Acids and bases neutralise to give a salt and water
Definition
Base
A substance that neutralises an acid, such as a metal oxide, a metal hydroxide or a metal carbonate.
Definition
Neutralisation
The reaction of an acid with a base in which hydrogen ions and hydroxide ions combine to form water.
- A base is a substance that neutralises an acid; a soluble base is called an alkali.
- In neutralisation, an acid reacts with a base to make a salt and water.
- acid+base→salt+water\text{acid} + \text{base} \rightarrow \text{salt} + \text{water}acid+base→salt+water
- Metal oxides and metal hydroxides are bases, so both follow this pattern.
- Sulfuric acid and copper oxide: H2SO4(aq)+CuO(s)→CuSO4(aq)+H2O(l)\text{H}_2\text{SO}_4(aq) + \text{CuO}(s) \rightarrow \text{CuSO}_4(aq) + \text{H}_2\text{O}(l)H2SO4(aq)+CuO(s)→CuSO4(aq)+H2O(l)
- Hydrochloric acid and sodium hydroxide: HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)\text{HCl}(aq) + \text{NaOH}(aq) \rightarrow \text{NaCl}(aq) + \text{H}_2\text{O}(l)HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)
Key Idea
- Acid + metal oxide gives a salt and water.
- Acid + metal hydroxide gives a salt and water.
Acids and carbonates also give off carbon dioxide
Definition
Metal carbonate
A compound of metal ions and carbonate ions that reacts with acids to give a salt, water and carbon dioxide.
- A metal carbonate reacts with an acid to give a salt, water and carbon dioxide.
- acid+metal carbonate→salt+water+carbon dioxide\text{acid} + \text{metal carbonate} \rightarrow \text{salt} + \text{water} + \text{carbon dioxide}acid+metal carbonate→salt+water+carbon dioxide
- Hydrochloric acid and calcium carbonate: 2HCl(aq)+CaCO3(s)→CaCl2(aq)+H2O(l)+CO2(g)2\text{HCl}(aq) + \text{CaCO}_3(s) \rightarrow \text{CaCl}_2(aq) + \text{H}_2\text{O}(l) + \text{CO}_2(g)2HCl(aq)+CaCO3(s)→CaCl2(aq)+H2O(l)+CO2(g)
- Test the gas by bubbling it through limewater, which turns milky if it is carbon dioxide.
Example
- Dropping marble chips (calcium carbonate) into dilute hydrochloric acid gives steady fizzing as carbon dioxide escapes.
- Bubbling that gas through limewater turns it cloudy white, confirming carbon dioxide.
The acid decides the ending of the salt name
Definition
Chloride ion
A negatively charged chlorine ion with the formula Cl−\text{Cl}^-Cl− that forms chloride salts.
Definition
Nitrate ion
A negatively charged group of atoms with the formula NO3−\text{NO}_3^-NO3− that forms nitrate salts.
- Hydrochloric acid makes chloride salts.
- Sulfuric acid makes sulfate salts.
- Nitric acid makes nitrate salts.
- The metal in the salt comes from the base, so copper oxide and sulfuric acid give copper sulfate.
Exam technique
- Build the salt name from two parts: the metal from the base and the ending from the acid.
- Nitric acid plus potassium hydroxide gives potassium nitrate, not potassium nitrite.
Choosing the right base for the salt you want
- Use a soluble alkali when both reactants are in solution, so the salt can be found by titration.
- Use an insoluble base, such as a metal oxide or carbonate, when you can add it in excess and filter off what is left.
- Either route gives the same salt; the choice depends on whether the base dissolves.
Self review
- What two products form when an acid neutralises a base?
- What extra product forms when an acid reacts with a metal carbonate?
- Which salt forms from nitric acid and sodium hydroxide?
- Describe the test for carbon dioxide and its result.
- Write the balanced equation for hydrochloric acid reacting with calcium carbonate.
