C4 : Acids and Alkalis

Question 1

Alkalis

Answer 1

Alkalis are substances that form hydroxide (OH-) ions when they dissolve in water (aqueous solutions).
An alkali is a type of base.

Question 2

Acids

Answer 2

Acids are substances that form hydrogen (H+) ions when they dissolve in water (aqueous solutions).

Question 3

Measuring using a pH scale

Answer 3

he pH scale is used to measure how acidic or alkaline a substance is.

Question 4

Oxidation

Answer 4

An oxidation reaction involves gaining oxygen.
Oxidation reactions can also involve losing electrons.
E.g. when metals react with oxygen, metal oxides are produced:
Calcium + oxygen → calcium oxide
2Ca + O2 → 2CaO

Question 5

Reduction

Answer 5

A reduction reaction involves losing oxygen.
Reduction reactions can also involve gaining electrons.
E.g. when metal oxides lose oxygen and return to their atomic form:
Iron oxide + carbon monoxide → iron + carbon dioxide
Fe2O3 + 3CO → 2Fe + 3CO2

Question 6

What happens to a substance when it is oxidised

Answer 6

When a substance is oxidised (gains oxygen), it always loses electrons.

Question 7

What happens to a substance when it is reduced

Answer 7

When a substance is reduced (loses oxygen), it always gains electrons.