Acids, bases and salts

Q: What is an acid in aqueous solution?

An acid is a substance that produces hydrogen ions (\(\text{H}^+\text{(aq)}\)) when dissolved in water.

Q: What is the difference between a base and an alkali?

A base neutralises an acid to form a salt and water, while an alkali is a soluble base that dissolves in water to produce hydroxide ions (\(\text{OH}^-\text{(aq)}\)).

Q: What is a salt and what is the ionic equation for neutralisation?

A salt is formed when the \(\text{H}^+\) ions of an acid are replaced by metal or ammonium ions; the ionic equation for neutralisation is \(\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}\).

Q: What distinguishes acid/alkali strength from concentration?

Strength refers to the degree of ionisation in water (complete vs. partial), whereas concentration refers to the amount of solute dissolved per volume of water.

Q: What are the pH ranges and Universal Indicator colours for strong acids, weak acids, neutral substances, weak alkalis, and strong alkalis?

\(\text{pH } 0-2\): strong acid ( Red ); \(\text{pH } 3-6\): weak acid ( Orange/Yellow ); \(\text{pH } 7\): neutral ( Green ); \(\text{pH } 8-11\): weak alkali ( Blue/Blue-green ); \(\text{pH } 12-14\): strong alkali ( Purple/Violet ).

Q: How does litmus paper behave in acidic, neutral, and alkaline solutions?

Red litmus stays red in acid and neutral, and turns blue in alkali; blue litmus turns red in acid, and stays blue in neutral and alkali.

Q: What are the products and observations when an acid reacts with a metal?

Products: Salt + Hydrogen gas ; Observations: effervescence/fizzing, metal dissolves/disappears, temperature rises, and a lit splint gives a squeaky pop.

Q: What are the products and gas test for the reaction of an acid with a metal carbonate?

Products: Salt + Water + Carbon dioxide ; Gas test: bubbling \(\text{CO}_2\) through limewater turns it from colourless to cloudy/milky .

Q: What is formed when an acid reacts with ammonia?

An ammonium salt only (no water is produced).

Q: What are the characteristic colours of key copper(II) compounds in GCSE Chemistry?

\(\text{CuO}\): black solid ; \(\text{CuCO}_3\): green solid ; hydrated \(\text{CuSO}_4\) and \(\text{Cu}^{2+}\text{(aq)}\): blue ; anhydrous \(\text{CuSO}_4\): white powder .

Science Double Award 1370 · CCEA GCSE · 10 min read · Updated: 31 Aug 2026

Welcome to Acids, Bases, and Salts!

Welcome to one of the most exciting and essential topics in your GCSE Chemistry course! Acids and bases are all around us, from the vinegar on your chips and the lemon juice in your drinks, to the soap you use to wash your hands and the indigestion tablets that soothe a stomach ache. In this chapter, you will discover what makes something acidic or alkaline, how these substances react with each other, and how we can use them in the lab to make useful chemical salts.

Don't worry if chemistry equations seem a bit tricky at first. We will break everything down into clear, step-by-step pieces so that you can tackle your exam with total confidence!

1. Key Definitions: What Are Acids, Bases, and Alkalis?

Let's begin with the fundamental building blocks of this topic. Examiners love testing these exact definitions, so getting them clear in your mind right away is a great boost for your revision.

Acids

An acid is a substance that produces hydrogen ions (\(\text{H}^+\text{(aq)}\)) when dissolved in water.
For example, when hydrogen chloride gas dissolves in water, it forms hydrochloric acid: \(\text{HCl(aq)} \rightarrow \text{H}^+\text{(aq)} + \text{Cl}^-\text{(aq)}\). It is the presence of these free \(\text{H}^+\text{(aq)}\) ions that gives acids their characteristic chemical properties.

Bases

A base is a substance that reacts with an acid to neutralise it, forming a salt and water.
Bases are typically metal oxides (such as copper(II) oxide, \(\text{CuO}\)), metal hydroxides (such as sodium hydroxide, \(\text{NaOH}\)), or ammonia (\(\text{NH}_3\)).

Alkalis

An alkali is a special type of base: it is a soluble base that dissolves in water to produce hydroxide ions (\(\text{OH}^-\text{(aq)}\)).
For example: \(\text{NaOH(s)} \rightarrow \text{Na}^+\text{(aq)} + \text{OH}^-\text{(aq)}\).

Important Memory Trick: Think of bases as a large family, and alkalis as the members of that family who know how to swim! "All alkalis are bases, but not all bases are alkalis." If a base dissolves in water, it is an alkali.

