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

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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:

1. Ripening of fruits

2. Fermentation of grapes

3. Digestion of food

4. 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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