Two gases combine and you get a liquid. That is what “chemically combined” means — and it is the whole difference between a compound and a mixture.
Water is the compound you meet most often: hydrogen and oxygen combined in a chemical reaction. Carbon dioxide is another — carbon and oxygen. It is in the air, plants need it for photosynthesis, and because it traps heat it is a greenhouse gas; too much of it, from human activity, is warming the Earth.
The difference between an element, a compound and a mixture is easiest to see if you look at the particles. Click through the six samples.
| Compound | Elements in it, and a use |
| Sodium chloride common salt | sodium + chlorine · food preservative |
| Sodium hydrogencarbonate baking soda | sodium + hydrogen + carbon + oxygen · baking; an antacid for excess stomach acid |
| Ammonia | nitrogen + hydrogen · cleaning |
| Calcium carbonate marble | calcium + carbon + oxygen · statues and floors |
| Glass | silicon, calcium, carbon, oxygen · bottles and containers (made by heating sand, limestone and other compounds) |
| Plastics | carbon, hydrogen, oxygen · almost everything; cause pollution if not disposed of properly |
Hydrogen and oxygen are both gases. Chemically combined, they make water — a liquid. Carbon is a black solid, oxygen a gas; combined, they make carbon dioxide, a colourless gas.
The most dramatic example is salt: sodium is a metal that fizzes violently in water, and chlorine is a poisonous green gas. Chemically combined, they are the white crystals you put on food.
Every sample of water, anywhere, has the same ratio of hydrogen to oxygen. You cannot make “water with a bit more oxygen in it” — if the ratio changes, it is a different compound (or no longer a compound at all). This is the property that most clearly separates a compound from a mixture.
Once hydrogen and oxygen are combined as water, pulling them apart takes a lot of energy — you have to pass an electric current through the water (electrolysis). Compare a mixture, where you can often just filter or evaporate.