SPI Chemistry · Acids, Bases and Salts

Acids, Bases and Indicators

An acid gives hydrogen ions in water and a base gives hydroxide ions; how fully they split decides their strength, and an indicator shows which one you have.

All Acids, Bases and Salts notes

Why this matters

Three questions in the bank, all EASY: what an acid contains that a base does not, the name for an acid that splits completely in water, and the indicator used most in the laboratory. Each is one definition, so the only risk is mixing up two similar words.

Concept 1 of 3: What acids and bases are

Acids taste sour and bases taste bitter and feel soapy, but the real test is what they give in water. An acid gives hydrogen ions. A base gives hydroxide ions. Put the two together and they cancel into a salt and water.

Definition

An acid contains replaceable hydrogen: a metal can take its place. In water it gives $\mathrm{H^+}$ ions (as $\mathrm{H_3O^+}$ ).
A base reacts with an acid to give a salt and water. A base that dissolves in water is an alkali and gives $\mathrm{OH^-}$ ions.
Neutralisation: acid + base gives salt + water.
Acid + reactive metal gives salt + hydrogen. Acid + carbonate or hydrogencarbonate gives salt + water + carbon dioxide.

Reactions of an acid

\text{acid} + \text{base} \rightarrow \text{salt} + \text{water} \qquad \text{acid} + \text{metal} \rightarrow \text{salt} + \mathrm{H_2}

Worked example

Hydrochloric acid reacts with sodium hydroxide. Write the balanced equation and name the products.

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The same idea in a practice question:

Practice questionEasy

Example 1 · Acids, Bases and Salts · Acids, Bases and Indicators

The compound which contains replaceable hydrogen is ____:-

Calling every base an alkali

An alkali is a base that dissolves in water. Copper oxide is a base but does not dissolve, so it is not an alkali. Every alkali is a base; not every base is an alkali.

Forgetting that acids need water

An acid shows its acidic nature only in water, where it gives

\mathrm{H^+}

ions. Dry hydrogen chloride gas does not change the colour of dry blue litmus.

Concept 2 of 3: Strong and weak acids and bases, and the pH scale

Strength is about how completely an acid or base splits into ions in water. A strong acid splits almost completely, a weak one only partly. The pH scale turns that into a number.

Definition

A strong acid dissociates completely in water: hydrochloric, sulphuric, nitric acid.
A weak acid dissociates only partly: ethanoic acid, carbonic acid.
Strong bases: sodium hydroxide, potassium hydroxide. Weak base: ammonium hydroxide.
pH runs from 0 to 14: below 7 acidic, 7 neutral, above 7 basic. The lower the pH, the stronger the acid.
A hydracid has no oxygen ( $\mathrm{HCl}$ ); an oxyacid has oxygen ( $\mathrm{H_2SO_4}$ ). This names what is in the acid, not its strength.

Substance	Strong or weak	Kind
Hydrochloric acid, $\mathrm{HCl}$	Strong acid	Hydracid (no oxygen)
Sulphuric acid, $\mathrm{H_2SO_4}$	Strong acid	Oxyacid
Nitric acid, $\mathrm{HNO_3}$	Strong acid	Oxyacid
Ethanoic acid, $\mathrm{CH_3COOH}$	Weak acid	Organic acid
Carbonic acid, $\mathrm{H_2CO_3}$	Weak acid	Oxyacid
Sodium hydroxide, $\mathrm{NaOH}$	Strong base	Alkali
Ammonium hydroxide, $\mathrm{NH_4OH}$	Weak base	Alkali The one weak base to remember.

Strong or weak is about splitting into ions, not about how much acid is in the water.

Practice this conceptself-check · 5 quick reps

The same idea in a practice question:

Practice questionEasy

Example 2 · Acids, Bases and Salts · Acids, Bases and Indicators

The acid which gets completely dissociates when dissolved in water is a ____ :-

Confusing concentrated with strong

Concentrated means a lot of acid in a little water. Strong means the acid splits completely into ions. Dilute hydrochloric acid is still a strong acid.

