Physical and Chemical Changes
ICSE Class 7 · Chemistry · Chapter 2
What this chapter covers
- Changes all around you
- Physical changes
- Chemical changes: a new substance with new properties
- Differences between physical and chemical changes
- Reversible and irreversible changes
- Desirable and undesirable changes
- Changes involving state and energy
Key definitions
Making a sugar solution is a physical change.
Burning of paper is a chemical change.
Ripening of fruit is a chemical change.
Spoiling of food is a chemical change.
Boiling an egg is a chemical change.
Key points
- A change is any difference in the state, colour, size, shape, smell or taste of a substance.
- A physical change forms no new substance; examples are melting, drying, evaporation and dissolving sugar.
- A chemical change forms one or more new substances with new properties; examples are rusting, burning, fireworks and curd from milk.
- Iron and sulphur powders only mix at first, but on heating they form black iron sulphide, a new compound.
- Respiration, burning of paper, ripening of fruit, spoiling of food and boiling an egg are chemical changes; making a sugar solution is a physical change.
- A reversible change can be undone; an irreversible change cannot. Most physical changes are reversible and most chemical changes are irreversible, but not all.
- A desirable change is useful to us, like cooking; an undesirable change is harmful, like rusting or spoiling of food.
- A change of state, including sublimation of iodine or camphor, is a physical change. Ammonium chloride only appears to sublime: on heating it splits into two gases that join again on cooling.
- Changes can give out heat (burning, quicklime in water) or take in heat (melting ice, dissolving ammonium chloride).
- Signs such as heat, light or bubbles show that a change has happened; only the formation of a new substance shows that it is chemical.
Questions answered
What is the one sure test that tells you a chemical change has taken place?
A new substance with new properties is formed.
Classify as physical or chemical: (a) melting of ice, (b) rusting of iron, (c) drying of clothes, (d) curd from milk.
(a) Physical, (b) chemical, (c) physical, (d) chemical.
Why is dissolving sugar in water a physical change?
The sugar remains sugar. The solution tastes sweet, and the sugar can be got back by evaporating the water, so no new substance is formed.
A mixture of iron powder and sulphur is heated and a black solid forms. Give two observations that show a new substance has formed.
The black solid is not attracted by a magnet, and it is neither grey like iron nor yellow like sulphur.
What is sublimation? Name two substances that sublime.
Sublimation is the change of a solid directly into a gas on heating, without becoming a liquid. Iodine and camphor sublime. (Ammonium chloride only appears to sublime: on heating it splits into two gases, which join again into ammonium chloride on cooling.)
Give one example each of a reversible and an irreversible change.
Reversible: melting of wax. Irreversible: burning of paper.
Is every irreversible change a chemical change? Explain with an example.
No. Breaking a glass tumbler cannot be undone, but the pieces are still glass, so it is a physical change.
Why is rusting called an undesirable change? Give one way to prevent it.
Rusting weakens and wastes iron objects such as gates and bridges. Painting or oiling iron keeps air and water away and prevents rusting.