Acids and alkalis, Year 9

So that no acid is left over

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

为什么The 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

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

为什么Copper 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

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

为什么As 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

提示Crystals build up layer by layer, and that takes time.

为什么When 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

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

为什么Every 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

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

为什么The 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

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

为什么Neutralising 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

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

为什么The 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

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

为什么Lime 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

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

为什么A 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

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

为什么A 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

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

为什么Your 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.