Acids and alkalis, Year 9

So that no acid is left over

HintThink which of the two starting chemicals you would least want in your finished crystals.

The whyThe spare powder is easy to remove afterwards because it does not dissolve, so adding too much does no harm. Leftover acid could not be filtered out, and it would stay in the dish with the salt as the water evaporated.

In the liquid that runs through

HintA dissolved substance behaves differently from the bits of powder you can still see.

The whyCopper sulfate dissolves in water, so its particles pass through the tiny holes in the paper and give the liquid its blue colour. Only the leftover black copper oxide, which does not dissolve, is held back.

Crystallisation

HintThe name is built from the word for the regular, flat-sided solids that appear.

The whyAs water leaves, the dissolved salt is crowded into less and less liquid until it can no longer stay dissolved, and it comes out as solid crystals. It is the last stage in making a salt.

The one left on the windowsill

HintCrystals build up layer by layer, and that takes time.

The whyWhen water leaves slowly, particles have time to join the growing crystals in a regular pattern, so a few crystals grow large. Fast boiling forces the salt out all at once as a fine powder.

Magnesium chloride

HintThe metal keeps its own name, and this acid always passes on an ending in -ide.

The whyEvery salt name has two parts: the first comes from the metal and the second from the acid. Hydrochloric acid always gives chlorides, and hydrogen gas bubbles off as the metal reacts.

Nitric acid

HintLook at the second word of the salt and find the bottle whose label starts the same way.

The whyThe second word of a salt points back to the acid that made it: chlorides come from hydrochloric acid, sulfates from sulfuric acid and nitrates from this one. The potassium comes from the alkali.

sodium chloride; water

HintAn alkali and an acid cancel out, leaving a salt named after both of them plus the commonest liquid on Earth.

The whyNeutralising an alkali with an acid always gives a salt and water. Here the first word of the salt comes from the alkali and the second from the acid, which makes ordinary table salt.

The metal

HintOne reactant is a solid element and the other is a solution; the name starts with the solid one.

The whyThe metal gives the first word and the acid gives the second. When the acid reacts with an alkali instead, the first word comes from the metal in the alkali: sodium hydroxide gives sodium salts.

Lime

HintA white powder made from a common rock; it shares its name with a green citrus fruit.

The whyLime is a base, so it neutralises some of the acid in the soil and brings the pH back towards 7. Most crops grow best in soil that is close to neutral.

It is corrosive

HintThink of the hazard symbol on a bottle of oven cleaner or drain cleaner.

The whyA strong alkali attacks living tissue just as a strong acid does, so it would burn the mouth and throat. A weak base cancels the extra stomach acid gently, and any that is left over does no harm.

It rises above 7, so the water becomes alkaline

HintNeutral is only the halfway point; keep adding and you overshoot it.

The whyA base cancels acid only until the acid has gone. Any added after that has nothing to react with, so the water ends up alkaline, which harms river life as much as acid does. Neutralising means adding just enough.

An antacid

HintThe name is "anti" joined to the thing it works against, with one letter squeezed out.

The whyYour stomach makes hydrochloric acid to help digest food, and too much of it causes the burning pain of indigestion. An antacid contains a mild base that neutralises the extra acid.