1.8 Acids, bases and salts 🧑‍🔬

Indicator

chemical that gives colour change in acidic, alkaline and neutral solutions

Examples of indicators

red/ blue litmus paper, phenolphthalein and methyl orange

Using litmus paper

use both to confirm if solution is neutral or acid/ alkali

Litmus paper results

Indicator	Acidic solution	Neutral solution	Alkaline solution
Red litmus paper	red	red	blue
Blue litmus paper	red	blue	blue

Phenolphthalein results

Indicator	Acidic solution	Neutral solution	Alkaline solution
Phenolphthalein	colourless	colourless	pink

Methyl orange results

Indicator	Acidic solution	Neutral solution	Alkaline solution
Methyl orange	red	orange	yellow

pH scale

gives a measure of the strength of an acid or alkali

Interpreting pH scale

• 0 – 2: strong acid

• 3 – 6: weak acid

• 7: neutral

• 8 – 11: weak alkali

• 12 – 14: strong alkali

pH meter

electronic device that measures pH and gives more accurate result

Acid

substance that dissolves in water to produce hydrogen ions, H⁺(aq)

Higher concentration of hydrogen ions

lower pH so stronger acid

Alkali

a soluble base, dissolves in water to produce hydroxide ions, OH^-(aq)

Strength of acids and alkalis

determined by how much they ionise in water, splitting into separate ions

Strong acid/ alkali

ionise completely in water giving high concentration of hydrogen or hydroxide ions

Examples of strong acids

• hydrochloric acid (HCl)

• sulfuric acid (H₂SO₄)

• nitric acid (HNO₃)

Examples of strong alkalis

• sodium hydroxide (NaOH)

• potassium hydroxide (KOH)

Weak acid/ alkali

ionise partially in water giving lower concentration of hydrogen or hydroxide ions and is reversible (⇌)

Examples of weak acids

• ethanoic acid (CH₃COOH)

• carbonic acid (H₂CO₃)

Examples of weak alkalis

ammonia (NH₃)

Base

substance that reacts with acid to produce a salt and water

Difference between alkali and base

alkalis are bases that dissolve in water

Concentration (mol/ dm³)

measurement of the amount of acid/ alkali dissolved in solution

Measuring concentration

mol/ dm³

Concentrated acid/ alkali

contains large number of particles dissolved per unit volume so is stronger

Dilute acid/ alkali

contains small number of particles dissolved per unit volume so is weaker

Neutralisation

reaction between an acid and an alkali producing a salt and water

Ionic equation for neutralisation

H⁺(aq) + OH^-(aq) → H₂O(l)

Temperature of neutralisation

exothermic reaction, maximum temperature reached when acid solution is completely neutralised by alkali

Salt

compound formed when some or all hydrogen ions in acid are replaced by metal ions (or ammonium ions)

Names of salts

first word is the metal and second is type of salt

Hydrochloric acid as salt

Chloride

Sulfuric acid as salt

Sulfate

Nitric acid as salt

Nitrate

Why add excess insoluble solid

ensures all the acid is converted to salt and can be removed via filtration

When do we prepare salts using excess insoluble solid

most Group 2, aluminium and transition metal salts

When do we prepare salts using acids and alkalis

sodium, potassium and ammonium salts

How to dry crystals

place in desiccator/ low temperature oven or between two sheets of filter paper

Observations in acid reactions

• when gas is produced, observations are ‘bubbles’ or ‘fizzing’

• if solid reacts observation is ‘solid disappears and solution is produced’

• most are exothermic except copper(II) oxide and sodium hydrogencarbonate

Colour of iron(II) salts in solution

green solution

Group 1, 2, aluminium and zinc compounds

white solids that dissolve in water to give colourless solutions

Acid + metal

→ salt + hydrogen

Test for hydrogen

apply lighted splint and popping sound results if present

Acid + metal carbonate (or hydrogencarbonate)

→ salt + water + carbon dioxide

Test for carbon dioxide

bubble gas into colourless limewater will change from colourless to milky if present

Limewater

calcium hydroxide solution

Acid + ammonia

→ ammonium salt

Corrosive

burns and destroys living tissue e.g concentrated acid

Explosive

explodes if exposed to flame, heat or knocked e.g potassium

Flammable

catches fire easily when in contact with air e.g ethanol

Toxic

can kill by poisoning e.g weed-killer or cyanide

Caution

may be harmful or an irritant e.g dilute sodium hydroxide

Risk assesssment

• PPE should be worn when handling any harmful substance

• contact should be minimised and use of fume cupboard with toxic substances

• make sure there are no naked flames when using flammable substances (safety flame)

• care should be taken when labelled ‘caution’ to avoid spills