Acids and alkalis, Year 10: neutralisation and making salts

Ca(NO3)2

HintThe charges have to cancel, and a group of atoms that is needed more than once goes inside brackets.

The whyOne Ca²⁺ carries two positive charges, so it needs two NO3⁻ ions to balance it. The whole nitrate group is doubled, so it is written in brackets with the 2 outside.

A sulfate ion is 2−, and each sodium ion is only 1+

HintA compound has no overall charge, so the pluses and minuses must add up to zero.

The whySodium ions are Na⁺ and sulfate ions are SO4²⁻. Two single positive charges are needed to cancel one double negative charge, which gives Na2SO4.

An alkali

HintSodium hydroxide solution is the usual laboratory example.

The whyA base is anything that neutralises an acid to give a salt and water: metal oxides and metal hydroxides are bases. Most are insoluble; the ones that dissolve, such as sodium hydroxide, have this special name.

zinc nitrate; water

HintA base and an acid always give the same two kinds of product; the first takes its second name from the acid.

The whyZinc oxide is an insoluble base, and a base neutralises an acid to give a salt and one other product. Nitric acid always gives nitrates, and the metal comes from the base.

2

HintCount the sodium atoms needed on the right-hand side.

The whySodium sulfate, Na2SO4, contains two sodium ions, so two units of NaOH are needed to supply them. The two OH groups and the two H atoms from the acid then make two molecules of water.

sodium nitrate

HintThe metal supplies the first word and the acid supplies the second.

The whyEvery carbonate reacts with an acid to give a salt, water and carbon dioxide. The salt takes its metal from the carbonate and its second name from the acid, and nitric acid gives nitrates.

Bubbles (fizzing)

HintThink of what happens when you open a bottle of lemonade.

The whyThe carbon dioxide forms on the surface of the chips and rises through the liquid. The proper word for this is effervescence, and the chips slowly get smaller as they react.

2

HintCount the chlorine atoms in the salt on the right.

The whyCalcium chloride, CaCl2, holds two chlorine atoms, and each HCl supplies only one. Two HCl also bring the two hydrogen atoms needed for one molecule of water.

The fizzing stops, even when more powder goes in

HintEach earlier spoonful made a gas; watch for the moment that changes.

The whyWhile acid remains, each addition of carbonate makes carbon dioxide. Once the acid has gone, extra carbonate cannot react, so it simply sits in the liquid.

7

HintAsk what a carbonate does to an acid, and whether the leftover powder can dissolve to push things further.

The whyThe carbonate neutralises the acid, so the pH rises from a low value towards neutral. Calcium carbonate is insoluble, so the excess cannot dissolve and make the liquid alkaline; the salt solution that is left is roughly neutral.

The reaction is faster in warm acid

HintThink about how temperature affects how quickly particles collide.

The whyCopper oxide reacts only slowly with cold dilute acid. Warming speeds things up so that the powder reacts in a reasonable time; the acid is warmed gently, never boiled.

Strong heating would spoil the crystals

HintGentle treatment gives large, well-shaped pieces; think what harsh treatment could do to them.

The whyBoiling to dryness can make the solid spit out of the dish, and can drive off water that is part of the solid or even break the salt down. Evaporating part of the water and then cooling lets pure, regular pieces grow slowly.

The excess would dissolve, so it could not be filtered out

HintThink about what a paper cone in a funnel can and cannot catch.

The whyThe method relies on the leftover solid being caught by the filter paper, leaving only salt solution. A dissolved excess would pass straight through with the salt and contaminate it.