VARDAAN LEARNING INSTITUTE | POWERED BY VARDAAN COMET
Physical and Chemical Changes
ICSE Class 7 Chemistry • Chapter 2
Chapter Overview
Changes are happening all around us continuously. A growing tree, melting ice, rusting iron, and cooking food are all changes. In chemistry, we systematically categorize these alterations into purely Physical Changes and definitive Chemical Changes based on whether new substances are formed.
2.1 Physical Changes
Physical Change: A temporary change in which only the physical properties (like shape, size, state, or color) of a substance are altered, and absolutely no completely new substance is formed.
Characteristics of Physical Changes
- No new substance is formed: The chemical composition remains totally identical. Water, whether as solid ice or hot gaseous steam, remains chemically $H_2O$.
- Reversible: They are generally temporary and can be easily reversed by simply altering the specific physical conditions. (E.g., Melting ice can be refrozen into solid ice by cooling).
- No overall change in mass: The total mass physically remains constant before and after the change.
- Energy changes are negligible: Very little heat or light energy is absorbed or released.
Common Examples of Physical Changes
- Melting of wax or freezing of water.
- Dissolving solid sugar or salt in water (you can get the sugar back strictly by evaporating the water).
- Glowing of an electric bulb (the tungsten filament physically glows hot when current passes, but cools down completely identical to before).
- Tearing a piece of paper into small bits.
2.2 Chemical Changes
Chemical Change: A permanent change in which the original substance completely loses its chemical identity and entirely new substances with entirely distinct chemical properties tightly form.
Characteristics of Chemical Changes
- New substances are formed: The original composition is completely permanently altered. (E.g., When hydrogen gas burns in oxygen gas, it powerfully forms liquid water, a totally new substance).
- Irreversible: They are permanent and cannot be cleanly undone by simply reversing the external conditions. (E.g., You cannot turn baked cake back into raw flour and eggs).
- Change in mass of individual substances: Though total mass is conserved, the mass of individual reactants clearly changes as they convert cleanly into new products.
- Energy changes are significant: A prominent amount of heat, light, or sound energy is either totally absorbed or visibly evolved during a chemical change.
AI Image Prompt: A vibrant side-by-side comparison diagram. On the left side (Physical Change): An ice cube melting gracefully into a puddle of water with a distinct blue arrow indicating "Reversible". On the right side (Chemical Change): A piece of wood brightly burning in a campfire, turning completely into grey ash and black smoke, with a thick red arrow indicating "Irreversible".
Comparing Physical and Chemical Changes
| Parameter | Physical Change | Chemical Change |
|---|---|---|
| New Substance | None formed. | One or more new substances formed. |
| Nature | Usually highly temporary. | Permanent strictly. |
| Reversibility | Mostly smoothly reversible. | Irreversible entirely. |
Q1. Why is the souring of milk strictly classified as a permanent chemical change?
Ans: When milk actively sours, a completely new substance (lactic acid) is firmly formed. The sour milk fundamentally cannot be magically converted back into fresh, sweet milk, making the process permanent and chemically irreversible.
Q2. Is burning of a wax candle a physical or chemical change?
Ans: It is actually both. The melting of the solid wax is a temporary physical change. But the actual burning of the wax vapor to boldly produce carbon dioxide and heavy soot particles is a permanent chemical change.