Acids and bases
Bases are substances that can react with acids and neutralise them. Alkalis are bases that are soluble in water. The pH scale measures how acidic or alkaline a substance is. Substances with a pH lower than 7 are acidic, those with a pH of 7 are neutral and those with a pH greater than 7 are alkaline.
The chemical properties of many solutions enable them to be divided into three categories - acids, alkalis and neutral solutions. The strength of the acidity or alkalinity is expressed by the pH scale.
If universal indicator is added to a solution it changes to a colour that shows the pH of the solution.
These are some examples of common substances and their pH values.
|strong acidic||0||battery acid|
|10||milk of magnesia|
|strongly alkaline||14||drain cleaner|
Bases are substances that can react with acids and neutralise them. Bases such as metal oxides and metal hydroxides react with acids to form neutral products.
Examples of bases include:
An alkali is a soluble base, a base that can dissolve in water. For example, copper(II) oxide is a base because it can neutralise acids but, because it does not dissolve in water, it is not an alkali.
Examples of alkalis include:
All alkalis are bases.
When an alkali is added to an acid the pH of the mixture rises. This is because the alkali reacts with the acid to form neutral products. The reverse situation also happens too: when an acid is added to an alkali the pH of the mixture falls. This is because the acid reacts with the alkali to form neutral products.
A reaction in which acidity or alkalinity is removed is called neutralisation. A neutralisation involving an acid and a base (or alkali) always produces salt and water.
acid + base → salt + water
In all solution, all acids contain hydrogen ions, H+. The greater the concentration of these hydrogen ions, the lower the pH.
The name of the salt produced in a neutralisation reaction can be predicted. The first part of the name is ‘ammonium’ if the base used is ammonia. Otherwise, it is the name of the metal in the base. The second part of the name comes from the acid used:
The table shows some examples.
|acid||+||base||→||salt + water|
|hydrochloric acid||+||copper oxide||→||copper chloride + water|
|sulfuric acid||+||sodium hydroxide||→||sodium sulfate + water|
|nitric acid||+||calcium hydroxide||→||calcium nitrate + water|
|phosphoric acid||+||iron(III) oxide||→||iron(III) phosphate + water|
Carbonates also neutralise acids. As well as a salt and water, carbon dioxide is also produced. The name of the salt can be predicted in just the same way.
hydrochloric acid + potassium carbonate → potassium chloride + water + carbon dioxide
acids in solution contain hydrogen ions, H+.
alkalis in solution contain hydroxide ions, OH-.
Neutralisation can be written as an ionic equation:
H+ + OH-H2O
You need to be able to write balanced symbol equations for neutralisation reactions between acids and bases, and between acids and carbonates. The equations will involve these acids:
The equations will involve these bases:
They will involve these carbonates:
sulfuric acid + potassium hydroxide → potassium sulfate + water
H2SO4 + 2KOH → K2SO4 + 2H2O
nitric acid + calcium carbonate → calcium nitrate + water + carbon dioxide
2HNO3 + CaCO3 → Ca(NO3)2 + H2O + CO2
hydrochloric acid + ammonium hydroxide → ammonium chloride + water
HCl + NH4OH → NH4Cl + H2