AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
– Aqueous solutions of alkalis contain hydroxide ions (OH ).
Full specification wording
The pH scale and neutralisation: – Aqueous solutions of alkalis contain hydroxide ions (OH ).
Alkalis are bases that dissolve in water to produce hydroxide ions, OH⁻(aq).
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 2 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Alkalis are bases that dissolve in water to produce hydroxide ions, OH⁻(aq). These ions are responsible for alkaline properties such as turning universal indicator blue or purple, feeling soapy, and neutralising acids to form a salt and water. For example, sodium hydroxide dissolves in water to form Na⁺(aq) and OH⁻(aq); the OH⁻ ions make the solution alkaline. The concentration of OH⁻ ions determines the pH: a higher concentration of OH⁻ gives a higher pH value. In neutralisation, H⁺(aq) from the acid reacts with OH⁻(aq) from the alkali to form water, H₂O(l). This ionic equation, H⁺(aq) + OH⁻(aq) → H₂O(l), is the essential neutralisation reaction and applies to any acid reacting with any alkali.
What earns the marks
- State that aqueous solutions of alkalis contain hydroxide ions, OH⁻.
- Use the term aqueous or the symbol (aq) to show that the ions are in solution.
- Link the presence of OH⁻ ions to alkaline properties such as high pH and neutralisation of acids.
- Explain that the concentration of OH⁻ ions determines the pH value, with more OH⁻ giving a higher pH.
- Write the ionic equation for neutralisation: H⁺(aq) + OH⁻(aq) → H₂O(l).
Common mistakes and how to put them right
Writing the hydroxide ion without a charge or with the charge in the wrong place.
Put it right: Show the negative charge as a superscript, OH⁻.
Confusing alkalis with bases.
Put it right: An alkali is a base that is soluble in water and produces OH⁻(aq); not all bases dissolve.
Forgetting that the hydroxide ions must be in aqueous solution.
Put it right: Include (aq) because the alkali must be dissolved in water to release OH⁻ ions.
Exam tips
- When describing an alkali, name the ions present and state that they are aqueous, for example OH⁻(aq).
- For neutralisation questions, use the ionic equation H⁺(aq) + OH⁻(aq) → H₂O(l) and balance charges.
- If comparing acidity and alkalinity, refer to the relative concentrations of H⁺ and OH⁻ ions rather than just colour changes.
Try the lesson
The start of the The pH scale and neutralisation: – Aqueous solutions of alkalis contain hydroxide ions (OH ). lesson, made from checked teaching notes for this specification point.
Have a go first
Which ion is present in all aqueous solutions of alkalis?
Listen
Alkalis are bases that dissolve in water to produce hydroxide ions, OH⁻(aq). These ions are responsible for alkaline properties such as turning universal indicator blue or purple, feeling soapy, and neutralising acids to form a salt and water. For example, sodium hydroxide dissolves in water to form Na⁺(aq) and OH⁻(aq); the OH⁻ ions make the solution alkaline. The concentration of OH⁻ ions determines the pH: a higher concentration of OH⁻ gives a higher pH value. In neutralisation, H⁺(aq) from the acid reacts with OH⁻(aq) from the alkali to form water, H₂O(l). This ionic equation, H⁺(aq) + OH⁻(aq) → H₂O(l), is the essential neutralisation reaction and applies to any acid reacting with any alkali. A strong answer earns marks for these points. State that aqueous solutions of alkalis contain hydroxide ions, OH⁻. Use the term aqueous or the symbol (aq) to show that the ions are in solution. Link the presence of OH⁻ ions to alkaline properties such as high pH and neutralisation of acids. Explain that the concentration of OH⁻ ions determines the pH value, with more OH⁻ giving a higher pH. A common slip: Writing the hydroxide ion without a charge or with the charge in the wrong place: students may write OH or OH- instead of OH⁻. Instead, show the negative charge as a superscript, OH⁻. Exam tip: When describing an alkali, name the ions present and state that they are aqueous, for example OH⁻(aq).
See it
Key words
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Ions in aqueous solution
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Alkaline properties and pH
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Neutralisation and ionic equations
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Solubility of bases and alkalis
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See every definition
- Ions in aqueous solution
- Aqueous solutions of alkalis contain hydroxide ions, OH⁻, and the term aqueous or the symbol (aq) shows that the ions are in solution.
- Alkaline properties and pH
- The presence of OH⁻ ions is linked to alkaline properties such as high pH, and the concentration of OH⁻ ions determines the pH value, with more OH⁻ giving a higher pH.
- Neutralisation and ionic equations
- In neutralisation, H⁺(aq) from the acid reacts with OH⁻(aq) from the alkali to form water, H₂O(l), and this ionic equation applies to any acid reacting with any alkali.
- Solubility of bases and alkalis
- An alkali is a base that is soluble in water and produces OH⁻(aq), and not all bases dissolve.
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