Salts

A salt is a compound formed when some or all of the hydrogen ions of an acid are replaced by metal ions or ammonium ions (\(\text{NH}_4^+\)).
For example, if the \(\text{H}^+\) in hydrochloric acid (\(\text{HCl}\)) is replaced by sodium ions (\(\text{Na}^+\)), we get the everyday salt sodium chloride (\(\text{NaCl}\)).

Neutralisation

Neutralisation is the reaction between an acid and a base to form a salt and water.
In terms of ions, the universal ionic equation for neutralisation is:
\(\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}\)

Quick Review: Acids provide \(\text{H}^+\text{(aq)}\), alkalis provide \(\text{OH}^-\text{(aq)}\), and neutralisation joins them together to make neutral water (\(\text{H}_2\text{O(l)}\)).

2. Strength vs. Concentration: Don't Mix Them Up!

One of the most common mistakes students make in GCSE exams is confusing how strong an acid is with how concentrated it is. Let's make sure you never get caught out!

Acid and Alkali Strength (Degree of Ionisation)

Strength describes how completely a substance breaks apart (ionises or dissociates) into ions when dissolved in water.

Strong Acid: An acid that is completely ionised in aqueous solution.
Examples to remember: Hydrochloric acid (\(\text{HCl}\)), Sulfuric acid (\(\text{H}_2\text{SO}_4\)), Nitric acid (\(\text{HNO}_3\)).
All the molecules break apart into ions in water.

Weak Acid: An acid that is only partially ionised in aqueous solution.
Examples to remember: Ethanoic acid (\(\text{CH}_3\text{COOH}\), found in vinegar), Carbonic acid (\(\text{H}_2\text{CO}_3\)).
Only a small fraction of the molecules break apart into ions.

Strong Alkali: An alkali that is completely ionised in aqueous solution (e.g. Sodium hydroxide, \(\text{NaOH}\), and Potassium hydroxide, \(\text{KOH}\)).

Weak Alkali: An alkali that is only partially ionised in aqueous solution (e.g. Ammonia solution, \(\text{NH}_3\text{(aq)}\)).

Concentration (Amount per Volume)

Concentration tells you how much solute (mass or number of moles) is dissolved in a given volume of solution.
A concentrated solution has a large amount of solute dissolved in a small volume of water.
A dilute solution has a small amount of solute dissolved in a large volume of water.

Everyday Analogy: Think of a cup of tea. If you brew it with a tiny splash of water, it is concentrated; if you fill a giant mug with lots of water, it is dilute. But whether the tea leaves themselves are strong or mild is built into the chemistry of the leaf (its strength)! You can have a dilute solution of a strong acid, or a concentrated solution of a weak acid.

3. Indicators and the pH Scale

To find out how acidic or alkaline a solution is, we use the pH scale and chemical indicators.

The pH Scale Classifications (with Universal Indicator)

CCEA exams use specific pH groupings and Universal Indicator colours. Make sure you learn these exact brackets:

\(\text{pH } 0 - 2\): Strong acid \(\rightarrow\) Red
\(\text{pH } 3 - 6\): Weak acid \(\rightarrow\) Orange / Yellow
\(\text{pH } 7\): Neutral \(\rightarrow\) Green
\(\text{pH } 8 - 11\): Weak alkali \(\rightarrow\) Blue / Blue-green
\(\text{pH } 12 - 14\): Strong alkali \(\rightarrow\) Purple / Violet

Litmus Paper

Litmus is a simple indicator that tells us whether a solution is acidic, alkaline, or neutral:
Red litmus paper: Stays red in acidic and neutral solutions; turns blue in alkaline solutions.
Blue litmus paper: Turns red in acidic solutions; stays blue in neutral and alkaline solutions.

pH Meters and Probes

While Universal Indicator gives a rough idea of pH by colour matching, a pH meter (or pH probe) measures the pH electronically to at least 1 decimal place.
Advantages of a pH meter:
1. It is more accurate (gives a precise numerical value).
2. It is objective (does not rely on a person judging and matching colours by eye).

Key Takeaway: Strong acids have the lowest pH values (\(0 - 2\)), strong alkalis have the highest pH values (\(12 - 14\)), and pure neutral substances sit right at \(\text{pH } 7\).

4. The Standard Reactions of Acids

Acids take part in predictable chemical reactions. For each reaction below, you need to know the general word equation, the key observations, and any gas tests involved.

