Chemical Reactions
Physical and chemical changes
Decide whether a change has made a new substance or merely rearranged an old one, say what combustion needs, and name the slow changes that turn rock into soil.
Fold a sheet of paper into a boat and you can unfold it back into the sheet. Crush a piece of chalk and you still have chalk, just in smaller pieces. Now blow through a straw into lime water: it goes milky, and a white powder settles out that was not there before. The first two changes rearranged something. The third one made something.
The one question that sorts a change
Has a new substance appeared? If no — if what changed was shape, size or state — it is a physical change. If yes, it is a chemical change, and the reaction behind it can be written as a chemical equation. That question does the work; whether the change is easy to undo is a different question with a different answer.
Have a play
Look closely at a burning candle. Two different kinds of change are going on at once.
Tap any part of the picture.
Worked example
Two identical candles are lit. One is left standing; the other has a glass tumbler put over it. The covered one goes out after a short while. Why?
Ask what the tumbler took away. It did not touch the wax and it did not cool the wick — it cut the candle off from the room's air.
Whatever explains the difference has to be something the tumbler changed, and air supply is the whole of it.
Try it together
A piece of paper is held under a magnifying glass in bright sunlight until it smokes and catches fire. Work out what happened.
No match was struck, and the paper had been sitting in the same air for hours.
1.The paper sat in the same air for hours without catching fire, though it is a fuel and the oxygen was there. Which of combustion's three requirements was missing? One word.
Have a go
Have a go on your own. Fireflies give out light in the evening with no heat, by a chemical change inside them. What is light made this way in a living thing called?
Ready to practice?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.