NCERT Class 7 Science Chapter 5
Changes Around Us: Physical and Chemical
This chapter teaches CBSE Class 7 students to distinguish between physical and chemical changes observed in everyday life. Physical changes alter the form or appearance of a substance without changing its composition, while chemical changes produce new substances with different properties. Students explore examples, identify reversible and irreversible changes, and understand the role of chemical reactions in nature and industry.
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Key Terms
- Physical Change
- A change in the physical properties of a substance such as shape, size, or state, without any change in its chemical composition. Examples include melting ice and tearing paper.
- Chemical Change
- A change in which new substances with different chemical properties are formed. Examples include burning of wood, rusting of iron, and curdling of milk.
- Reversible Change
- A change that can be undone and the original substance recovered. Melting and freezing of water are reversible changes.
- Irreversible Change
- A change that cannot be undone and the original substance cannot be recovered. Burning of paper and cooking food are irreversible changes.
- Chemical Reaction
- A process in which one or more substances (reactants) are transformed into one or more different substances (products) with new properties.
- Precipitation
- The formation of a solid (precipitate) from a solution during a chemical reaction, such as when solutions of two salts are mixed and form an insoluble product.
Frequently Asked Questions
What is the difference between physical and chemical changes?▾
A physical change only alters the appearance or state of a substance without changing its chemical composition, like cutting paper or melting wax. A chemical change produces entirely new substances, like burning paper or rusting iron.
Give examples of reversible and irreversible changes.▾
Reversible changes can be undone, such as melting ice to get water and then freezing it back to ice. Irreversible changes cannot be undone, such as baking a cake or burning wood, because new substances are formed.
How do you identify a chemical change has occurred?▾
Signs of a chemical change include change in colour, production of gas (bubbles), change in smell, formation of a precipitate (solid in a liquid), or release or absorption of energy (heat or light).
Is dissolving salt in water a physical or chemical change?▾
Dissolving salt in water is a physical change because no new substance is formed. The salt can be recovered by evaporating the water, so the process is also reversible.
Why is burning of wood considered a chemical change?▾
Burning of wood is a chemical change because it produces new substances such as carbon dioxide, water vapour, and ash. These products cannot be converted back into wood, making the change irreversible.
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