Course 2.3Lesson 1 of 3
In a physical change, the physical properties such as state, shape, size, colour or density change, and no new substance is made, so the change can often be reversed. In a chemical change, one or more new substances with new properties are made, and it is usually hard to reverse. A chemical change often shows a gas, a change of colour, a new smell, heat or light, or a new solid. Some changes need a careful look, such as a burning candle, which has both kinds.
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Ice melts. A piece of paper burns. A glass breaks. Milk turns sour. Water boils. Iron rusts. All of these are changes. But they are not all the same kind.
Chemistry begins with a simple question about any change: is a new substance made? If the answer is no, the change is physical. If the answer is yes, it is chemical.
In a physical change, the physical properties change, such as the state, the shape, the size, the colour or the density. No new substance is made. The substance is the same before and after.
Examples:
Many physical changes can be reversed easily, by changing the conditions. But not all: a broken glass cannot easily be put together again, and it is still a physical change, because the glass is still glass.
In a chemical change (or a chemical reaction), one or more new substances are made, with new properties. The original substances are changed into something else.
Examples:
A chemical change often shows one or more of these signs.
| Sign | Example |
|---|---|
| A new gas is made | Bubbles when a tablet is dropped in water |
| A change of colour | Rusting, or a cut apple turning brown |
| A new smell | Burnt toast, or food that has gone bad |
| Heat or light is given out (or taken in) | A burning candle |
| A solid is formed in a liquid | Lime water turning milky with carbon dioxide |
A chemical change is usually hard to reverse.
Be careful: these signs are clues, not proofs. Boiling water also makes bubbles, but it is a physical change, because the bubbles are steam, which is still water. A hot iron rod glows red, giving off light, but when it cools it is the same iron. The real test is always the first question: is a new substance made?
Light a candle. Near the flame, the wax melts and becomes liquid. This is a physical change, because the liquid wax is still wax, and it becomes solid again when it cools. But the wax vapour that burns in the flame forms carbon dioxide and water vapour. This is a chemical change, because new substances are made.
| Change | Kind | Reason |
|---|---|---|
| Melting of butter | physical | the same substance, only the state changes |
| Burning of a match | chemical | new substances, with heat and light |
| Dissolving salt in water | physical | the salt can be got back by evaporation |
| Frying an egg | chemical | the egg cannot be turned back into a raw egg |
| Tearing a paper | physical | it is still paper |
| Baking a cake | chemical | new substances are made, with a new smell |
| Mixing sand and iron filings | physical | each is still the same, and a magnet separates them |
| Rusting of a gate | chemical | iron turns into a new substance |
You should find that a nail that rusts away completely becomes about times as heavy: the oxygen it took from the air is now part of the rust.
We use the difference all the time. To separate salt from sea water, we use a physical change (evaporation). To get iron from its ore, we use a chemical change. To make soap, medicines, fertilisers and plastics, we use chemical changes. A good chemist chooses the right kind of change for the job.
The program sorts six changes with the one question that matters: is a new substance made?
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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