AQA · GCSE · Combined Science

    Course version: 8464 · Version from Sept 2018

    The pH scale and neutralisation: Aqueous solutions of alkalis contain hydroxide ions (OH–).

    Alkalis are soluble bases, and when dissolved in water they produce hydroxide ions, OH⁻.

    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Alkalis are soluble bases, and when dissolved in water they produce hydroxide ions, OH⁻. This ion is what makes a solution alkaline: the greater the concentration of OH⁻, the higher the pH. For example, sodium hydroxide dissolves to give Na⁺ and OH⁻ ions, so the solution turns universal indicator blue or purple. The word 'aqueous' is essential because it means dissolved in water; solid alkalis such as sodium hydroxide pellets do not contain free hydroxide ions in the same way. Understanding this ion explains why alkalis neutralise acids: OH⁻ ions react with H⁺ ions to form water, H⁺ + OH⁻ → H₂O. This links the pH scale to ionic theory and to neutralisation reactions.

    What earns the marks

    • States that alkalis are soluble bases that produce hydroxide ions, OH⁻, when dissolved in water.
    • Identifies OH⁻ as the ion responsible for alkaline properties and for pH values above 7.
    • Explains that the concentration of OH⁻ ions determines how strongly alkaline a solution is.
    • Uses the example of sodium hydroxide dissolving to form Na⁺ and OH⁻ ions.
    • Links hydroxide ions to neutralisation: H⁺ + OH⁻ → H₂O.
    • Distinguishes aqueous alkali solutions from solid bases, which do not contain free hydroxide ions.

    Common mistakes and how to put them right

    • Writing the hydroxide ion as OH+ or OH- without the correct Unicode superscript sign.

      Put it right: The correct ion is OH⁻.

    • Confusing alkalis with bases generally.

      Put it right: All alkalis are bases, but not all bases are alkalis because only soluble bases form OH⁻ in water.

    • Thinking that the OH⁻ ion is present in solid sodium hydroxide in the same free form as in solution.

      Put it right: The ions are released when the solid dissolves.

    Exam tips

    • Underline the word 'aqueous' in the question and make sure your answer refers to ions in solution.
    • When asked why a solution is alkaline, name OH⁻ and link its concentration to pH rather than just saying 'it is an alkali'.
    • Use the ionic equation H⁺ + OH⁻ → H₂O to show how hydroxide ions cause neutralisation.

    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 soluble bases, and when dissolved in water they produce hydroxide ions, OH⁻. This ion is what makes a solution alkaline: the greater the concentration of OH⁻, the higher the pH. For example, sodium hydroxide dissolves to give Na⁺ and OH⁻ ions, so the solution turns universal indicator blue or purple. The word 'aqueous' is essential because it means dissolved in water; solid alkalis such as sodium hydroxide pellets do not contain free hydroxide ions in the same way. Understanding this ion explains why alkalis neutralise acids: OH⁻ ions react with H⁺ ions to form water, H⁺ + OH⁻ → H₂O. This links the pH scale to ionic theory and to neutralisation reactions. A strong answer earns marks for these points. States that alkalis are soluble bases that produce hydroxide ions, OH⁻, when dissolved in water. Identifies OH⁻ as the ion responsible for alkaline properties and for pH values above 7. Explains that the concentration of OH⁻ ions determines how strongly alkaline a solution is. Uses the example of sodium hydroxide dissolving to form Na⁺ and OH⁻ ions. Exam tip: Underline the word 'aqueous' in the question and make sure your answer refers to ions in solution.

    See it

    A flowchart running from top to bottom. The first box says alkalis are soluble bases and points to a box saying dissolved in water (aqueous). That points to a box saying produce hydroxide ions, OH⁻. From this box three arrows lead out: one to a box saying OH⁻ makes the solution alkaline, which leads to a box saying greater OH⁻ concentration gives higher pH; one to a box saying example: sodium hydroxide gives Na⁺ and OH⁻; and one to a box saying neutralisation: H⁺ + OH⁻ → H₂O. A separate box saying solid alkalis do not contain free hydroxide ions in the same way points into the dissolved in water box.
    It shows how soluble bases dissolved in water produce hydroxide ions, which make a solution alkaline, raise the pH and neutralise acids.

    Key words

    Say what each word means before you turn the card.

    Aqueous

    Try answering aloud or in your head before turning the card.

    Alkalis

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    Hydroxide ions

    Try answering aloud or in your head before turning the card.

    Neutralisation

    Try answering aloud or in your head before turning the card.

    See every definition
    Aqueous
    Describes a solution in which a substance is dissolved in water, so that ions are free in solution.
    Alkalis
    Soluble bases that produce hydroxide ions, OH⁻, when dissolved in water.
    Hydroxide ions
    Ions written OH⁻ that are responsible for alkaline properties and for pH values above 7.
    Neutralisation
    The reaction in which OH⁻ ions react with H⁺ ions to form water, H⁺ + OH⁻ → H₂O.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A flowchart running from top to bottom. The first box says alkalis are soluble bases and points to a box saying dissolved in water (aqueous). That points to a box saying produce hydroxide ions, OH⁻. From this box three arrows lead out: one to a box saying OH⁻ makes the solution alkaline, which leads to a box saying greater OH⁻ concentration gives higher pH; one to a box saying example: sodium hydroxide gives Na⁺ and OH⁻; and one to a box saying neutralisation: H⁺ + OH⁻ → H₂O. A separate box saying solid alkalis do not contain free hydroxide ions in the same way points into the dissolved in water box.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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