Physical and Chemical Changes
Subject: Science (Chemistry) | Topic: Physical and Chemical Changes
For TLT, ECT, PST, JEST & All Competitive Exams
Matter around us is constantly changing form — ice melts, wood burns, milk turns sour, and iron rusts. Some of these changes are temporary and easily reversed, while others create something completely new that cannot be undone. Understanding the difference between physical and chemical changes is one of the most important and most frequently tested topics in TLT, ECT, PST, and JEST exams.
What is a Physical Change?
A physical change is a change in which the physical properties of a substance — such as its shape, size, or state — change, but no new substance is formed. The substance keeps its original chemical composition, and in most cases, the change can be reversed.
Examples: melting of ice, boiling of water, cutting of paper, dissolving of sugar in water, breaking of glass, freezing of water into ice.
What is a Chemical Change?
A chemical change is a change in which one or more new substances are formed, having properties different from the original substance. Chemical changes involve a chemical reaction, and they are usually permanent — they cannot be reversed by simple physical means.
Examples: burning of wood, rusting of iron, digestion of food, souring of milk, cooking of food, burning of a candle wax's wick, baking of a cake.
Physical Change vs Chemical Change
Key Differences Between Physical and Chemical Changes
| Basis | Physical Change | Chemical Change |
|---|---|---|
| New Substance | No new substance is formed | A new substance is always formed |
| Reversibility | Usually reversible | Usually irreversible |
| Chemical Composition | Remains the same | Changes completely |
| Energy Change | Small or no energy change | Involves noticeable energy change (heat, light, gas) |
| Examples | Melting, freezing, boiling, cutting, dissolving | Burning, rusting, cooking, digestion, souring of milk |
Signs of a Chemical Change
A chemical change can often be identified by one or more of the following signs:
- Formation of a gas (bubbles), such as in baking soda reacting with vinegar
- Change in color, such as an apple turning brown or iron turning reddish-brown when rusting
- Formation of a precipitate (solid) in a liquid mixture
- Release or absorption of heat and light, such as burning a candle
- Production of a smell that was not there before, such as souring milk
Reversible and Irreversible Changes
Physical changes are generally reversible — ice can be melted back into water and refrozen into ice again. Chemical changes are generally irreversible — once wood is burned into ash and gases, it cannot be turned back into wood by any physical method.
Why This Topic Matters
Recognizing physical and chemical changes helps students understand chemical reactions, states of matter, and the conservation of mass — all foundational ideas that connect directly to atomic structure, compounds, and reactions studied later in chemistry. This topic is a favorite area for MCQs in TLT, ECT, PST, and JEST exams because everyday examples are easy to test.
Practice MCQs
1. Which of the following is a physical change?
a) Burning of paper b) Melting of ice c) Rusting of iron d) Digestion of food
2. Which of the following is a chemical change?
a) Cutting of cloth b) Dissolving of salt in water c) Souring of milk d) Breaking of glass
3. A chemical change always results in the formation of:
a) A new state of matter only b) A new substance c) A bigger amount of the same substance d) No change at all
4. Which of these changes is generally irreversible?
a) Freezing of water b) Melting of wax c) Burning of wood d) Boiling of water
5. Which of the following is NOT a sign of a chemical change?
a) Formation of gas b) Change in color c) Change in shape only d) Production of a new smell
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