Chemical Reactions · Grade 10

How can you tell a chemical change from a physical one?

Ask whether a new substance has appeared. In a physical change the substance stays what it was and only its shape or state altered, and you can usually reverse it. In a chemical change different substances form with different properties, and getting back needs another reaction rather than just cooling or drying.

Learning objectives

The only question that settles it

Ice that melts becomes water. The molecules are identical before and after and only the spacing between them changed, so it is a physical change. The same is true of sugar dissolving in tea: the sugar molecules are still there, and evaporating the water brings them back.

Paper that burns does not become hot paper. Carbon dioxide, water vapour and ash form — substances whose properties are nothing like the paper's — and no amount of cooling returns the paper.

So the distinction is not about how dramatic it looks. Dissolving can look impressive and be physical; rust forming slowly over months is unmistakably chemical.

Four signs worth knowing

A gas appearing that was not there before, like the bubbles when vinegar meets bicarbonate of soda. A colour change that is not two existing colours mixing. A solid precipitate forming in a clear solution. And a temperature change that did not come from outside.

None of them proves anything on its own. Boiling water releases gas and is not reacting, and mixing two paints changes colour without a reaction. The signs tell you where to look; the question about the new substance is what decides.

Where the mass went

A candle burns down and mass appears to vanish. It has not: it left as gas. Weigh everything that went in and everything that came out and the totals match exactly.

You can show this in a sealed container. Steel wool burnt inside a sealed flask does not change the flask's mass at all, because the oxygen that reacted was already inside and the product stayed inside.

The same steel wool burnt in open air gets heavier, which surprises almost everyone the first time. The reason is identical: the iron bonds to oxygen from the air, and that oxygen joined the mass. Atoms are neither destroyed nor created — they are only rearranged.

Worked examples

  1. Classify these: melting copper, rust on a nail, dissolving salt, burning cooking gas.

    1. Melting changes state only
    2. Rust forms iron oxide, a new substance
    3. Dissolved salt is still salt and the water can be evaporated off
    4. Burning forms carbon dioxide and water

    Answer: Physical, chemical, physical, chemical

  2. 10 g of steel wool is burnt inside a sealed flask weighing 90 g. What does the flask weigh afterwards?

    1. The flask is sealed, so nothing entered and nothing left
    2. The atoms only rearranged inside it
    3. The total mass cannot change

    Answer: 100 g, exactly as before

  3. Why does that same steel wool get heavier when burnt in open air?

    1. The iron reacts with oxygen from the air
    2. Oxygen atoms bond to the iron and stay in the product
    3. That oxygen was never on the balance before

    Answer: The added mass is the oxygen that joined it

Common mistakes

Assuming mass disappears when a candle or paper burns
It leaves as gas, so it cannot be seen. Weigh the gases too and the totals match exactly — that is the whole of conservation of mass.
Treating any dramatic change as chemical
Boiling, dissolving and crushing all look striking and make no new substance. The question is not how it looks but what is in the container at the end.
Taking one sign as proof
Boiling water releases gas without reacting. The signs point you where to look; the test is whether the substance's properties have changed.

What to remember

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