Acids and alkalis

5

HintEach whole step along the scale stands for a factor of ten, so a hundredfold change shifts it two places.

The whyThe pH scale counts in tens: pH 4 is ten times less acidic than pH 3, and pH 5 a hundred times less. That is why a change of a single pH unit matters so much in a river or a fish tank, and why a spilled acid needs a great deal of water before it is safe to mop up.

Carbon dioxide

HintIt is the gas plants take in through their leaves, and it makes a very weak acid in water.

The whyDissolved in water it forms carbonic acid, so weak that clean rain only reaches about pH 5.6. Acid rain is a separate problem: sulfur dioxide and nitrogen oxides from burning fuels can drive rain below pH 4, acidic enough to eat into limestone buildings and to harm life in lakes and rivers.

Red

HintIt goes the colour of a warning light, while an alkali sends the same paper blue.

The whyLitmus answers only one question: acid or alkali. Universal indicator goes further, giving a range of colours that each match a number on the pH scale.

Purple

HintIt is the colour at the very top of the chart, one step past blue.

The whyThe chart runs red, orange and yellow for acids, green at neutral, then blue to the darkest shade for the strongest alkalis such as oven cleaner.

Attack and destroy living tissue

HintThe picture shows drips eating holes in a hand and in a bench top.

The whyCorrosive substances damage skin and eat into metal, which is why concentrated acids and oven cleaner both carry this symbol and both demand eye protection.

Universal indicator

HintIts colour chart runs the whole way from red through green to purple.

The whyMatching a colour by eye gives a rough figure; a pH probe and meter gives a precise reading, which matters when a small difference changes the outcome.

salt

HintIt is the general name for the compound the acid leaves behind, not just the white grains on your chips.

The whyThe acid and the alkali cancel each other out, leaving a neutral solution. Every acid makes its own family of these, so sodium chloride is one of thousands.

Hydrogen

HintHold a lighted splint over the tube and you will hear a squeaky pop.

The whyThe metal reacts with the acid to make a salt and this gas. The more reactive the metal, the more furious the fizzing — copper does not react at all.

Carbon dioxide

HintBubble it through limewater and the clear liquid turns milky.

The whyThis is why chalk and marble fizz in acid, and why acid rain slowly eats away limestone statues. Limewater turning cloudy is the standard test for it.

Sulfates

HintThe acid passes on its own surname, with the ending altered to show oxygen is present.

The whyHydrochloric acid makes chlorides and nitric acid makes nitrates. The metal supplies the first half of the name, so magnesium plus sulfuric acid gives magnesium sulfate.

filter, then evaporate

HintSolid bits come out first; after that, warmth does the slow work of taking the liquid away.

The whyDo the steps in that order — solid out first, then warm the solution gently in a shallow dish and let it dry. Heating too fiercely spits the mixture out and spoils the crystals.

A base, such as a mild alkali or a carbonate

HintTo cancel out an acid you add its chemical opposite — but gently, since this is going into a stomach.

The whyAnything that cancels an acid is a base; a base that dissolves in water is also called an alkali. Most tablets use calcium carbonate or magnesium hydroxide. Toothpaste does the same job for the acid bacteria make on your teeth, and farmers spread lime on soil that has turned too acidic. All four are one piece of chemistry.