1. Acids turn indicators characteristic colours (this depends on the indicator) representing.
2. Acids react with bases to form salt and water (neutralisation). The base may be a metal oxide or a metal hydroxide.
3. Acids react with carbonates forming a salt, carbon dioxide and water (neutralisation)
4. Acids react with active metals to form salt and hydrogen (this is not actually a neutralisation reaction although the acid does get used up)

These properties are only expressed in aqueous solution (i.e. a solution in water). The reason for this is that acids can only release their hydrogen ions when they interact with water molecules.

hydrochloric acid	+	water		hydrogen ions	+	chloride ions
HCl		(aq)		H+(aq)		Cl-(aq)

Consequently, hydrochloric acid provides a solution that contains hydrogen ions and chloride ions - It is only the hydrogen ions that react with bases the chloride ions remain in the solution as spectator ions

Other common strong acids:

• Sulphuric acid - the solution contains hydrogen ions and sulphate ions
• Nitric acid - the solution contains hydrogen ions and nitrate ions
• Phosphoric acid - the solution contains hydrogen ions and phosphate ions

In all cases the acid provides a source of hydrogen ions

Bases

These are the chemical opposites of acids. The property of bases is caused by the presence of OH- (hydroxide) ions in solution.

• Metal oxides
• Metal hydroxides
• Metal carbonates
• Metal hydrogen carbonates

The bases either produce these hydroxide ions directly by dissolving or they assist the water molecules in breaking up by removing H+ ions from the water. (The situation is a little more complicated than explained as the water is actually in equilibrium with its own ions)

1. The effect of acids and bases on indicators

The colour change depends on the type of indicator used. The most common indicators are universal indicator, litmus indicator and phenolphthalein. The colour changes are:

The degree of acidity or basicity of a solution can be measured by the pH scale. This gives a value proportional to the concentration of H+ ions in the solution

2. Reaction with bases (neutralisation)

Before going over these reactions it is important to understand that bases are the chemical opposites of acids. This means that they contain ions that can neutralise (react with and cancel out) the H+ ions of the acids.

The following types of compounds are classified as bases:

Example Neutralisation reactions

sulphuric acid	+	magnesium oxide		magnesium sulphate	+	water
H2SO4		MgO		MgSO4		H2O
hydrochloric acid	+	calcium oxide		calcium chloride	+	water
2HCl		CaO		CaCl2		H2O
sulphuric acid	+	sodium hydroxide		sodium sulphate	+	water
H2SO4		2NaOH		Na2SO4		2H2O
hydrochloric acid	+	calcium hydroxide		calcium chloride	+	water
2HCl		Ca(OH)2		CaCl2		2H2O
sulphuric acid	+	zinc carbonate		zinc sulphate	+	water	+	carbon dioxide
H2SO4		Zn CO3		ZnSO4		H2O		CO2
hydrochloric acid	+	potassium carbonate		potassium chloride	+	water	+	carbon dioxide
2HCl		K2CO3		2KCl		H2O		CO2
hydrochloric acid	+	sodium hydrogen carbonate		sodium chloride	+	water	+	carbon dioxide
HCl		NaHCO3		NaCl		H2O		CO2

Note: Sulphuric acid always makes salts called sulphates, hydrochloric acid always makes salts called chlorides ( nitric acid makes nitrates, ethanoic acid makes ethanoates - etc.)

Ammonia as a weak base

Ammonia has the molecular formula NH₃. It is a pungent (sharp) smelling gas that is used in household cleaners. It is very soluble in water giving a solution of about pH 10.

Ammonia gas dissoves in water to give a solution that behaves as a base in that it neutralises acids producing a salt and water. The reason for this is that the ammonia molecule partially removes some of the hydrogen ions from water leaving the solution with an excess of OH´(hydroxide ) ions. These ions can then react with acids.

ammonia	+	water		ammonium ions	+	hydroxide ions
NH3		H2O		NH4+		OH-

Examples of ammonia behaving as a base

sulphuric acid	+	ammonia		ammonium sulphate
H2SO4		2NH3		(NH4)2SO4
hydrochloric acid	+	ammonia		ammonium chloride
HCl		NH3		NH4Cl
nitric acid	+	ammonia		ammonium nitrate
HNO3		NH3		NH4NO3
ethanoic acid	+	ammonia		ammonium ethanoate
CH3COOH		NH3		CH3COONH4

Note: In this case there is no water formed in the reaction. Some textbooks refer to a solution of ammonia in water as 'ammonium hydroxide' solution, NH₄OH, (the ions on the right hand side of the equation when it dissolves in water) in which case there will be water formed when it reacts with acids.

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