Reaction 1: Acid + Metal \(\rightarrow\) Salt + Hydrogen gas

Example: \(\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}\)
Observations:
- Bubbles of gas / effervescence / fizzing.
- The metal decreases in size and eventually disappears.
- The reaction is exothermic (temperature of the mixture increases).
- A colourless solution is formed.
Test for Hydrogen gas (\(\text{H}_2\)): Apply a burning (lit) splint to the gas. It burns with a characteristic squeaky pop.

Reaction 2: Acid + Metal Oxide (Base) \(\rightarrow\) Salt + Water

Example: \(\text{CuO(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{CuSO}_4\text{(aq)} + \text{H}_2\text{O(l)}\)
Observations:
- Black solid (\(\text{CuO}\)) dissolves/disappears.
- Solution changes from colourless to blue (due to formation of copper(II) sulfate).
- Temperature rises (exothermic).

Reaction 3: Acid + Metal Hydroxide (Alkali/Base) \(\rightarrow\) Salt + Water

Example: \(\text{HCl(aq)} + \text{NaOH(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)}\)
Observations: Temperature rises (exothermic). Both reactants are colourless solutions and react to produce a colourless solution.
Ionic Equation: \(\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}\)

Reaction 4: Acid + Metal Carbonate \(\rightarrow\) Salt + Water + Carbon Dioxide

Example: \(\text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}\)
Observations:
- Rapid effervescence / bubbling.
- Solid carbonate dissolves/disappears.
- Temperature rises.
Test for Carbon Dioxide gas (\(\text{CO}_2\)): Bubble the gas through limewater. The limewater turns from colourless to cloudy / milky (due to the formation of a white precipitate of calcium carbonate).

Reaction 5: Acid + Metal Hydrogencarbonate \(\rightarrow\) Salt + Water + Carbon Dioxide

Example: \(\text{NaHCO}_3\text{(s)} + \text{HCl(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}\)
Like carbonates, hydrogencarbonates produce effervescence and carbon dioxide gas.

Reaction 6: Acid + Ammonia \(\rightarrow\) Ammonium Salt

Example: \(\text{NH}_3\text{(aq)} + \text{HCl(aq)} \rightarrow \text{NH}_4\text{Cl(aq)}\)
Examiner Warning: Notice that no water is produced in this reaction! The ammonia simply accepts a hydrogen ion from the acid to form an ammonium salt.

5. Characteristic Chemical Colours (CCEA Core Set)

Examiners frequently ask you to describe what you see before and after a reaction. Memorise these core appearances:

Copper(II) oxide (\(\text{CuO}\)): Black solid.
Copper(II) carbonate (\(\text{CuCO}_3\)): Green solid.
Hydrated copper(II) sulfate (\(\text{CuSO}_4\cdot 5\text{H}_2\text{O}\)): Blue crystalline solid.
Anhydrous copper(II) sulfate (\(\text{CuSO}_4\)): White powder (turns blue when water is added).
Copper(II) solutions (\(\text{Cu}^{2+}\text{(aq)}\)): Blue solution.
Group 1, Group 2, and Aluminium compounds: White solids that dissolve to form colourless solutions.

6. Prescribed Practical C3: Preparing a Soluble Salt

In this essential practical, you prepare pure, dry crystals of a soluble salt (such as copper(II) sulfate) starting from an acid and an insoluble base or carbonate (such as copper(II) oxide or copper(II) carbonate).

Step-by-Step Method:

Step 1: Heating the acid
Measure a fixed volume of dilute acid (e.g. sulfuric acid, \(\text{H}_2\text{SO}_4\)) into a beaker. Warm it gently using a Bunsen burner. Do not boil the acid!

Step 2: Adding base in excess
Add the insoluble base (e.g. black \(\text{CuO}\)) a spatula at a time, stirring continuously.
Why in excess? We keep adding solid until no more dissolves and solid remains at the bottom. This guarantees that all of the acid has completely reacted and been used up.

Step 3: Filtration
Filter the warm mixture using a funnel and filter paper into a conical flask or evaporating basin.
Why filter? This removes the unreacted excess insoluble solid, leaving a pure solution of the salt (filtrate).

Step 4: Evaporation to the crystallisation point
Pour the filtrate into an evaporating basin. Heat gently over a water bath or Bunsen burner to evaporate some of the water until the point of crystallisation is reached (test by dipping a cold glass rod into the solution; small crystals will quickly form on it).
Warning: Do NOT boil the solution completely dry, or you will destroy the crystal structure and decompose the hydrated salt into a powder!

Step 5: Cooling and Crystallisation
Remove from the heat and leave the concentrated solution in a warm place to cool and crystallise slowly (slow cooling produces larger, well-formed crystals).

