Acids and alkalis, Year 8: more practice

Lemon juice, water, bleach

提示The lower the pH number, the more acidic the substance.

为什么Acids 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

提示The middle of the scale, 7, is neutral.

为什么The 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

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

为什么Hydrochloric 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

提示The alkali is gradually cancelling out the acid.

为什么As 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

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

为什么The 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

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

为什么A 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

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

为什么Left-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

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

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

Zinc chloride

提示Name the metal first, then use the ending that matches the acid.

为什么Zinc + hydrochloric acid → zinc chloride + hydrogen. Hydrochloric acid always makes chlorides.

Magnesium, because it is more reactive

提示A metal higher up the reactivity series reacts eagerly.

为什么Magnesium sits above iron in the reactivity series, so it reacts faster and gives off hydrogen more vigorously.