The world is made of things, and those things change. Sometimes they change on their own, and sometimes they change because some outside force, like a person, acts on them. When we talk about changes to things, we can group them into two kinds: physical changes and chemical changes.

Physical Changes

Physical changes are events that change what the substance looks like but do not change what it is made of. All objects and everything we use is made up of molecules, which are made up of atoms of different elements. Remember that an element is a kind of atom with a certain number of protons and electrons and certain chemical properties. Objects have two components to them: the combination of elements that is in that object, called the chemical formula, and the structure of those atoms and molecules. A physical change will alter the structure of the object, not the chemical formula. This is a little confusing, so let's have some examples:
  • An ice cube melting. The water is going from a solid to a liquid but is not changing the chemical formula to something other than water.
  • Nuts being crushed up. The structure of the nuts is changing, but they are still nuts. They didn't become something else like apples or carrots.
  • Molten (liquid) iron being poured into molds to harden. The liquid iron doesn't have much shape, but when it hardens in the molds, it freezes and becomes solid. It's in a different shape than it was before, but it's still iron.

Chemical Changes

Chemical changes are events that change the chemical formula of a substance. Often they also change the structure of the object as well, but not always. If physical changes just change how an object looks, then chemical changes affect what an object is made of. Sometimes it is hard to tell if something is a physical or chemical change, but there are ways to tell when a chemical change is occurring, but first let's have some examples:
  • Wood burning into ashes. The wood has a very complicated chemical formula, but because of the burning, it is changed into something else. Ashes are not wood at all and they're different from ground-up-wood, too. They don't burn like wood does and so they must be a different compound.
  • Your stomach acid digesting a cheeseburger. When you eat a cheeseburger, your stomach acid breaks it down into small molecules of sugar and nutrients, tearing apart the original molecules of the cheeseburger. Since the end products have a different chemical formula, this is a chemical change.
  • An iron nail rusting. The nail was pure iron, but now it is a new compound of iron and oxygen. Unlike iron, which you can melt and reform, you can't melt rust as easily and it doesn't hold together like iron does. There was a chemical change here, because you might still have a nail, but it's not a nail made of iron, now it's made of rust.

Telling them apart

Physical changes are pretty easy to identify:
  • Tearing something apart or putting it together either with hands or machines.
  • A substance changing state by melting, freezing, boiling, evaporating, condensing, or sublimating.
  • The end products behave similarly to the beginning products (if the start could be attacked by acid, than the end could also be attacked by acid).
Chemical changes always occur in a chemical reaction and also have a few distinctive signs:
  • The reaction gets hot or cold on its own, as opposed to heat being added or taken away from a machine or device
  • Some light appears on its own, but not because of the stuff being heated to glowing.
  • Gas is produced, but not because the substance is boiling.
  • A solid is produced out of two liquids mixing together, but there is no freezing going on. This process is called precipitation.
  • Something is burning, which means it is reacting with oxygen to produce a compound called an oxide, which is usually dusty like ashes or rust.