Atoms, elements and compounds, Year 7: more practice

Water

HintA compound has two or more different elements joined together chemically.

The whyWater is hydrogen and oxygen chemically joined in a fixed ratio. Iron is an element and air is a mixture of gases.

atom

HintEvery particle in a sample of the element is the same kind.

The whyCopper contains only copper atoms and oxygen gas contains only oxygen atoms. A compound contains different types of atom chemically joined.

K

HintIt does not start with P, because P stands for phosphorus.

The whyThe symbol K comes from the Latin name kalium. Several symbols come from Latin names, such as Na for sodium and Fe for iron.

Iron

HintThe symbol comes from its Latin name, not its English one.

The whyFe is short for ferrum, the Latin word for iron. Fe is also in the word ferrous, as in iron-rich foods.

CO2

HintLook at how many different elements each formula contains.

The whyCO2 has two different elements, carbon and oxygen, joined together. O2 is also a molecule, but of an element, because both atoms are oxygen.

Evaporating the water

HintHeat gently until the liquid has gone.

The whyIn a mixture the substances keep their own properties, so they can be separated by physical methods. No new substance is made.

A mixture

HintThe gases in air are not chemically joined to each other.

The whyThe gases in air are just mixed together and can be present in different amounts. In a compound, the elements are chemically joined in a fixed ratio.

A chemical change

HintCan you turn a cooked egg back into a runny egg?

The whyHeating changes the proteins in the egg and a new substance forms, so it cannot be undone. A physical change, such as melting, keeps the same substance.

Water

HintThink about what firefighters spray.

The whyHydrogen and oxygen join to make water, which behaves nothing like either gas. A compound has its own properties.

It is a new substance with its own properties

HintCompare table salt against the metal and the poisonous gas it is made from.

The whyWhen elements join chemically they make a new substance with different properties from the elements. That is why each new compound needs its own safety check.