 6 – Physical and Chemical Changes

NCERT Solutions for Class 7 Science Chapter 6

We notice various changes around us every day, such as the setting of curd from milk, dissolving sugar in water, and many more. These changes can broadly be characterized as chemical and physical changes. In this chapter, we shall learn about various chemical as well as physical changes in detail.

6.1 Physical Changes

Activity 6.1, 6.2, 6.3, 6.4, and 6.5 in this section explains the concept of physical changes. Before starting with the practical experiments, you must know some basic terms and definitions that will help you understand the concept in a better way. 

Physical Properties: Shape, size, colour, and state of any substance are known as its physical properties.

Physical Change: When any substance undergoes a change in its physical properties, it is said to undergo a physical change. It is worth noting that a physical change is generally reversible. In a physical change, no new substance is formed.

Perform all the experiments discussed in this section and you will surely have a better understanding of the physical properties of a substance and physical changes.

6.2 Chemical Changes

Have you ever noticed a film of brownish substance on any iron substance that you left open for some time? This substance is known as rust and the process is known as rusting. Rusting of iron is a chemical change. In this section activity 6.6, 6.7, and 6.8 illustrate the experiments of various chemical changes. You must be thinking, what is a chemical change and how is it different from a physical change? 

A change in which one or more new substances are formed is known as a chemical change. A chemical change is commonly termed as a chemical reaction. In the rusting of Iron, we noticed that a new substance that is rust had formed. Similarly, in other experiments discussed in this section, you will notice the formation of one or more chemical substances. Some other examples of chemical change are:

a. Ripening of fruits

b. Fermentation of grapes

c. Digestion of food

d. Burning of coal, wood, or leaves. In fact, the burning of a substance is a chemical change because burning is always accompanied by the production of heat and other gases.

In a chemical change, a gas may be formed, a sound might be produced, a new substance may be formed, or heat is produced. A change in smell can also be observed to be a chemical change.

6.3 Rusting of Iron

Rusting of iron is a chemical alteration that not only affects iron articles but slowly and gradually destroys them, as well. In this section, you will study in detail the rusting of iron, its properties, and the cause. Note the following word equation for rusting –

Iron + Oxygen + water → Rust

Note: The presence of oxygen and water is an essential condition for the process of rusting to take place. You can always prevent rusting of iron by preventing the iron article from coming in contact with oxygen, water, or both. 

Another way of preventing rusting is by depositing a layer of metal like chromium or zinc on iron. The process of depositing a layer of zinc on iron is known as galvanization.

6.4 Crystallization

The process of crystallization is a physical change. You must have studied in junior classes that the salt obtained by the evaporation of seawater is not pure. So, the process of forming large crystals of a pure substance is known as crystallization. Activity 6.9 is based on the process of crystallization. It is advised that students must perform this experiment in the science lab under the teacher’s supervision. 

Now, you are well aware of physical and chemical changes. Make a list of some changes happening in your surroundings and try to classify them as physical and chemical changes. This small activity will help you understand the concept in a much better way. In case of any doubts, you can always reach out to the experts at Vedantu on our official website.

Exercise Solutions: 12 Questions (3 short questions and 9 long questions).

Extended learning: Activities and Projects (4 Questions)

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