Compounds

Why can't we pull hydrogen and oxygen out of water by sieving, filtering or a magnet? Because in water their particles are joined so tightly that physical methods can't separate them. That makes water a compound.

Compounds form when different elements join chemically in fixed ratios to make something entirely new. A compound's properties are different from those of its elements, and its elements can't be separated by any physical method.

You can get salt back out of salty water by evaporation, which is a physical process. But that only separates salt from water; it doesn't split salt into sodium and chlorine.

Is sugar an element or a compound? (Activity 8.4, with a teacher.) Put a teaspoon of sugar in a boiling tube and heat it gently. It turns brown, then starts to char and goes blackish. Tiny water droplets appear near the open, cooler end of the tube.

Where did the water come from? We're heating the tube, so it isn't dew from the air: it came out of the dry sugar. What's left is black charcoal, which is carbon (scoop it onto a watch glass and see if it burns like coal).

So sugar breaks down on heating into carbon and water, and water is hydrogen and oxygen. Sugar can't be an element. It is a compound of carbon, hydrogen and oxygen.