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Physical and Chemical Changes
ICSE Class 8 Chemistry • Chapter 2 (Detailed Master Notes)
Chapter Overview
Changes are happening everywhere, all the time. Sometimes iron rusts, sometimes milk turns into curd, and sometimes ice melts into water. In chemistry, we classify all changes into two major categories: Physical and Chemical. Understanding the difference is the foundation of chemical reactions.
2.1 Physical Changes
A physical change is a temporary change. The internal chemical composition of the substance remains exactly the same before and after the change.
Physical Change: A true change in which only the physical properties (like state, shape, color, or size) of the substance alter, but no completely new chemical substance is formed.
Characteristics of a Physical Change:
- No new substance: When ice melts, it becomes water. Both ice and water consist of $H_2O$ molecules. No new molecules are created.
- Temporary and Reversible: It can usually be reversed easily by altering the conditions. You can freeze water back into ice.
- No change in total mass: The mass of the substance remains unchanged.
- Energy changes are small: Very little heat or light is absorbed or released compared to chemical changes.
Examples: Tearing paper, melting of wax, dissolving sugar in water, magnetization of iron, glowing of an electric bulb.
2.2 Chemical Changes
A chemical change is a permanent alteration. It is the very heart of chemistry, where atoms rearrange themselves.
Chemical Change: A permanent change in which the original substance loses its identity and nature, and one or more completely new chemical substances with distinct properties are formed.
Characteristics of a Chemical Change:
- New substance is formed: When carbon burns in oxygen, it creates carbon dioxide gas. The products are entirely different from the reactants.
- Permanent and Irreversible: You cannot easily turn ash back into wood, or curd back into milk.
- Energy is always involved: A huge amount of energy (heat, light, or sound) is either absorbed or released during the reaction.
- Change in mass (of individual substances): Although total mass is conserved, the mass of specific reactants or products changes during the reaction.
Examples: Rusting of iron, burning a piece of paper, digestion of food, respiration, souring of milk.
AI Image Prompt: A side-by-side illustration. On the left (Physical Change): A glass of water with ice cubes melting. On the right (Chemical Change): A rusty iron nail sitting next to a burning wooden matchstick. Clear, educational labels below each scenario.
2.3 Differences at a Glance
| Parameter | Physical Change | Chemical Change |
|---|---|---|
| Formation of Substance | No new substance is formed. | One or more completely new substances are formed. |
| Nature | It is temporary and mostly reversible. | It is permanent and generally irreversible. |
| Properties | Only physical properties change. Chemical composition is identical. | Both physical and chemical properties completely change. |
| Energy | Very small energy changes occur. | Significant energy (heat/light) is absorbed or evolved. |
2.4 Chemical Reactions
When a chemical change occurs, we say a chemical reaction has taken place.
- Reactants: The original starting substances that participate in the reaction.
- Products: The new chemical substances formed as a result of the reaction.
Example: $Carbon + Oxygen \rightarrow Carbon Dioxide$
Here, Carbon and Oxygen are Reactants. Carbon Dioxide is the Product.
Q1. Is the dissolving of salt in water a physical or a chemical change?
Answer: It is a physical change. No new chemical substance is formed. The salty water still contains salt and water molecules. The salt can be easily recovered by evaporating the water, showing the change is completely reversible.
Q2. Why is burning a candle considered both a physical and a chemical change?
Answer: When a candle burns, the melting of solid wax into liquid wax is a physical change (state change, no new substance). However, the burning of the wax vapor producing heat, light, carbon dioxide, and water vapor is a chemical change (new substances formed, irreversible).