Physical and chemical change

Here's a lump of sugar. It's made of one kind of substance - sugar. If we crush it, it's still the same substance - sugar. We can mix the sugar with sand. It's still the same substance.

We can melt the sugar. That still doesn't change what substance it is. When the melted sugar cools down, it will return to solid form again, looking just like before. But if you eat the sugar, something else will happen. It will be broken down in your body.

Inside the cells, the sugar will undergo chemical reactions. The sugar is changed into other substances. It's not sugar anymore. Do you see the difference between these examples? If you: ...

crush the sugar lump, ... mix the sugar with something else or, ... melt it, ... it's still sugar. But in the fourth example, when sugar reacts inside the body, it ceases to be sugar and is changed into other substances.

In this case, carbon dioxide and water are formed. So there are two kinds of change that a substance can undergo. In a physical change, we still have the same substance before and after. Even if it sometimes looks different. After a chemical change, we no longer have the substance we started with.

It has changed into other substances. A physical change is usually easy to reverse. We can pick the sugar crystals from the sand, or let the melted sugar cool so it solidifies again. A chemical change is usually more difficult to reverse. Let's look at two more examples.

Let's melt the sugar again, but this time, keep heating it. The sugar starts changing its colour, and smells differently, and becomes: burned sugar. Is this a physical change or chemical? If we let the burned sugar cool down, it will turn into a solid again. But it retains its brown colour and its burned smell.

It's not going to be that easy, getting the white sugar back. If the sugar has been burned, it's gone through a chemical reaction. What about if we dissolve the sugar in water? Physical or chemical change? The sugar doesn't look the same as before.

In fact, we can't see it at all! But if we let the water evaporate... We get the white sugar back again. Dissolving sugar in water is a process that is pretty easy to reverse - it is a physical change. Crushing, mixing, dissolving, melting, or boiling something are examples of physical changes.

We still have the same substance as before, and it's fairly easy to return it to the form we started with. The other type of change is chemical change, where the original substance is changed into other substances. After a chemical change has taken place, it is usually difficult to go back to the substances we started out with.