Physical & Chemical Changes – NCERT Solutions Class 7/ Class 9

Physical and chemical changes – Definition and Examples

Introduction of physical and chemical changes

We have seen our moms putting water in the ice tray to freeze it into ice cubes. But we have also seen her heat it up to boil it to make tea. Are these acts of changing water into different forms a physical change or a chemical change? Further, we have also seen our moms add milk and tea leaves to the water. Is that a physical change or a chemical change?

 

Let us find out!

Physical Change

What is a physical change?

Physical Change Definition: A physical change is any such change that does not produce a new substance as a result. The original object can change in terms of its shape, size or form. But, it does not involve any change in the chemical composition or properties of the initial substance. The nature of the object, the chemical particles, and the number of particles stays the same.

 

Physical Changes can be of two types –

 

1. Reversible – In such changes, restoring the original substance is possible.

Example – You first freeze ice from water. Therefore, water is the original substance. After freezing, it turns into ice cubes. The chemical composition of water/ice is the same. To reverse the change, you let the ice cubes stay out in the open. And voila! It turns back to the water. And that, my friends, is called a reversible change.

 

2. Irreversible – In such changes, restoring the original substance is impossible.

For example, you take a pencil and sharpen its tip. The length of the pencil would naturally decrease, but its chemical composition will not change. It was formed by led earlier, and led makes it up still. But can you increase the length of the pencil again and reverse the impact of sharpening it? Sadly, no! And that, my friends, is called an irreversible change.

 

Therefore, physical changes do not change the identity of the matter.

Physical change examples:  Tearing of paper, Chopping of Wood, Melting Ice, Boiling Water

Let us consider another example. Say, you have a model – We’ll call her Alpha. She has weak eyesight, so you provide her with pair of fancy blue-coloured lenses. That makes her look completely different but is there any change in Alpha? No.

 

Also, give some thought if you can reverse this change!

 

Chemical Change

What is a chemical change?

Chemical Change Definition: A chemical change is any such change that produces a new substance as a result. The original object gets changed in terms of its shape, size or form. But, it also involves a change in the chemical composition or properties of the initial substance. The nature of the object, the chemical particles, and the number of particles can change. They are also called chemical reactions. Any substances that help cause such a change are called Reactants, and the results are called the Products.

 

Therefore, chemical reactions change the chemical identity of the matter and cannot get reversed.

Chemical change examples: Burning of paper, Rusting of Metal, Digestion of food, Rotting of Fruit

 

Coming back to Alpha, she still has weak eyesight. But instead of lenses, you decide to sign her up for laser surgery for her eyes. She goes for the surgery and comes back with perfect eyesight. Even though she looks the same, her sight is now perfect. Hence, you just caused a chemical change.

 

Signs of a chemical change

While all of the following changes may not occur for all chemical changes, some of these definitely would. If none of the changes gets spotted, most likely, there was a physical change.

 

1. Formation of gas. In liquids, bubbles may form.
2. Formulation of odour.
3. Colour changes.
4. The presence of sound.
5. Change in temperature.
6. Presence of light.
7. Formation of a precipitate.

 

However, in some cases, you can get thoroughly confused about the change that occurred. For example, dissolve sugar in water in one glass and dissolve salt in water in another glass. You will notice that in both cases, a white solid dissolves into a clear liquid.

 

However, in the sugar solution, the composition of sugars stay the same (sucrose molecules). Therefore, a physical change occurs.

However, the salt solution dissociates salt into its ions (from NaCl into Na+ and Cl-), so a chemical change occurs.

 

Now let us come back to our initial example about our moms boiling water and making tea. That, too, is a chemical change because it involves the reaction between water and tea leaves.

The reaction results in tasty tea, and your mom officially becomes a scientist for doing such an awesome chemical reaction every day!

 

Now let us understand the difference between physical and chemical changes.

Difference between physical and chemical change

Basis	Physical Changes	Chemical Changes
Composition	Physical properties change. However, the chemical composition stays the same.	Physical properties change. Additionally, chemical compositions also change.
Longevity	Temporary changes	Permanent changes
Change in energy	Very little to no absorption of energy.	More absorption and creation of energy occurs.
Creation of Products	No new products get formed.	New products always get formed.
Reversibility	Can or cannot be reversed.	Reversal is extremely difficult or impossible.
Examples	Tearing of paper	Burning of paper

 

To get a better understanding of this concept, check out the video below.

 

Further, to test your knowledge, answer the NCERT questions for Physical & Chemical Changes Class 7.

 

We hope you enjoyed this fun distinction between physical and chemical changes.

 

But the next time your mom is cooking, try to evaluate if she is making chemical or physical changes. And while you are at it, feel free to flaunt your knowledge! 😉

 

Till then, keep learning, keep enjoying!

 

 

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