Acids

OCR

GCSE

Acids are defined as proton donors that release hydrogen ions (H+) in aqueous solution, resulting in pH values below 7. Their chemical behaviour is dominated by neutralisation reactions with bases, including metal oxides, hydroxides, and carbonates, to produce specific salts. Candidates must distinguish between acid strength, which refers to the degree of dissociation, and concentration, which refers to the molarity of the solution. Advanced understanding involves the logarithmic relationship of the pH scale and the quantitative analysis of neutralisation via titrations.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

What You Need to Demonstrate

Key skills and knowledge for this topic

Award 1 mark for stating that acids release hydrogen ions (H+) in aqueous solution
Credit responses that write the ionic equation for neutralisation as H+(aq) + OH-(aq) → H2O(l)
Award 1 mark for explaining that strong acids completely ionise/dissociate in water, while weak acids only partially dissociate
For Higher Tier, credit the calculation that a decrease of 1 unit on the pH scale represents a 10x increase in H+ ion concentration
Award 1 mark for correctly identifying the salt produced based on the parent acid (e.g., hydrochloric acid forms chlorides)

Example Examiner Feedback

Real feedback patterns examiners use when marking

"You correctly identified the products, but check the formula of the salt — remember to balance the charges of the ions."
"Good definition of an acid. To access higher marks, explain what happens to the acid molecules in water (dissociation)."
"You have confused strength with concentration. Remember: strength is about how many molecules split up; concentration is how many molecules are in the volume."
"Excellent use of the ionic equation. Ensure you always include state symbols to show the formation of liquid water."

Marking Points

Key points examiners look for in your answers

Award 1 mark for stating that acids release hydrogen ions (H+) in aqueous solution
Credit responses that write the ionic equation for neutralisation as H+(aq) + OH-(aq) → H2O(l)
Award 1 mark for explaining that strong acids completely ionise/dissociate in water, while weak acids only partially dissociate
For Higher Tier, credit the calculation that a decrease of 1 unit on the pH scale represents a 10x increase in H+ ion concentration
Award 1 mark for correctly identifying the salt produced based on the parent acid (e.g., hydrochloric acid forms chlorides)

Examiner Tips

Expert advice for maximising your marks

💡When describing salt preparation, explicitly state 'add excess base' to ensure all acid reacts, then 'filter' to remove the excess.
💡For 6-mark questions on acid strength, structure your answer to compare dissociation, pH, and reaction rate.
💡Memorise the general formula: Acid + Metal Carbonate → Salt + Water + Carbon Dioxide. Apply this to any specific example given.

Common Mistakes

Pitfalls to avoid in your exam answers

Confusing 'strong' (fully dissociated) with 'concentrated' (high moles per unit volume)
Writing the formula for salts with incorrect charges, such as NaSO4 instead of Na2SO4
Failing to include state symbols in ionic equations, specifically omitting (aq) for ions and (l) for water
Assuming that a pH of 1 implies a weak acid because the number is small

Key Terminology

Essential terms to know

Ionisation and the pH scale

Neutralisation reactions and salt formation

Strong vs Weak acids (dissociation)

Titrations and volumetric analysis

Likely Command Words

How questions on this topic are typically asked

Define

Explain

Calculate

Describe

Write

Suggest

Practical Links

Related required practicals

•{"code":"PAG C4","title":"Preparation of salts","relevance":"Technique for creating soluble salts from acids and insoluble bases"}
•{"code":"PAG C3","title":"Titration","relevance":"Quantitative analysis of acid-base neutralisation"}

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