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

The hydrogen ion, H⁺

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

为什么Hydrochloric, 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

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

为什么Sodium 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

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

为什么Whatever 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

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

为什么Universal 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

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

为什么At 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

提示Hydrochloric, nitric and sulfuric acids all belong in this class.

为什么In 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

提示Each single step down the scale multiplies the concentration by ten.

为什么A 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

提示The label describes how much, not how fully it splits up.

为什么Dilute 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

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

为什么Hydrochloric 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

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

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