Acids, bases, alkalis and salts

Acids

Produce hydrogen ions (H⁺) in aqueous solutions. They have a pH range of 0–6.

Alkaline

Produce hydroxide ions (OH⁻) in aqueous solutions. They have a pH range of 8–14. Alkalis are soluble bases.

Common lab acids

Hydrochloric acid: HCl. Sulfuric acid: H₂SO₄. Nitric acid: HNO₃. Ethanoic acid: CH₃COOH.

Common bases

Hydroxides: OH⁻ ions. Oxides: O²⁻ ions. Carbonates: CO₃²⁻ ions. Ammonia: NH₃.

Indicators

Substances that change colour depending on whether the solution is acidic or basic.

What are the state symbols

(aq) dissolved in water, (s) solid, (g) gas, (l) pure liquid.

Indicators – universal indicator, litmus, phenolphthalein, methyl orange

Four common indicators used to test if a solution is acidic or alkaline.

pH scale

Stands for “potential of hydrogen”. Shows how acidic or alkaline a solution is.

Neutralisation

Reactions where an acid reacts with a base to produce water (and a salt).

Equation for neutralisation

acid + base → salt + water

Advantages & disadvantages of universal indicator

Advantage: easy to use. Disadvantage: colour reading is subjective and not very precise.

Advantages & disadvantages of pH meter

Advantage: very precise numerical value. Disadvantages: expensive and needs skill/calibration.

Things to label on a graph

Acid in excess, neutralisation point, alkali in excess.

Neutralisation with a metal oxide

metal oxide + acid → metal salt + water

Experiment

Gently heat the acid. Add small portions of powdered metal oxide with a spatula, stirring each time, until some solid remains at the bottom (excess). Filter to remove excess solid and keep the solution. Heat the filtrate in an evaporating basin over a water bath to make a saturated solution. Leave to cool and evaporate so crystals form. If needed, filter and dry crystals with filter paper.

Colours of litmus paper

Red in acids, purple when neutral, blue in alkalis.

Colours of methyl orange

Red from pH 0–3, orange from pH 4, yellow from pH 5–14.

Colours of phenolphthalein

Colourless from pH 0–8, pink from pH 8–10, deep pink/red from pH 10–14.

What is a saturated solution

A solution in which no more solute (e.g. salt) can dissolve at a given temperature and extra solute starts to crystallise.

Equation for metal carbonate

acid + metal carbonate → salt + water + carbon dioxide