Step 6: Separating and Drying
Filter off the crystals, wash them with a tiny amount of cold distilled water to remove impurities, and gently dry them between two sheets of filter paper (or in a desiccator / low-temperature oven).

7. Common Exam Pitfalls and How to Avoid Them

Make sure you review these classic traps before your exam:

Pitfall 1: Confusing "Strong" with "Concentrated".
Remember: "Strong" means 100% ionised. "Concentrated" means a lot of particles packed into a small volume.

Pitfall 2: Saying all bases are alkalis.
Remember: An alkali is strictly a soluble base that releases \(\text{OH}^-\text{(aq)}\) ions in water.

Pitfall 3: Adding water as a product with ammonia.
\(\text{Acid} + \text{Ammonia} \rightarrow \text{Ammonium Salt}\) (No water is made!).

Pitfall 4: Forgetting state symbols in the neutralisation ionic equation.
Always write: \(\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}\).

Pitfall 5: Incomplete descriptions of gas tests.
Do not just write "squeaky pop" — write: "tested with a burning (or lit) splint, burns with a squeaky pop".
Do not just write "turns cloudy" — write: "bubble through limewater, turns colourless to cloudy/milky".

Pitfall 6: Boiling the salt solution to complete dryness.
Always state that you heat only to the point of crystallisation and then leave it to cool slowly.

Chapter Summary Checklist

Before moving on, check that you can:
- Define acid, base, alkali, salt, and neutralisation.
- Distinguish between strong vs. weak and concentrated vs. dilute.
- Recall pH scale values and colour changes for Universal Indicator and Litmus.
- State the 6 standard acid reactions and their general word equations.
- Describe the gas tests for hydrogen (\(\text{H}_2\)) and carbon dioxide (\(\text{CO}_2\)).
- Recall the characteristic colours of copper compounds.
- Describe the 6 practical steps to prepare a pure, dry sample of a soluble salt from an insoluble base.

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What is an acid in aqueous solution?

An acid is a substance that produces hydrogen ions (\(\text{H}^+\text{(aq)}\)) when dissolved in water.

What is the difference between a base and an alkali?

A base neutralises an acid to form a salt and water, while an alkali is a soluble base that dissolves in water to produce hydroxide ions (\(\text{OH}^-\text{(aq)}\)).

What is a salt and what is the ionic equation for neutralisation?

A salt is formed when the \(\text{H}^+\) ions of an acid are replaced by metal or ammonium ions; the ionic equation for neutralisation is \(\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}\).

What distinguishes acid/alkali strength from concentration?

Strength refers to the degree of ionisation in water (complete vs. partial), whereas concentration refers to the amount of solute dissolved per volume of water.

What are the pH ranges and Universal Indicator colours for strong acids, weak acids, neutral substances, weak alkalis, and strong alkalis?

\(\text{pH } 0-2\): strong acid (Red); \(\text{pH } 3-6\): weak acid (Orange/Yellow); \(\text{pH } 7\): neutral (Green); \(\text{pH } 8-11\): weak alkali (Blue/Blue-green); \(\text{pH } 12-14\): strong alkali (Purple/Violet).

How does litmus paper behave in acidic, neutral, and alkaline solutions?

Red litmus stays red in acid and neutral, and turns blue in alkali; blue litmus turns red in acid, and stays blue in neutral and alkali.

What are the products and observations when an acid reacts with a metal?

Products: Salt + Hydrogen gas; Observations: effervescence/fizzing, metal dissolves/disappears, temperature rises, and a lit splint gives a squeaky pop.

What are the products and gas test for the reaction of an acid with a metal carbonate?

Products: Salt + Water + Carbon dioxide; Gas test: bubbling \(\text{CO}_2\) through limewater turns it from colourless to cloudy/milky.

What is formed when an acid reacts with ammonia?

An ammonium salt only (no water is produced).

What are the characteristic colours of key copper(II) compounds in GCSE Chemistry?

\(\text{CuO}\): black solid; \(\text{CuCO}_3\): green solid; hydrated \(\text{CuSO}_4\) and \(\text{Cu}^{2+}\text{(aq)}\): blue; anhydrous \(\text{CuSO}_4\): white powder.

Why is excess insoluble base added when preparing a soluble salt from an acid?

To ensure that all the acid has completely reacted and been used up.

What are the key steps to crystallise and dry a soluble salt after filtering the reaction mixture?

Heat to the point of crystallisation, cool slowly to form crystals, filter, rinse with cold distilled water, and dry between filter paper.

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