Acids and alkalis, Year 8: more practice

Lemon juice, water, bleach

HintThe lower the pH number, the more acidic the substance.

The whyAcids have a pH below 7, neutral is exactly 7, and alkalis are above 7. The further from 7, the stronger the acid or alkali.

14

HintThe middle of the scale, 7, is neutral.

The whyThe pH scale runs from 0, strongly acidic, to 14, strongly alkaline, with 7 as neutral. It measures how acidic or alkaline a solution is.

Sodium chloride and water

HintThe salt is named from the metal in the alkali and the acid used; a familiar liquid forms too.

The whyHydrochloric acid makes chlorides, so the salt is sodium chloride. This is table salt, though the word salt in chemistry covers many compounds.

It rises towards 7

HintThe alkali is gradually cancelling out the acid.

The whyAs alkali is added the acid is used up, so the pH goes up. At the exact balance point the mixture is neutral at pH 7, and more alkali pushes it above 7.

Neutralisation

HintThe tablet cancels out the acid rather than just covering it up.

The whyThe tablets contain a base, such as magnesium hydroxide, which reacts with the extra hydrochloric acid and brings its strength down towards neutral.

A pH probe

HintOne method depends on your eye matching a colour; the other gives a number on a screen.

The whyA pH probe reads a value, often to one decimal place. Matching colours by eye is quick and cheap but can be a judgement call.

So that the last solution does not contaminate the next reading

HintThink about what is still on the probe after it has been in the first liquid.

The whyLeft-over liquid on the probe would mix into the next solution and shift its reading. Rinsing makes each reading fair and repeatable.

It goes out with a squeaky pop

HintHydrogen burns very quickly, using oxygen from the air in the tube.

The whyThe squeaky pop is the small explosion of hydrogen burning with oxygen. It is the standard test for hydrogen gas.

Zinc chloride

HintName the metal first, then use the ending that matches the acid.

The whyZinc + hydrochloric acid → zinc chloride + hydrogen. Hydrochloric acid always makes chlorides.

Magnesium, because it is more reactive

HintA metal higher up the reactivity series reacts eagerly.

The whyMagnesium sits above iron in the reactivity series, so it reacts faster and gives off hydrogen more vigorously.