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

1. Gently heat the acid (the solvent) in a beaker to increase the rate of reaction.

2. Add small portions of the insoluble metal oxide (the solute) using a spatula, stirring continuously.

3. Continue adding the metal oxide until it is in excess, shown by some solid remaining at the bottom. This ensures all the acid has reacted.

4. Filter the mixture to remove the excess, unreacted metal oxide. The filtrate is a solution of the soluble salt in the solvent.

5. Heat the filtrate in an evaporating basin over a water bath to evaporate some of the solvent until a saturated solution is formed.

6. Leave the saturated solution to cool so crystallisation occurs and crystals of the solute form.

7. Filter to collect the crystals and dry them gently using 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

what is an acid

an atom donor