Acids and alkalis are all around you. Lemon juice is an acid. Soap and bleach are alkalis. In this lesson you will see how chemists tell them apart and what happens when they meet.
It matters because acid reactions make salts, and salts are very useful. Table salt, fertilisers and many medicines are salts. Neutralising acid also helps farmers and eases indigestion.
By the end you will be able to read the pH scale and name the products of acid reactions. You will also describe how to make pure, dry crystals of a salt.
Acids, alkalis and pH
An acid is a substance that makes hydrogen ions, H⁺, when it dissolves in water. A base is a substance that neutralises an acid. An alkali is a base that dissolves in water and makes hydroxide ions, OH⁻.
The pH scale shows how acidic or alkaline a solution is. It runs from 0 to 14. Below 7 is acidic, exactly 7 is neutral and above 7 is alkaline.
Universal indicator is a mixture of dyes. Its colour shows the pH: red for strong acid, green for neutral and purple for strong alkali.
Other indicators have two colours. Litmus is red in acid and blue in alkali. Methyl orange is red in acid and yellow in alkali. Phenolphthalein is colourless in acid and pink in alkali. Universal indicator gives a range of colours, so it shows the pH.
Bars showing approximate pH values: lemon juice 2, water 7, baking soda solution 9 and bleach 12.
Neutralisation
Neutralisation is what happens when an acid reacts with a base. The products are a salt and water. A salt is a compound where the hydrogen of an acid has been replaced by a metal.
With an alkali, the hydrogen ions and hydroxide ions join to make water: H⁺ + OH⁻ → H₂O. For example, HCl + NaOH → NaCl + H₂O. The salt is sodium chloride.
Think of two puzzle pieces clicking together. The acid and the alkali cancel each other out, and the pH moves towards 7. Indigestion tablets use this to calm stomach acid.
Three more acid reactions
Acids react with metals, metal oxides and metal carbonates. Each one follows a pattern. Learn the patterns and you can predict the products.
Acid + metal → salt + hydrogen. For example, Mg + H₂SO₄ → MgSO₄ + H₂. Acid + metal oxide → salt + water. For example, CuO + H₂SO₄ → CuSO₄ + H₂O. The black powder disappears and a blue solution forms.
Acid + metal carbonate → salt + water + carbon dioxide. For example, CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. The carbon dioxide fizzes out and turns limewater milky.
Naming salts
A salt has a two-part name. The first part comes from the metal in the base, metal or carbonate. The second part comes from the acid.
Hydrochloric acid makes chlorides. Sulfuric acid makes sulfates. Nitric acid makes nitrates. Think of a first name and a surname. The metal is the first name and the acid is the surname.
Zinc and hydrochloric acid make zinc chloride. Magnesium oxide and nitric acid make magnesium nitrate. Potassium hydroxide and sulfuric acid make potassium sulfate.
Making a soluble salt
Soluble means it dissolves in water. To make pure, dry crystals of copper sulfate, warm dilute sulfuric acid in a beaker. Add copper oxide a little at a time and stir. Keep going until some solid will not dissolve. That leftover solid is the excess.
Filter the mixture to remove the excess copper oxide. The blue liquid that passes through is the filtrate. Heat it gently to evaporate some of the water. Stop when crystals start to form.
Leave it to cool so crystals grow. Filter them off and dry them between filter paper. The oxide is in excess so that all the acid is used up. Leftover solid is easy to filter off.
To find out exactly how much acid and alkali react, use a titration. A pipette measures a fixed volume of alkali into a flask with a few drops of indicator. Acid is added slowly from a burette until the indicator just changes colour, and the volume used is read from the burette scale. Repeat until two results agree closely.
Insoluble salts and solubility
Some salts do not dissolve in water. Silver chloride and barium sulfate are examples. To make one, mix two solutions that hold the right ions. The salt appears as a solid called a precipitate. Filter it, wash it and dry it.
Silver nitrate and sodium chloride make a white precipitate of silver chloride. All sodium, potassium and ammonium salts dissolve. All nitrates dissolve too.
Most chlorides and sulfates dissolve. Silver chloride, lead chloride, lead sulfate, barium sulfate and calcium sulfate do not. Most carbonates and hydroxides do not dissolve, except sodium, potassium and ammonium ones.
Recap
Acids make H⁺ and alkalis make OH⁻. Below pH 7 is acid, 7 is neutral and above 7 is alkaline.
Acid + base → salt + water. Acid + metal → salt + hydrogen. Acid + carbonate → salt + water + carbon dioxide.
Hydrochloric acid makes chlorides, sulfuric acid makes sulfates and nitric acid makes nitrates.
Insoluble base or metal: add in excess, filter, evaporate some water, then crystallise. Soluble alkali: use a titration.
Insoluble salt: mix two solutions to make a precipitate, then filter.
Key points
Acid + metal → salt + hydrogen. Acid + base → salt + water. Acid + carbonate → salt + water + carbon dioxide.
Neutralisation is H⁺ + OH⁻ → H₂O.
To make a soluble salt, use excess insoluble solid, filter, evaporate some water, crystallise.
pH below 7 is acid, 7 is neutral and above 7 is alkali.
Worked examples
Example 1
Name the salt made when sodium hydroxide reacts with hydrochloric acid.
The first part comes from the alkali, which has sodium.
The second part comes from hydrochloric acid, which makes a chloride.
The salt is sodium chloride.
Answer: sodium chloride
Common slips
Slip: The salt is sodium hydroxide. That is the reactant. The salt is a new substance.
Slip: The salt is sodium sulfate. Hydrochloric acid makes chlorides.
Slip: The salt is hydrogen chloride. The salt contains the metal, not hydrogen.
