Acids, Bases and Salts

7.1 The characteristic properties of acids and bases

• An acid is a substance that forms $\ce{H+}$ ions when it dissolves in water. Aqueous solutions of acids contain $\ce{H+}$ ions.
• An alkali is a soluble base. Aqueous solutions of alkalis contain $\ce{OH-}$ ions.

Bases are oxides or hydroxides of metals. Most bases are insoluble in water. The bases that dissolve in water are the alkalis, such as sodium hydroxide, potassium hydroxide and calcium hydroxide (partially).

Reactions of acids

• A reaction with a metal gives off hydrogen: it burns with a squeaky pop.
• A reaction with a carbonate fizzes and the gas turns limewater milky (carbon dioxide).

Reactions of bases

Ammonia is a gas with a sharp smell that turns damp red litmus paper blue.

Neutralisation

A neutralisation reaction occurs between an acid and a base to make a salt and water. When an acid reacts with an alkali, the ionic equation is the same for every pair:

$\ce{H+(aq) + OH-(aq) -> H2O(l)}$

Indicators and pH

Universal indicator (a mixture) gives a range of colours that match the pH number, and so it shows how acidic or alkaline a solution is. It is often used as paper or as a solution.

• pH $< 7$: acid. The lower the pH, the higher the concentration of $\ce{H+}$ ions and the more acidic the solution.
• pH $= 7$: neutral (pure water).
• pH $> 7$: alkaline. The higher the pH, the more alkaline.

Strong and weak acids

Strong acids include hydrochloric acid, nitric acid and sulfuric acid. All of their molecules split into ions, shown with a single arrow:

$\ce{HCl(aq) -> H+(aq) + Cl-(aq)}$

$\ce{H2SO4(aq) -> 2H+(aq) + SO4^2-(aq)}$

Weak acids include the carboxylic acids such as ethanoic acid. Only a small fraction of the molecules split into ions, and an equilibrium is set up, shown with a reversible arrow:

$\ce{CH3COOH(aq) <=> H+(aq) + CH3COO-(aq)}$

At the same concentration, a strong acid has a lower pH (more $\ce{H+}$ ions), a faster reaction with a metal, and conducts electricity better than a weak acid.

7.2 Oxides

Oxides are classified by how they react with acids and bases.

$\ce{CaO(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l)} \quad \text{(basic oxide with an acid)}$

$\ce{CO2(g) + Ca(OH)2(aq) -> CaCO3(s) + H2O(l)} \quad \text{(acidic oxide with an alkali)}$

$\ce{ZnO(s) + 2HCl(aq) -> ZnCl2(aq) + H2O(l)} \quad \text{(amphoteric oxide with an acid)}$

Zinc oxide also reacts with sodium hydroxide solution to give a salt and water, which shows that it is amphoteric. Aluminium oxide does the same.

The oxides of metals are usually basic, and the oxides of non-metals are usually acidic. Aluminium and zinc are in between, so their oxides are amphoteric.

7.3 Preparation of salts

A salt is a compound made when the $\ce{H+}$ of an acid is replaced by a metal ion or by $\ce{NH4+}$. The first part of the name comes from the base or metal and the second part from the acid.

Solubility rules

Making a soluble salt

The method depends on whether the other reactant is soluble.

The steps (after the reaction using an excess of a solid):

1. Filter to remove the excess solid. The filtrate is the salt solution.
2. Heat the filtrate in an evaporating basin to evaporate some of the water, until the solution is saturated (a drop on a glass rod crystallises when cooled).
3. Leave it to cool and crystallise.
4. Remove the crystals by filtering, wash them with a little cold distilled water, and dry them between filter papers.

Warm the dilute sulfuric acid and add copper(II) oxide a little at a time, stirring, until no more dissolves (the oxide is in excess). Filter to remove the excess copper(II) oxide. Heat the filtrate to evaporate some of the water until it is saturated, then leave it to cool and crystallise. Filter off the blue crystals, wash with a little cold distilled water, and dry between filter papers.

$\ce{CuO(s) + H2SO4(aq) -> CuSO4(aq) + H2O(l)}$

Making an insoluble salt by precipitation

An insoluble salt is made by mixing two soluble compounds that contain its ions. The insoluble salt forms as a precipitate.

Mix a solution of a soluble barium salt, such as barium chloride, with a solution of a soluble sulfate, such as sodium sulfate. A white precipitate of barium sulfate forms. Filter it, wash it with distilled water to remove the soluble impurities, and dry it.

$\ce{Ba^2+(aq) + SO4^2-(aq) -> BaSO4(s)}$

Hydrated and anhydrous salts

Examples: hydrated copper(II) sulfate, $\ce{CuSO4.5H2O}$ (blue), and hydrated cobalt(II) chloride, $\ce{CoCl2.6H2O}$ (pink). The "$5$" and the "$6$" show the number of moles of water for each mole of salt.