Acids are chemical compounds that when dissolved in water, create a solution with hydrogen ion activity greater than in pure water; less than pH7. Alkalis are an ionic salt of an alkali metal that dissolve to give a pH greater than 7.
As the most common molecule in the universe, water (H2O) is made up of one atom of oxygen and two atoms of hydrogen.
Water molecules are polarised, meaning that the oxygen atom pulls on the hydrogen atoms leaving them with a more positive charge, whilst the oxygen becomes more negative. This polarity allows water to easily dissolve other polarised molecules into ions. Water itself is constantly dissociating into H+(hydrogen or ‘proton’) and OH- (hydroxide) ions.
Acids are compounds which donate hydrogen ions in solution, free-ing up hydrogen gas as they attack metals.
Alkalis (or bases) are compounds that accept hydrogen ions and include such compounds as metal oxides, hyrdoxides and amines.
Alkalis and acids combine to form a salt and water.
HCl + NaOH = NaCl + H2O
Hydrochloric acid + Sodium hydroxide = Sodium chloride (common salt) + Water