Physical and Chemical Changes
ICSE Class 8 · Chemistry · Chapter 2
What this chapter covers
- Changes all around you
- Ways of classifying changes
- Physical changes
- Chemical changes
- Differences between physical and chemical changes
- Changes of state
- Energy changes in physical and chemical changes
- Classifying changes with examples
Key points
- A change is any alteration in the size, shape, colour, state, position or make-up of a substance.
- Changes can be sorted as slow or fast, reversible or irreversible, periodic or non-periodic, natural or man-made, and desirable or undesirable.
- A physical change forms no new substance; it is usually reversible and involves a small energy change.
- A chemical change forms one or more new substances with new properties; it is usually irreversible.
- Signs of a chemical change include a change of colour, smell or taste, a gas given off, heat or light given out, and a solid forming in a liquid.
- Melting of ice, formation of vapours, sublimation of camphor and ammonium chloride, dissolving sugar and a balloon bursting near a hot bulb are physical changes.
- Curd from milk, curdling of milk, rotting of eggs, rusting of iron, burning of fuels and heating iron with sulphur are chemical changes.
- Every change of state is a physical change.
- Irreversible does not always mean chemical; the test is whether a new substance is formed.
- In both physical and chemical changes the total mass stays the same.
- Changes may give out energy, as in burning, respiration and quicklime with water, or take in energy, as in melting and dissolving ammonium chloride.
- Some events, such as a burning candle, include both physical and chemical changes.
Questions answered
Define a physical change and give two examples from your kitchen.
A physical change is a change in which no new substance is formed and only properties such as size, shape or state change. Examples: melting of butter and dissolving sugar in milk.
Define a chemical change. Name three signs that one has taken place.
A chemical change is a change in which one or more new substances with new properties are formed. Signs include a change of colour, a gas given off and heat or light given out. A change of smell or the formation of a solid in a liquid are further signs.
Why is the bursting of a balloon near a lighted bulb a physical change even though it cannot be reversed?
The heat makes the air inside expand and the rubber weaken, but the air and rubber remain the same substances. No new substance is formed, so the change is physical. Irreversibility alone does not make a change chemical.
Classify the formation of curd from milk in three different ways.
It is a chemical change, because curd is a new substance. It is slow, because it takes several hours. It is desirable, because curd is a useful food.
Why is sublimation of camphor a physical change?
Camphor turns directly from solid to vapour, and the vapour turns back into solid camphor on cooling. No new substance is formed.
Iron filings and sulphur are mixed and then heated. How can a magnet show that a chemical change happened on heating?
Before heating, a magnet pulls the iron out of the mixture. After heating, the black product, iron sulphide, is not attracted by the magnet, which shows the iron has become part of a new compound.
Give one example each of a periodic and a non-periodic change.
Periodic: day and night, or the phases of the Moon. Non-periodic: an earthquake, or a hailstorm.
Water is poured on quicklime and the container becomes hot. Is the change physical or chemical? Does it give out or take in heat?
It is a chemical change, because a new substance, slaked lime, forms. It gives out heat.