AQA · GCSE · Combined Science

    Course version: 8464 · Version from Sept 2018

    Strong and weak acids (HT only): describe neutrality and relative acidity in terms of the effect on hydrogen ion concentration and the numerical value of pH (whole numbers only).

    The pH scale is a numerical way to express the relative acidity or alkalinity of an aqueous solution based on hydrogen ion (H⁺) concentration.

    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    The pH scale is a numerical way to express the relative acidity or alkalinity of an aqueous solution based on hydrogen ion (H⁺) concentration. A neutral solution, such as pure water, has a pH of exactly 7. Acidic solutions have a pH below 7; the lower the pH number, the higher the H⁺ concentration and the more acidic the solution. For example, a solution with pH 3 has a higher H⁺ concentration than one with pH 5. Alkaline solutions have a pH above 7, meaning their H⁺ concentration is lower than that of neutral water.

    What earns the marks

    • Neutral solutions have a pH of 7.
    • Acidic solutions have a pH below 7 and a higher hydrogen ion concentration than neutral solutions.
    • Alkaline solutions have a pH above 7 and a lower hydrogen ion concentration than neutral solutions.
    • The lower the pH number, the greater the hydrogen ion concentration and the more acidic the solution.
    • Whole-number pH values are sufficient when comparing acidity or neutrality in this specification.

    Common mistakes and how to put them right

    • Thinking that a higher pH number means a more acidic solution.

      Put it right: Lower pH numbers correspond to higher H⁺ concentrations and greater acidity.

    • Confusing the terms 'strong' and 'concentrated'.

      Put it right: pH measures H⁺ concentration, not acid strength; a dilute strong acid can have a higher pH than a concentrated weak acid.

    • Believing that neutral solutions contain no ions.

      Put it right: Neutral water contains H⁺ and OH⁻ ions in equal, very small concentrations.

    Exam tips

    • When comparing two acidic solutions, state both the pH values and the relative H⁺ concentrations, for example 'pH 3 has a higher H⁺ concentration than pH 5'.
    • Use the words 'hydrogen ion concentration' rather than 'amount of acid' when explaining pH.

    Try the lesson

    The start of the Strong and weak acids (HT only): describe neutrality and relative acidity in terms of the effect on hydrogen ion concentration and the numerical value of pH (whole numbers only). lesson, made from checked teaching notes for this specification point.

    Have a go first

    Solution X has a pH of 3 and Solution Y has a pH of 7. Which statement correctly compares their hydrogen ion concentrations?

    Listen

    The pH scale is a numerical way to express the relative acidity or alkalinity of an aqueous solution based on hydrogen ion (H⁺) concentration. A neutral solution, such as pure water, has a pH of exactly 7. Acidic solutions have a pH below 7; the lower the pH number, the higher the H⁺ concentration and the more acidic the solution. For example, a solution with pH 3 has a higher H⁺ concentration than one with pH 5. Alkaline solutions have a pH above 7, meaning their H⁺ concentration is lower than that of neutral water. A strong answer earns marks for these points. Neutral solutions have a pH of 7. Acidic solutions have a pH below 7 and a higher hydrogen ion concentration than neutral solutions. Alkaline solutions have a pH above 7 and a lower hydrogen ion concentration than neutral solutions. The lower the pH number, the greater the hydrogen ion concentration and the more acidic the solution. A common slip: Thinking that a higher pH number means a more acidic solution. Instead, lower pH numbers correspond to higher H⁺ concentrations and greater acidity. Exam tip: When comparing two acidic solutions, state both the pH values and the relative H⁺ concentrations, for example 'pH 3 has a higher H⁺ concentration than pH 5'.

    See it

    A top-down flowchart. One branch starts at a node labelled pH below 7, which leads to Acidic, which leads to Higher hydrogen ion concentration than neutral. A second branch starts at pH of 7, which leads to Neutral. A third branch starts at pH above 7, which leads to Alkaline, which leads to Lower hydrogen ion concentration than neutral. A fourth branch starts at Lower pH number, which leads to Greater hydrogen ion concentration, which leads to More acidic.
    It shows how pH values below, at and above 7 relate to acidic, neutral and alkaline solutions and their hydrogen ion concentrations.

    Key words

    Say what each word means before you turn the card.

    The pH scale

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

    Neutral solutions

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

    Acidic solutions

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    See every definition
    The pH scale
    A numerical way to express the relative acidity or alkalinity of an aqueous solution, using whole numbers only in this specification.
    Neutral solutions
    Solutions, such as pure water, that have a pH of exactly 7.
    Acidic solutions
    Solutions that have a pH below 7 and a higher hydrogen ion concentration than neutral solutions.

    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 top-down flowchart. One branch starts at a node labelled pH below 7, which leads to Acidic, which leads to Higher hydrogen ion concentration than neutral. A second branch starts at pH of 7, which leads to Neutral. A third branch starts at pH above 7, which leads to Alkaline, which leads to Lower hydrogen ion concentration than neutral. A fourth branch starts at Lower pH number, which leads to Greater hydrogen ion concentration, which leads to More acidic.

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    • 2Box 2

    • 3Box 3

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