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GCSE Chemistry Revision

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GCSE Chemistry revision

Strong and weak acids

Reactions of acids

AQA 4.4.2.6 Higher Tier only
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Key knowledge

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Neutralisation Reactions Overview

  • Neutralisation reactions occur when an acid reacts with a base, always producing a salt as one of the products.
  • Metal oxides, metal hydroxides, and metal carbonates all act as bases and undergo neutralisation with acids.
  • The general pattern is to combine the negative ion from the acid with the positive ion from the base to form the salt.

Acids Reacting with Metal Oxides

  • When an acid reacts with a metal oxide, the products are always a salt and water.
  • For example, hydrochloric acid reacting with sodium oxide produces sodium chloride (the salt) and water: Na₂O + 2HCl → 2NaCl + H₂O The salt formula must be written correctly by balancing the charges of the ions before balancing the overall equation.

Acids Reacting with Metal Hydroxides

  • Metal hydroxides also react with acids to form a salt and water, following the same neutralisation pattern.
  • For example, sulfuric acid reacting with potassium hydroxide produces potassium sulfate and water: 2KOH + H₂SO₄ → K₂SO₄ + 2H₂O Care must be taken with ion charges — the sulfate ion is 2− so two potassium ions (each 1+) are needed to balance the formula.

Acids Reacting with Metal Carbonates

  • When an acid reacts with a metal carbonate, the products are a salt, water, and carbon dioxide gas.
  • For example, nitric acid reacting with calcium carbonate produces calcium nitrate, water, and carbon dioxide: 2HNO₃ + CaCO₃ → Ca(NO₃₎₂ + H₂O + CO₂ Because nitrate has a 1− charge and calcium has a 2+ charge, two nitrate ions are needed to form the correct salt formula, calcium nitrate.

Making Soluble Salts Using an Insoluble Base

  • A soluble salt can be made by reacting a dilute acid with an excess of an insoluble base such as a metal oxide, hydroxide, or carbonate.
  • The acid is gently heated in a beaker, and the insoluble base is added a little at a time until it is no longer dissolving, indicating the acid has been fully neutralised and the base is in excess.
  • The excess insoluble base is removed by filtering the mixture through filter paper and a filter funnel, leaving behind the dissolved salt solution.

Crystallisation to Obtain Pure Salt Crystals

  • The filtered salt solution is gently heated using a water bath or electric heater (not a Bunsen burner) to evaporate some of the water and begin crystallisation.
  • Heating is stopped once crystals begin to form, and the solution is left to cool so that further crystals develop.
  • The crystals are collected by filtering again and then dried by dabbing with filter paper or leaving them in a warm place.

Key Salt Formulae and Ion Charges

  • The name of a salt is determined by the metal from the base and the negative ion from the acid (e.g. hydrochloric acid chloride salts, sulfuric acid sulfate salts, nitric acid nitrate salts).
  • Ion charges must be balanced in the salt formula — for example, calcium (2+) requires two nitrate ions (each 1−) to form calcium nitrate, Ca(NO₃)₂.
  • Common soluble salts produced in these reactions include sodium chloride, potassium sulfate, calcium nitrate, and copper chloride.

Why Use a Water Bath Instead of a Bunsen Burner?

  • A water bath or electric heater provides gentle, controlled heating to avoid overheating the salt solution.
  • Excessive heat from a Bunsen burner could decompose or damage the salt crystals, reducing the purity of the final product.
  • Gentle evaporation ensures that well-formed, pure crystals of the soluble salt are obtained.