4 · Acids, alkalis and indicators

Acids

Have a sour taste.
May be dangerous — some acids can burn the skin.
Turn blue litmus paper red.
React with alkalis, metals and carbonates.
Commonly found in the laboratory: hydrochloric acid, sulfuric acid, nitric acid.
Commonly found at home: acetic acid (vinegar), ascorbic acid (vitamin C), citric acid (citrus fruit).

Alkalis

Have a bitter taste and a soapy feel.
May be dangerous — some alkalis can burn the skin.
Turn red litmus paper blue.
React with acids in neutralisation reactions.
Commonly found in the laboratory: sodium hydroxide, calcium hydroxide, potassium hydroxide.
Commonly found at home: sodium hydroxide (in soaps), baking soda, ammonia (in cleaning liquids).

Where alkalis come from An alkali is the solution formed when ammonia, or a soluble metallic oxide, dissolves in water:

ammonia + water → ammonia solution

sodium oxide + water → sodium hydroxide

calcium oxide + water → calcium hydroxide (limewater)

Safety first Some acids and alkalis are toxic or corrosive and are stored in bottles labelled with hazard symbols — such as the skull-and-crossbones for acute toxicity, and the hand-and-surface symbol for corrosives. It is never safe to taste or touch an unknown solution in the laboratory. That is exactly why indicators exist.

The pH scale

The pH scale tells you how acidic or alkaline a solution is, and runs from 0 to 14:

pH less than 7 — acidic. The lower the value, the more strongly acidic.
pH exactly 7 — neutral. Pure water sits here.
pH greater than 7 — alkaline. The higher the value, the more strongly alkaline.

Substances at the extreme ends of the scale are very acidic or very alkaline, and can burn your skin.

Test ten substances

Pick a substance, then choose which indicator to dip into it. The pH values are the ones printed in Figure 11.20 of your textbook.

Indicator bench

1 · Choose a substance

2 · Choose an indicator

SubstancePure water

pH valuepH 7

Classified asNeutral

Universal Indicator shows Green

0 · most acidic7 · neutral14 · most alkaline

Litmus paper always behaves the same way, whatever the indicator above says:

Red litmus paper: stays red

Blue litmus paper: stays blue

Table 11.2 — what litmus tells you

Type of solution	Red litmus paper	Blue litmus paper
Acidic	remains red	turns red
Alkaline	turns blue	remains blue
Neutral	remains red	remains blue

Why Universal Indicator beats litmus Litmus only tells you acid or alkali. Universal Indicator is a mixture of dyes that gives a whole range of colours, so it tells you how acidic or alkaline — an approximate pH value rather than just a category. For an accurate reading, a pH meter with an electrode placed in the solution gives a numerical value directly.

Acids

Alkalis

The pH scale

Test ten substances

Indicator bench

Table 11.2 — what litmus tells you

Check yourself