Summary Grade 9: NS: Revision

Testing acidity & basicity – using pH value
• If we want to know whether something is an acid or a base, we can measure
its pH:
– Acids have pH values below 7. The lower the pH value, the more acidic
the substance.
– Bases have pH values above 7. The higher the pH value, the more basic
the substance.
– Neutral substances have a pH equal to 7.

Acids
When soluble non-metal oxides such asCo2, So2, No 2, p2o5 react with water they
can form acidic solutions such as:
Co 2 + H 2 O  H 2 Co3 (carbonic acid)
So2 + H 2 O  H 2 So3 (sulphurous acid)
P2 O5 + H 2 O  H 3 Po 4 (phosphoric acid)
Bases
• Metal oxides are bases.
• When soluble metal oxides such as MgO, ZnO, CaO react with water they can
form hydroxide solutions.
Mgo + H 2 O  mg (oh)2 (magnesium hydroxide)
Zno + H 2 O  zn(oh)2 (zinc hydroxide)
Cao + H 2 O  ca (oh)2 (calcium hydroxide)
– When hydroxides dissolve in water, they are called alkali solutions.
– When a metal hydroxide reacts with carbon dioxide a metal carbonate is
produced.
– When carbonic acid reacts with a metal, a metal carbonate is produced.
Common acids and bases