An alkali is a base that dissolves in water. Soluble hydroxides such as sodium hydroxide, NaOH, and potassium hydroxide, KOH, are alkalis. They release hydroxide ions, OH-(aq), in water, and alkaline solutions have a pH greater than 7.

How does ammonia act as a base?

Ammonia gas dissolves in water to make an alkaline solution, which reacts with acids to make ammonium salts. Ammonia, NH3, is used to make ammonium salts, important as fertilisers. For example NH3(aq) + HNO3(aq) - NH4NO3(aq) gives ammonium nitrate, a common nitrogen fertiliser.

Alkalis & Bases Notes

Oxford AQA IGCSE Chemistry: Revision notes

Key facts

A base neutralises an acid to make a salt and water. Metal oxides and metal hydroxides are bases.
An alkali is a base that dissolves in water, releasing $\ce{OH-(aq)}$ ions.
All alkalis are bases, but not all bases are alkalis.
Ammonia dissolves in water to make an alkaline solution used to make ammonium salts.
Limewater is calcium hydroxide solution; it turns cloudy with carbon dioxide.

Bases

A base is a substance that neutralises an acid, forming a salt and water.

Metal oxides (copper oxide, magnesium oxide) and metal hydroxides (sodium hydroxide, calcium hydroxide) are both bases. Most bases are solids that do not dissolve in water.

1
Base
for example a metal oxide, hydroxide or carbonate
2
Acid
is neutralised by the base
3
Salt and water
are formed

A base neutralises an acid

Base + acid $\to$ salt + water

Which of these is a base?

Alkalis

An alkali is a base that dissolves in water.

Soluble hydroxides such as sodium hydroxide, $\ce{NaOH}$ , and potassium hydroxide, $\ce{KOH}$ , are alkalis. They release hydroxide ions, $\ce{OH-(aq)}$ , in water, and alkaline solutions have a pH greater than 7.

Sodium hydroxide Copper oxide Magnesium oxide Base? Yes Yes Yes Alkali? Yes No No

	Sodium hydroxide	Copper oxide	Magnesium oxide
Base?	Yes	Yes	Yes
Alkali?	Yes	No	No

Sodium hydroxide

Base?:: Yes
Alkali?:: Yes

Copper oxide

Base?:: Yes
Alkali?:: No

Magnesium oxide

Base?:: Yes
Alkali?:: No

Which statement is correct?

Ammonia as a base

Ammonia gas dissolves in water to make an alkaline solution, which reacts with acids to make ammonium salts.

Ammonia, $\ce{NH3}$ , is used to make ammonium salts, important as fertilisers. For example $\ce{NH3(aq) + HNO3(aq) -> NH4NO3(aq)}$ gives ammonium nitrate, a common nitrogen fertiliser.

1
Dissolves in water
ammonia gives an alkaline solution
2
Reacts with an acid
neutralisation
3
Ammonium salt
for example ammonium nitrate, a fertiliser

Ammonia as a base

Ammonia reacts with sulfuric acid. What type of salt forms?

Limewater and carbon dioxide

Carbon dioxide reacts with limewater to make insoluble calcium carbonate, which turns it cloudy.

Limewater is a solution of calcium hydroxide. It is weakly soluble, so it is an alkali. When carbon dioxide is bubbled through it, insoluble calcium carbonate forms and the solution turns cloudy (milky white). This is the standard test for carbon dioxide.

1
Bubble the gas through limewater
calcium hydroxide solution, which is colourless
2
Carbon dioxide reacts
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
3
Insoluble calcium carbonate forms
the limewater turns cloudy

Limewater test for carbon dioxide

Calcium hydroxide + carbon dioxide $\to$ calcium carbonate + water

A gas is bubbled through limewater and the limewater turns cloudy. Which gas is it?

Try an exam question

State the difference between a base and an alkali, and describe the test for carbon dioxide using limewater.

[3 marks]

That's the notes covered.

Carry on to the next subtopic.