In several reactions a colour change indicates the formation of new products. For example, green copper carbonate reacts, when heated, to form blackcopperoxide and carbon dioxide gas.
As the copper cools, it shrinks. The black cupric oxide also shrinks. But they shrink at different rates, which makes the black cupric oxide flake off.
由于铜冷却，它缩小。黑色铜氧化物也缩小。但是，他们在不同的收缩率，从而使黑色铜氧化物片赶走。
3
If you cool it too quickly, the blackoxide will stay stuck to the copper.