AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
+ Acids produce hydrogen ions (H ) in aqueous solutions.
Full specification wording
The pH scale and neutralisation: + Acids produce hydrogen ions (H ) in aqueous solutions.
When an acid dissolves in water, it ionises to release hydrogen ions, H⁺.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 1 of 6
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
When an acid dissolves in water, it ionises to release hydrogen ions, H⁺. These are the species responsible for acidic properties such as turning universal indicator red or orange, reacting with metals to produce hydrogen gas, and reacting with bases to form salts and water. For example, hydrogen chloride gas dissolves in water to form hydrochloric acid, which contains H⁺(aq) and Cl⁻(aq); the H⁺ ions are produced because the polar water molecules pull the HCl molecule apart. The subscript (aq) shows that the ions are dissolved in water. In aqueous solution, H⁺ ions are strongly attracted to water molecules, but for GCSE it is sufficient to represent them as H⁺(aq). The concentration of H⁺ ions determines the pH: a higher concentration of H⁺ gives a lower pH value.
What earns the marks
- State that acids produce hydrogen ions, H⁺, when dissolved in water.
- Use the term aqueous or the symbol (aq) to show that the ions are in solution.
- Link the presence of H⁺ ions to acidic properties such as low pH, reaction with metals, and neutralisation.
- Explain that the concentration of H⁺ ions determines the pH value, with more H⁺ giving a lower pH.
- Write balanced ionic equations for acid reactions, showing H⁺(aq) as a reactant where appropriate.
Common mistakes and how to put them right
Writing the hydrogen ion without a charge or with the charge in the wrong place.
Put it right: Show the positive charge as a superscript, H⁺.
Confusing hydrogen ions with hydrogen atoms or hydrogen molecules.
Put it right: Acids produce H⁺ ions in aqueous solution; hydrogen gas, H₂, is only produced when some acids react with metals.
Omitting the aqueous state symbol.
Put it right: Include (aq) to show the ion is dissolved in water, because acids only produce H⁺ ions in aqueous solution.
Exam tips
- When explaining acidic properties, always refer to H⁺(aq) rather than just 'hydrogen' to show precise chemical understanding.
- If asked why a solution is acidic, link your answer to the presence and concentration of H⁺ ions.
- In ionic equations, balance both atoms and charge, and include state symbols where they are given or required.
Try the lesson
The start of the The pH scale and neutralisation: + Acids produce hydrogen ions (H ) in aqueous solutions. lesson, made from checked teaching notes for this specification point.
Have a go first
Which ion is produced by acids when they are added to water?
Listen
When an acid dissolves in water, it ionises to release hydrogen ions, H⁺. These are the species responsible for acidic properties such as turning universal indicator red or orange, reacting with metals to produce hydrogen gas, and reacting with bases to form salts and water. For example, hydrogen chloride gas dissolves in water to form hydrochloric acid, which contains H⁺(aq) and Cl⁻(aq); the H⁺ ions are produced because the polar water molecules pull the HCl molecule apart. The subscript (aq) shows that the ions are dissolved in water. In aqueous solution, H⁺ ions are strongly attracted to water molecules, but for GCSE it is sufficient to represent them as H⁺(aq). The concentration of H⁺ ions determines the pH: a higher concentration of H⁺ gives a lower pH value. A strong answer earns marks for these points. State that acids produce hydrogen ions, H⁺, when dissolved in water. Use the term aqueous or the symbol (aq) to show that the ions are in solution. Link the presence of H⁺ ions to acidic properties such as low pH, reaction with metals, and neutralisation. Explain that the concentration of H⁺ ions determines the pH value, with more H⁺ giving a lower pH. A common slip: Writing the hydrogen ion without a charge or with the charge in the wrong place: students may write H or H+ instead of H⁺. Instead, show the positive charge as a superscript, H⁺. Exam tip: When explaining acidic properties, always refer to H⁺(aq) rather than just 'hydrogen' to show precise chemical understanding.
See it
Key words
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Ions in aqueous solution
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Ionic equations and state symbols
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Concentration and pH
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See every definition
- Ions in aqueous solution
- When an acid dissolves in water, it ionises to release hydrogen ions, H⁺, and the subscript (aq) shows that the ions are dissolved in water.
- Ionic equations and state symbols
- Balanced ionic equations show H⁺(aq) as a reactant where appropriate, balancing both atoms and charge and including state symbols where they are given or required.
- Concentration and pH
- The concentration of H⁺ ions determines the pH, so a higher concentration of H⁺ gives a lower pH value.
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