Example 2
Write a balanced symbol equation for magnesium oxide reacting with sulfuric acid.
A metal oxide and an acid make a salt and water.
The salt is magnesium sulfate, MgSO₄.
MgO + H₂SO₄ → MgSO₄ + H₂O
Answer: MgO + H₂SO₄ → MgSO₄ + H₂O
Common slips
Slip: MgO + H₂SO₄ → MgSO₄ + H₂ A base and an acid make water, not hydrogen.
Slip: MgO + H₂SO₄ → Mg₂SO₄ + H₂O Magnesium sulfate is MgSO₄ because Mg²⁺ balances SO₄²⁻.
Slip: MgO + HCl → MgCl + H₂O The acid is sulfuric acid, so the salt is a sulfate.
Example 3
Describe how to make pure, dry crystals of zinc sulfate from zinc oxide and dilute sulfuric acid.
Warm the acid and add zinc oxide a little at a time, stirring, until no more reacts so that some is in excess.
Filter to remove the excess zinc oxide, keeping the filtrate.
Heat the filtrate to evaporate some of the water, leave to cool to crystallise, filter the crystals and dry them.
Answer: Add zinc oxide to warm acid until in excess, filter, evaporate some water, cool to crystallise, filter and dry the crystals.
Common slips
Slip: Add the acid to the zinc oxide until all the oxide has gone. Add the oxide in excess, so that all the acid is used up.
Slip: Heat until all the water has gone. This gives a powder. Evaporate some water and let crystals form on cooling.
Slip: Use a titration to find the volumes. Zinc oxide is insoluble, so use excess and filter.
Questions and answers
Acids, bases and making salts practice questions for GCSE Combined Science Chemistry. Try each one, then open the answer. Easy questions come first.
Question 1 Easy
What is the name of the salt made from hydrochloric acid and magnesium oxide?
Show answer
Answer: magnesium chloride
Hydrochloric acid makes chlorides.
The metal is magnesium.
Question 2 Easy
What is the pH of a strong alkali?
Show answer
Answer: 14
Alkalis have a pH above 7.
A strong alkali is near 14.
Question 3 Easy
Name the gas that is made when a carbonate reacts with an acid.
Show answer
Answer: carbon dioxide
Carbonates make carbon dioxide.
It turns limewater milky.
Question 4 Easy
Which ions make a solution acidic?
Show answer
Answer: hydrogen ions
Acids make hydrogen ions.
The symbol is H⁺.
Question 5 Easy
A solution has a pH of 3. Is it acidic, neutral or alkaline?
Show answer
Answer: acidic
Below pH 7 means acidic.
pH 3 is a fairly strong acid.
Question 6 Easy
Name the salt made when sulfuric acid reacts with potassium hydroxide.
Show answer
Answer: potassium sulfate
The metal is potassium.
Sulfuric acid makes sulfates.
Question 7 Medium
Name the salt made when zinc reacts with sulfuric acid.
Show answer
Answer: zinc sulfate
Zinc forms zinc ions.
Sulfuric acid makes sulfates.
Question 8 Medium
Write the balanced equation for sodium hydroxide reacting with hydrochloric acid.
Show answer
Answer: NaOH + HCl → NaCl + H₂O
Acid + alkali → salt + water.
NaOH + HCl → NaCl + H₂O
Exam-style questions
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
Exam-style question 14 marks
A student reacts magnesium carbonate with dilute nitric acid.
(a) Name the salt formed. [1 mark]
(b) Describe a test to show the gas made is carbon dioxide. [2 marks]
(c) Write a word equation for the reaction. [1 mark]
Show answer and marking points
Answer: (a) magnesium nitrate (b) Bubble the gas through limewater. It turns milky or cloudy. (c) magnesium carbonate + nitric acid → magnesium nitrate + water + carbon dioxide
(a) magnesium nitrate (1)
(b) bubble through limewater (1)
(b) turns milky or cloudy (1)
(c) correct word equation with salt, water and carbon dioxide (1)
Exam-style question 26 marks
A student has a beaker of dilute sulfuric acid and a bottle of copper oxide powder. Describe how the student can make pure, dry crystals of copper sulfate. [6 marks]
Show answer and marking points
Answer: Warm the dilute sulfuric acid in a beaker. Add copper oxide a little at a time, stirring, until some solid is left over, so that all the acid has reacted. Filter the mixture to remove the excess copper oxide. Heat the filtrate to evaporate some of the water until crystals begin to form on a glass rod. Leave the solution to cool so that crystals form. Filter off the crystals and dry them between filter paper.
warm the acid (1)
add copper oxide in small amounts until in excess, so all the acid reacts (1)
filter off the excess copper oxide (1)
heat the filtrate to evaporate some of the water (1)
leave to cool so crystals form (1)
filter and dry the crystals (1)
Common mistakes
Saying a metal and acid make water
A metal and acid make a salt and hydrogen. A base and acid make a salt and water.
Using the wrong anion for the acid
Hydrochloric makes chloride, sulfuric makes sulfate and nitric makes nitrate.
Boiling all the water off to get crystals
Evaporate part of the water, leave to cool and crystallise. Boiling to dryness leaves a powder.
Using the wrong pH ranges
Acid is below 7, neutral is 7, alkali is above 7.
Questions students ask
Is Acids, bases and making salts on the Foundation or Higher tier?
Acids, bases and making salts is on both Foundation and Higher papers for GCSE Combined Science Chemistry, aimed at around grade 4.
What is a common mistake on Acids, bases and making salts?
Saying a metal and acid make water. A metal and acid make a salt and hydrogen. A base and acid make a salt and water.
Where are the answers to the Acids, bases and making salts questions?
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.