Acids and alkalis, Year 10: the pH scale, strong and weak acids

The hydrogen ion, H⁺

HintIt is what is left of the lightest atom of all after it loses its one electron.

The whyHydrochloric, nitric and sulfuric acids are different compounds, but in water each releases H⁺ ions. It is these ions that make a solution acidic and give acids their shared reactions.

hydroxide

HintThe formula of this ion has one oxygen atom and one hydrogen atom, with a single negative charge.

The whySodium hydroxide, potassium hydroxide and other alkalis all release OH⁻ ions when they dissolve. These ions are what make a solution alkaline, with a pH above 7.

H⁺ + OH⁻ → H2O

HintOne ion comes from the acid, one from the alkali, and together they make the commonest liquid on Earth.

The whyWhatever the acid and alkali, the same thing happens: a hydrogen ion and a hydroxide ion join to make a water molecule. The other ions stay in solution and make up the salt. With state symbols it is H⁺(aq) + OH⁻(aq) → H2O(l).

A precise numerical reading

HintWith the indicator you have to match a colour against a chart by eye.

The whyUniversal indicator shows a colour that is compared with a chart, so it gives only an approximate pH, usually to the nearest whole number. A pH probe measures electronically and displays the value, often to one or two decimal places.

It rises from below 7, through 7, to above 7

HintStart with what the flask holds at the beginning, and end with what is left over at the finish.

The whyAt first the flask holds acid, so the pH is low. As hydroxide ions remove hydrogen ions the pH climbs, reaching 7 when the acid is exactly neutralised, and then goes higher as extra alkali is added.

A strong acid

HintHydrochloric, nitric and sulfuric acids all belong in this class.

The whyIn an acid of this kind every molecule splits up to release its hydrogen ions when dissolved. In a weak acid, such as ethanoic acid, only a small fraction of the molecules are ionised at any moment.

1000

HintEach single step down the scale multiplies the concentration by ten.

The whyA fall of one pH unit means ten times the hydrogen ion concentration. From 5 to 2 is three steps, so the factor is 10 × 10 × 10 = 1000.

Only a small amount of acid is dissolved in each dm³

HintThe label describes how much, not how fully it splits up.

The whyDilute and concentrated describe the amount of acid in a given volume of solution. Strong and weak describe how completely the acid ionises. So an acid can be strong and dilute at the same time, or weak and concentrated.

The hydrochloric acid

HintThe lower pH belongs to whichever releases more hydrogen ions; compare how fully each one ionises.

The whyHydrochloric acid is strong, so it is completely ionised; ethanoic acid is weak, so only some of its molecules release hydrogen ions. At equal concentration the strong acid has more hydrogen ions in each dm³, and so a lower pH.

nitric; sulfuric

HintThey are the other two acids found on almost every school laboratory shelf.

The whyAll three are completely ionised in water. The weak acids to know are ethanoic, citric and carbonic acids, which are only partly ionised.