Acids, bases and salts SA - (CCEA)

• Indicators are chemicals that can change colour to identify acidic, alkaline and neutral solutions.

• Universal indicator can determine the strength of an acidic or alkaline solution.

• Acids and alkalis can be strong or weak depending on how much they ionise in water.

• Acids can react with metals, metal carbonates, metal hydrogencarbonates, bases (metal oxides/hydroxides) and ammonia.

• Practical methods can be used to prepare pure salts from acids.

You should always take care in the laboratory when using chemicals, but strong acids and alkalis are particularly dangerous.

Hazard symbols

Take care with chemicals carrying international chemical hazard labelling.

Hazard symbols are better than using words because they are:

• internationally recognised (understood in all languages);

• eye-catching;

• useful for people who are unable to read.

An indicator is a chemical that changes colour in acidic, alkaline and neutral solutions.

Some common indicators include red litmus paper, blue litmus paper and universal indicator.

These indicators allow you to easily work out if a substance is an acid or an alkali.

However, none of these indicators tell you about the strength of an acid or alkali.

Indicators can be made from plants such as beetroot or red cabbage.

How to make an indicator from beetroot

The steps to make an indicator from beetroot are:

1. Chop or crush the beetroot.
2. Place the beetroot into a beaker and add water.
3. Boil the mixture gently to remove the colour from the beetroot.
4. Allow the solution to cool and filter off the beetroot and discard it.
5. The filtrate (liquid) can now be used as an indicator.

The pH scale gives a measure of the strength of an acidic or alkaline solution.

The range for the pH scale is 0 (strong acid) to 14 (strong alkali).

• pH 0 – 2: Strong acid (for example hydrochloric acid or sulfuric acid).
• pH 3 – 6: Weak acid (for example ethanoic acid - which is vinegar).
• pH 7: Neutral (for example, water).
• pH 8 – 11: Weak alkali (for example ammonia).
• pH 12 – 14: Strong alkali (for example sodium hydroxide or potassium hydroxide).

Universal indicator solution or paper turns a range of colours, depending on the strength of the acid or alkali.

pH can be accurately measured using a pH meter or pH probe.

This is an electronic device that can measure the pH of a solution to at least one decimal place.

An acid is a substance that dissolves in water to produce hydrogen ions.

Acids can be strong or weak

Strong acids include:

• hydrochloric acid (HCl)

• sulfuric acid (H₂SO₄)

Weak acids include:

• ethanoic acid (CH₃COOH)

A base is a metal oxide or metal hydroxide that neutralises an acid to produce a salt and water.

An alkali is a soluble base.

Strong alkalis include:

• sodium hydroxide (NaOH)
• potassium hydroxide (KOH)

Weak alkalis include:

• Ammonia (NH₃)

Acids can react with bases, metals, metal carbonates and metal hydrogencarbonates.

Reaction observations

Some general observations can be made during acid reactions:

• if a gas is produced, the observation is ‘bubbles’;
• if a solid such as a solid metal, base, carbonate or hydrogencarbonate reacts with the acid then the observation is ‘solid disappears and a solution is produced’;
• most acid reactions are exothermic – the observation is ‘heat released’. The exception is sodium hydrogencarbonate with acid.

What is a neutralisation reaction?

A neutralisation reaction is a reaction between an acid and a base producing a salt and water.

Neutralisation is an exothermic reaction – one in which heat is given out.

The maximum temperature is reached when the acid solution is completely neutralised by the alkali.

How to work out the name of the salt produced during a reaction

When an acid reacts it is converted into a salt.

You need to be able to work out the name (and formula for higher tier only) of a salt produced in a reaction.

Salts are compounds composed of two parts.

The first word in a salt name is always the name of the metal in the equation.

The second word in a salt name comes from the acid in the equation:

For example, the salt produced in the reaction of sodium hydroxide with hydrochloric acid is sodium chloride.

Please use the link below to access the article on: Prescribed practical (C3) - Temperature changes during a reaction

(https://www.bbc.co.uk/bitesize/articles/zq936rd)

Acids react with metals to produce a salt and hydrogen.

acid + metal → salt + hydrogen

Example:

hydrochloric acid + magnesium → magnesium chloride + hydrogen

2HCl(aq) + Mg(s) → MgCl₂(aq) + H₂(g)

Observations: grey solid magnesium disappears, colourless solution produced, heat released, bubbles.

The hydrogen in these reactions can be tested.

The test for hydrogen is:

• apply a lighted splint,
• a popping sound results.

Acids react with bases to form a salt and water.

acid + base → salt + water

Example

sulfuric acid + copper(II) oxide → copper(II) sulfate + water

H₂SO₄(aq) + CuO(s) → CuSO₄(aq) + H₂O(l)

Observations: black solid copper(II) oxide disappears, blue solution produced.

Acids react with metal carbonates and metal hydrogencarbonates in the same way.

These reactions produce a salt, water and carbon dioxide.

acid + metal carbonate → salt + water + carbon dioxide

or

acid + metal hydrogencarbonate → salt + water + carbon dioxide

Example - carbonate

hydrochloric acid + copper(II) carbonate → copper(II) chloride + water + carbon dioxide

2HCl(aq) + CuCO₃(s) → CuCl₂(aq) + H₂O(l) + CO₂(g)

Observations: green solid copper(II) carbonate disappears, blue solution produced, heat released, bubbles.

Example - hydrogencarbonate

hydrochloric acid + sodium hydrogencarbonate → sodium chloride + water + carbon dioxide

HCl(aq) + NaHCO₃ (s) → NaCl(aq) + H₂O(l) + CO₂(g)

Observations: white solid sodium hydrogencarbonate disappears, colourless solution produced, bubbles.

The carbon dioxide gas produced in these reactions can be tested.

The test for carbon dioxide is:

• bubble gas into colourless limewater (calcium hydroxide solution),
• the solution will change from colourless to milky if the gas is carbon dioxide.

How to treat indigestion

Your stomach contains hydrochloric acid to help prevent infection and digest food.

Sometimes there can be too much acid in your stomach, this is called indigestion.

Indigestion medicines contain an alkali that helps to neutralise the excess acid.

For example, Milk of Magnesia contains magnesium hydroxide.

Magnesium hydroxide reacts with hydrochloric acid to produce magnesium chloride and water.

What happens when we use toothpaste?

Bacteria in our mouths break down sugar in food, this produces acid.

Many foods we eat contain weak acids.

If the pH of the mouth is 5.5 then tooth decay can happen.

Toothpaste contains an alkali such as sodium hydrogencarbonate, which can neutralise the acid and help prevent tooth decay.