What are acids, bases and alkalis, how does the pH scale work, and what happens when acids react?
Acids, bases and alkalis in terms of hydrogen and hydroxide ions, the pH scale and indicators, neutralisation to make a salt and water, the reactions of acids with metals, oxides, hydroxides and carbonates, and simple tests for hydrogen and carbon dioxide.
A focused CCEA GCSE Single Award Science answer on acids, bases and salts, covering hydrogen and hydroxide ions, the pH scale and indicators, neutralisation, the reactions of acids with metals, oxides, hydroxides and carbonates, and the tests for hydrogen and carbon dioxide.
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What this dot point is asking
CCEA wants you to define acids, bases and alkalis using hydrogen and hydroxide ions, use the pH scale and indicators, explain neutralisation, give the products of acids reacting with metals, oxides, hydroxides and carbonates, and test for hydrogen and carbon dioxide.
Acids, bases and alkalis
Common acids are hydrochloric, sulfuric and nitric acid. Common alkalis are sodium hydroxide and potassium hydroxide.
The pH scale and indicators
Indicators show pH by colour. Universal indicator gives a range of colours (red for strong acid, green at neutral, purple for strong alkali) and so shows roughly how strong the acid or alkali is. Litmus turns red in acid and blue in alkali, but only tells you whether a solution is acidic or alkaline, not how strong it is. A pH meter gives a precise number.
Neutralisation and the reactions of acids
The standard reactions of acids are:
- Acid + metal gives a salt + hydrogen.
- Acid + metal oxide gives a salt + water.
- Acid + metal hydroxide gives a salt + water.
- Acid + carbonate gives a salt + water + carbon dioxide.
Testing for the gases
Examples in context
- Example 1. Curing an acidic soil
- Farmers add lime (calcium oxide or calcium hydroxide, both bases) to soil that is too acidic. The base neutralises the excess acid, raising the pH towards neutral so crops grow better. This is neutralisation in everyday use, and it explains why the products of the reaction are a salt and water.
- Example 2. Indigestion remedies
- The stomach contains hydrochloric acid, and too much causes indigestion. Antacids contain bases such as magnesium hydroxide or calcium carbonate, which neutralise the excess acid. A carbonate antacid also fizzes, releasing carbon dioxide, while a hydroxide antacid just makes a salt and water. This links the reactions of acids directly to a familiar product.
- Example 3. Choosing the right acid to make a salt
- To make a named salt, you pick the acid that supplies the correct non-metal part. Copper sulfate is made from sulfuric acid and copper oxide; sodium chloride is made from hydrochloric acid and sodium hydroxide; potassium nitrate is made from nitric acid and potassium hydroxide. In each case the acid decides the salt (sulfate, chloride or nitrate) and the base decides the metal. Reacting an acid with a metal oxide or carbonate, filtering off the excess solid, then crystallising the solution is a standard way to make a pure salt, and CCEA often asks you to name the acid and base needed for a given salt.
Try this
Q1. State the pH range of an alkaline solution. [1 mark]
- Cue. Above 7 (up to 14).
Q2. Name the gas given off when an acid reacts with a metal, and the test for it. [2 marks]
- Cue. Hydrogen; it pops with a lighted splint.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA SAS 20204 marksGive the products and write a word equation for the reaction of hydrochloric acid with calcium carbonate, and describe a test that would confirm the gas produced.Show worked answer →
Four marks for the products, the equation and the gas test.
An acid reacting with a carbonate gives a salt, water and carbon dioxide.
Word equation: hydrochloric acid + calcium carbonate gives calcium chloride + water + carbon dioxide.
Test for the gas: bubble it through limewater. Carbon dioxide turns limewater milky (cloudy).
Markers reward the three products, the correct salt (calcium chloride from hydrochloric acid), and the limewater test going milky.
CCEA SAS 20183 marksExplain what is meant by neutralisation and state the pH of a neutral solution.Show worked answer →
Three marks for the definition, the products and the pH.
Neutralisation is the reaction of an acid with a base (or alkali) to make a salt and water.
The essential change is that hydrogen ions from the acid join hydroxide ions from the alkali to make water.
A neutral solution has a pH of 7.
Markers reward acid plus base gives salt and water, the joining of H+ and OH-, and pH 7 for neutral.
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Sources & how we know this
- CCEA GCSE Science: Single Award specification — CCEA (2017)