C8 (8.1~8.4)

Question 1

Describe neutrality and relative acidity and alkalinity in terms of pH

Answer 1

You can use litmus paper or universal indicator to check a substance’s pH.

Neutral: pH 7, turns UI green, turns litmus paper purple

Acid: low pH, turns UI red-yellow, turns litmus paper red

Alkali: high pH, turns UI blue-purple, turns litmus paper blue

Question 2

Describe the characteristic properties of acids (exemplified by dilute hydrochloric acid and dilute sulfuric acid) including their effect on litmus paper and their reactions with metals,
bases and carbonates

Answer 2

Acid + Alkali -> Salt + Water
Acid + Metal -> Salt + Hydrogen
Acid + Carbonate -> Salt + Water + Carbon Dioxide
Acid + Oxides (bases) -> Salt + Water

Question 3

Describe the characteristic properties of bases including their effect on litmus paper and their reactions with acids and ammonium salts

Answer 3

Bases have pH values above 7.

A base which is water soluble is referred to as an alkali.

In basic (alkaline) conditions, red litmus paper turns blue and methyl orange indicator turns yellow.

Bases are substances which can neutralise an acid, forming a salt and water.

Bases are usually oxides or hydroxides of metals.

When alkalis react, they gain electrons to form negative hydroxide ions (OH-).
The presence of the OH- ions is what makes the aqueous solution an alkali.
When they react with an acid, a neutralisation reaction occurs.
Acids and bases react together in a neutralisation reaction and produce a salt and water: Acid + Base → Salt + Water
Ammonium salts undergo decomposition when warmed with an alkali.
Even though ammonia is itself a weak base, it is very volatile and can easily by displaced from the salt by another alkali.
A salt, water and ammonia are produced.