Reading oxyacid or hydracid as a strength

Those words only say whether the acid contains oxygen. Hydrochloric acid is a hydracid and strong; carbonic acid is an oxyacid and weak.

Concept 3 of 3: Indicators

An indicator is a dye that changes colour with acids and bases. Litmus is the everyday laboratory choice; others are used when you need a clearer colour change or a pH value.

Definition

Litmus is a purple dye from lichens. It is the indicator used most in the laboratory: blue litmus turns red in an acid, red litmus turns blue in a base.
Phenolphthalein is colourless in acid and pink in base. Methyl orange is red in acid and yellow in base. Both are used in titrations.
Turmeric is yellow and turns reddish brown in a base.
Universal indicator shows a range of colours, so it gives the pH value, not just acid or base.
Olfactory indicators (onion, vanilla, clove) lose their smell in a base.

Indicator	In an acid	In a base
Blue litmus	Turns red	Stays blue
Red litmus	Stays red	Turns blue
Phenolphthalein	Colourless	Pink Colourless with phenolphthalein can mean acidic or neutral.
Methyl orange	Red	Yellow
Turmeric	Stays yellow	Reddish brown
Universal indicator	Red to orange (pH below 7)	Blue to violet (pH above 7)

Universal indicator is green at pH 7.

Practice this conceptself-check · 5 quick reps

The same idea in a practice question:

Practice questionEasy

Example 3 · Acids, Bases and Salts · Acids, Bases and Indicators

The most commonly used indicator in the laboratory is

Testing an acid with red litmus

Red litmus stays red in an acid, so it shows nothing. Use blue litmus to detect an acid and red litmus to detect a base.

Taking 'no colour' with phenolphthalein as neutral

Phenolphthalein is colourless in both acids and neutral solutions. Only pink tells you something for certain: the solution is basic.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

What acids and bases are
Reactions of an acid
$\text{acid} + \text{base} \rightarrow \text{salt} + \text{water} \qquad \text{acid} + \text{metal} \rightarrow \text{salt} + \mathrm{H_2}$

Reference tables (2)

Strong and weak acids and bases, and the pH scale7 rows

Substance	Strong or weak	Kind
Hydrochloric acid, $\mathrm{HCl}$	Strong acid	Hydracid (no oxygen)
Sulphuric acid, $\mathrm{H_2SO_4}$	Strong acid	Oxyacid
Nitric acid, $\mathrm{HNO_3}$	Strong acid	Oxyacid
Ethanoic acid, $\mathrm{CH_3COOH}$	Weak acid	Organic acid
Carbonic acid, $\mathrm{H_2CO_3}$	Weak acid	Oxyacid
Sodium hydroxide, $\mathrm{NaOH}$	Strong base	Alkali
Ammonium hydroxide, $\mathrm{NH_4OH}$	Weak base	Alkali The one weak base to remember.

Strong or weak is about splitting into ions, not about how much acid is in the water.

Indicators6 rows

Indicator	In an acid	In a base
Blue litmus	Turns red	Stays blue
Red litmus	Stays red	Turns blue
Phenolphthalein	Colourless	Pink Colourless with phenolphthalein can mean acidic or neutral.
Methyl orange	Red	Yellow
Turmeric	Stays yellow	Reddish brown
Universal indicator	Red to orange (pH below 7)	Blue to violet (pH above 7)

Universal indicator is green at pH 7.

Watch out for (6)

Calling every base an alkali
→ What acids and bases are
Forgetting that acids need water
→ What acids and bases are
Confusing concentrated with strong
→ Strong and weak acids and bases, and the pH scale
Reading oxyacid or hydracid as a strength
→ Strong and weak acids and bases, and the pH scale
Testing an acid with red litmus
→ Indicators
Taking 'no colour' with phenolphthalein as neutral
→ Indicators

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