Atoms, elements and compounds, Year 7: more practice

Water

提示A compound has two or more different elements joined together chemically.

为什么Water is hydrogen and oxygen chemically joined in a fixed ratio. Iron is an element and air is a mixture of gases.

atom

提示Every particle in a sample of the element is the same kind.

为什么Copper contains only copper atoms and oxygen gas contains only oxygen atoms. A compound contains different types of atom chemically joined.

K

提示It does not start with P, because P stands for phosphorus.

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

Iron

提示The symbol comes from its Latin name, not its English one.

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

CO2

提示Look at how many different elements each formula contains.

为什么CO2 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

提示Heat gently until the liquid has gone.

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

A mixture

提示The gases in air are not chemically joined to each other.

为什么The 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

提示Can you turn a cooked egg back into a runny egg?

为什么Heating 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

提示Think about what firefighters spray.

为什么Hydrogen 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

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

为什么When 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.