Neutralisation
Bases and alkalis
A baseA substance that reacts with an acid to neutralise it and produce a salt. is any substance that reacts with an acidSubstance producing more hydrogen ions than hydroxide ions when dissolved in water. to form a saltThe substance formed when the hydrogen ion in an acid is replaced by a metal ion. and water only. This means that metal oxides and metal hydroxides are bases.
Bases that are solubleAble to dissolve in solvent. For example, sugar is soluble in water because it dissolves to form sugar solution. in water are called alkaliSubstance producing more hydroxide ions than hydrogen ions when dissolved in water. and they dissolveWhen a substance breaks up and mixes completely with a solvent to produce a solution. in water to form alkalineHaving a pH greater than 7. solutionMixture formed by a solute and a solvent.. For example:
- copper oxide is a base, but it is not an alkali because it is insolubleUnable to dissolve in a particular solvent. For example, sand is insoluble in water. in water
- sodium hydroxide is a base, and it dissolves in water so it is also an alkali
Question
Explain why all alkalis are bases, but not all bases are alkalis.
Alkalis are soluble bases, but only some bases are soluble in water. Bases that dissolve in water are alkalis.
Neutralisation reactions
A neutralisationThe reaction between an acid and a base to form a salt plus water. reaction is a reaction between an acid and a base. Remember:
- acids in solution are sources of hydrogen ionElectrically charged particle, formed when an atom or molecule gains or loses electrons., H+
- alkalis in solution are sources of hydroxide ions, OH-
In acid-alkali neutralisation reactions, hydrogen ions from the acid react with hydroxide ions from the alkali. Here are the ions involved in the reaction of hydrochloric acid with sodium hydroxide solution:
H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) → Na+(aq) + Cl-(aq) + H2O(l)
Now remove the spectator ionAn ion that is exactly the same on both sides of an ionic equation., Cl-(aq) and Na+(aq). This leaves:
H+(aq) + OH-(aq) → H2O(l)
Pure water is neutralWhen a substance is neither acidic nor alkaline, and has a pH of 7. (its pHScale of acidity or alkalinity. A pH (power of hydrogen) value below 7 is acidic, a pH value above 7 is alkaline. is 7). A neutral solution can be produced if the correct amounts of acid and alkali react together. The change in pH during a neutralisation reaction can be measured using a pH probe and meter, or estimated using universal indicatorA chemical solution that produces many different colour changes corresponding to different pH levels. solution and a pH colour chart.