Physical or Chemical Change? How to Decide

Ice melts. Milk turns sour. A piece of paper is torn, and another is burnt. Some of these changes are physical, and some are chemical. The key to deciding is one question.

The one question

Is a new substance formed?

• In a physical change, the substance stays the same. Only the state, the shape, the size or the position changes. It is usually easy to get the original back.
• In a chemical change, a new substance with new properties is formed. The change is usually hard to undo.

Clues that a new substance may have formed

• A gas is given off (bubbles, a new smell).
• A solid appears in a liquid (a precipitate).
• A permanent colour change.
• Heat, light or a sound is given out.
• The temperature of the mixture changes by itself.

A clue is only a clue. The real test is the new substance.

Cases that confuse

Mistakes to avoid

• "If a change can be reversed, it is physical. If not, it is chemical." Smashing a bottle cannot be undone, but it is physical. Some chemical changes can be reversed. Ask about the new substance instead.
• "A colour change always means a chemical change." A coloured solid dissolving in water changes the colour of the water, but no new substance is made.
• "Burning is the same as melting." A candle does both: melting is physical, and burning is chemical.

Think it through

Write your answers in full sentences. Try each one before you read the pointers.

1. A candle burns. The wax first melts near the flame, and then the vapour burns. Which of these are physical changes and which are chemical? Say what evidence you use for each.
2. Smashing a glass bottle cannot be undone. Cooling steam to get water can be undone. A student says, "Cannot be undone means chemical." Give a case that shows this is wrong, and say what a better test is.
3. Sugar dissolves in water. How could you show, with a simple experiment, that no new substance is made? What would you find?
4. Milk left out of the fridge turns sour. A friend says it is "only going bad" and not a chemical change. Give two observations that support a chemical change, and one test you could do.
5. A sealed bottle holds some vinegar and, in a balloon on its neck, some baking soda. You tip the soda into the vinegar. The balloon inflates. List all the evidence for a chemical change, and predict what a balance would show for the mass of the whole sealed system before and after.

Pointers for your answers

1. Melting is physical, because it is still wax. Burning is chemical, because carbon dioxide and water vapour are new substances. The evidence is the flame, the heat and the gas formed.
2. Smashing glass is physical, because glass stays glass. The better test is whether a new substance forms.
3. Evaporate the water. You get sugar back. The taste and the other properties stay the same.
4. A sour taste and smell, and a change in texture. A test is to measure the acidity of the milk, which changes.
5. The evidence is the gas (bubbles and the inflating balloon), and perhaps a fall in temperature. The mass of the sealed system does not change, because nothing escapes (the law of conservation of mass).