Neutralisation of acids and salt production: the positive ions in the base, alkali or carbonate.

The idea

The positive ion in a base, alkali or carbonate determines the first part of the salt name. In sodium hydroxide the positive ion is Na⁺, so salts formed are sodium salts. In copper(II) oxide the positive ion is Cu²⁺, so salts are copper(II) salts. In calcium carbonate the positive ion is Ca²⁺, so salts are calcium salts. The acid supplies the negative ion, but the positive ion comes from the base, alkali or carbonate. For example, magnesium oxide reacting with hydrochloric acid gives magnesium chloride, because Mg²⁺ comes from the oxide and Cl⁻ comes from the acid. Identifying the positive ion correctly, including its charge where relevant, allows the salt to be named and its formula to be deduced.

What earns the marks

• Identify the positive ion present in a named base, alkali or carbonate, such as Na⁺ in sodium hydroxide or Cu²⁺ in copper(II) oxide.
• Use that positive ion to write the first part of the salt name, for example sodium in sodium sulfate or copper(II) in copper(II) nitrate.
• Recognise that the positive ion may be a metal ion or the ammonium ion, NH₄⁺, when an ammonium compound is used.
• Deduce the salt formula by balancing the charge on the positive ion from the base, alkali or carbonate with the charge on the negative ion from the acid.
• Explain that the positive ion is unchanged during neutralisation and appears in the salt product.

Common mistakes and how to put them right

• Taking the positive ion from the acid instead of the base, alkali or carbonate.

  Put it right by identifying the metal or ammonium ion in the base, alkali or carbonate first.

• Ignoring the charge on a variable-charge ion and writing an incorrect formula, for example CuCl for copper(II) chloride.

  Put it right by using Cu²⁺ and two Cl⁻ to give CuCl₂.

• Treating the hydroxide ion, OH⁻, as the positive ion in an alkali.

  Put it right by recognising that OH⁻ is negative and the positive ion is the metal or ammonium ion present.

Exam tips

• Write the positive ion symbol and charge above the base, alkali or carbonate before attempting the salt name or formula.
• For variable-charge metals, read the Roman numeral in the name and use it as the ion charge, for example iron(III) means Fe³⁺.
• Cross-check the final formula so the total positive charge equals the total negative charge.