Acids, alkalis and indicatorsIB MYP Chemistry: Revision notes
Section 1
Acids, bases and alkalis
An acid is a substance that forms hydrogen ions, H⁺, when it dissolves in water. For example, hydrochloric acid: HCl → H⁺ + Cl⁻. Acids taste sour (never taste chemicals in the lab) and have a pH below 7.
A base is a substance that can neutralise an acid. Metal oxides and hydroxides are bases. A base that dissolves in water is called an alkali, and it forms hydroxide ions, OH⁻. For example, sodium hydroxide: NaOH → Na⁺ + OH⁻. Alkalis feel soapy and have a pH above 7.
A solution that is neither acidic nor alkaline is neutral, with equal H⁺ and OH⁻ (pure water).
All alkalis are bases, but not all bases are alkalis. Only the bases that dissolve in water are called alkalis.
Section 2
Properties of acids and bases
Acids react with:
- reactive metals (such as magnesium) to give a salt and hydrogen. The hydrogen burns with a squeaky pop.
- carbonates to give a salt, water and carbon dioxide, which turns limewater milky.
- bases and alkalis in neutralisation, giving a salt and water.
Alkalis neutralise acids, and they also feel soapy. Strong acids and alkalis are corrosive, so safety glasses must be worn.
Section 3
Indicators: litmus and universal indicator
An indicator is a substance that changes colour depending on whether a solution is acidic or alkaline.
Litmus has two colours: red in acid, blue in alkali (and purple in a neutral solution). It only tells you acid or alkali.
Universal indicator is a mixture of dyes that gives a range of colours, so it shows how acidic or alkaline a solution is:
- red/orange: strongly/weakly acidic
- yellow: very weakly acidic
- green: neutral
- blue/purple: weakly/strongly alkaline
Some natural dyes, such as red cabbage extract, also work as indicators. Indicators can be used as a solution or as paper strips.
Litmus tells you acid or alkali. Universal indicator also tells you how strong.
Section 4
Hydrogen and hydroxide ions
The difference between acids and alkalis comes down to ions in water:
- Acidic solutions contain more H⁺ than OH⁻.
- Alkaline solutions contain more OH⁻ than H⁺.
- Neutral solutions have equal amounts.
When an acid reacts with an alkali, H⁺ and OH⁻ ions join to form water: H⁺ + OH⁻ → H₂O. This is why the solution becomes neutral.
Section 5
Strong and weak acids
A strong acid ionises completely in water, so almost every molecule gives up its hydrogen ion. Examples are hydrochloric, sulfuric and nitric acid.
A weak acid ionises only partly, so most of its molecules stay as they are. Examples are ethanoic acid (in vinegar) and citric acid (in lemons).
At the same concentration, a strong acid has a lower pH, gives a redder colour with universal indicator and reacts faster with magnesium than a weak acid.
Strong and weak describe how much an acid ionises. Concentrated and dilute describe how much acid is dissolved. They are different ideas.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Acids, alkalis and indicators
- Students at a school in Singapore test four household liquids with litmus paper: lemon juice, vinegar, soapy water and oven cleaner.Soapy water is an alkali. State what happens to red litmus paper in soapy water and name the ion that makes the solution alkaline.2 marks
- A student compares two acids of the same concentration. Hydrochloric acid turns universal indicator red, but ethanoic acid turns it orange-yellow. When identical strips of magnesium ribbon are added, bubbles of gas appear much faster in the hydrochloric acid than in the ethanoic acid.Identify two control variables that the student must keep the same for a fair comparison of the two acids.2 marks
- A science teacher in Nairobi makes an indicator by boiling red cabbage leaves in water. She adds a few drops to three colourless liquids: lemon juice turns it pink, distilled water leaves it purple, and a solution of washing soda turns it green. Later, the class compares the cabbage indicator with litmus paper and universal indicator.State what the results show about each of the three liquids.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).