Chemical Reactions and Substances
Acids, bases and salts
Predict what an acid gives with a metal, a carbonate and a base, read a solution's nature off the pH scale, and name the salts that common salt is turned into.
A curry stain on a white shirt turns reddish brown the moment soap is scrubbed on it, and goes yellow again under the tap. Nothing has been added to the stain that you can see. The turmeric in it is an indicator: a dye whose colour reports whether what touches it is acidic or basic. Litmus does the same job in a test tube, and phenolphthalein and methyl orange are the made-up ones a laboratory keeps on the shelf.
What acids and bases have in common
An acid in water gives hydrogen ions; a base in water gives hydroxide ions. That is the whole story behind the behaviour. It is also why an acid solution conducts electricity and a glucose solution does not — ions carry the current, and glucose makes none. A base that dissolves in water has its own name: an alkali.
Worked example
A metal compound is dropped into dilute hydrochloric acid. It fizzes, and the gas put through lime water turns the lime water milky. What was the compound, and what came out?
The gas turned lime water milky, so the gas is carbon dioxide.
Carbon dioxide throws down a white precipitate of calcium carbonate in lime water. That is the test, and it is what makes the liquid go cloudy.
Try it together
Two solutions sit on the bench. A reads pH 4 on the paper; B reads pH 9.
Work through what the two readings tell you.
1.A neutral solution reads 7. Is solution A acidic or basic?
Have a go
Have a go on your own. A gardener's soil tests at pH 8.5. What is the soil?
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.