Bases and Alkalis

The oxides and hydroxides of metals are called bases. Sodium oxide, sodium hydroxide, potassium oxide, potassium hydroxide and magnesium oxide are all examples of bases. Every one of these compounds begins with a metal. When oxygen joins that metal the compound is a metal oxide, and when a hydroxide group joins it the compound is a metal hydroxide. So the first question to ask about any compound is whether a metal is present in it, because a base always carries one.

• Sodium oxide \( \mathrm{Na_2O} \)
• Sodium hydroxide \( \mathrm{NaOH} \)
• Potassium oxide \( \mathrm{K_2O} \)
• Potassium hydroxide \( \mathrm{KOH} \)
• Magnesium oxide \( \mathrm{MgO} \)

Not every metal oxide dissolves in water. Some dissolve readily and disappear into the water, while others simply sink and stay at the bottom of the vessel. This single difference, dissolving or not dissolving, splits bases into two groups that behave quite differently in the laboratory.

A base that dissolves in water is called an alkali. When an alkali dissolves it releases hydroxide ions into the water. Sodium hydroxide and potassium hydroxide are alkalis, because both dissolve and both fill the solution with hydroxide ions. Magnesium oxide, on the other hand, does not dissolve in water, so it is a base but it is not an alkali.

Alkalis are divided into strong and weak according to the number of hydroxide ions they release in water. A strong alkali gives a large number of hydroxide ions when it dissolves, while a weak alkali gives only a few. Sodium hydroxide and potassium hydroxide are strong alkalis, and ammonium hydroxide is a weak alkali.

You do not need a laboratory chemical to meet the physical properties of a base. Wood ash stirred into water gives a solution that is itself alkaline, and that solution shows every property a base has. Make ash water in the order below and watch what happens at each stage.

• Put a little wood ash in a glass, add water and stir it well.
• Let the mixture settle for a while, then pour the clear water on top into a second glass.
• Rub a drop of that ash water between your fingers.
• Touch a very small amount to the tip of your tongue and note the taste.
• Dip a strip of red litmus paper into the ash water.
• Now dip a strip of blue litmus paper into the same water.
• Add two drops of methyl orange to a little of the ash water.
• Add two drops of phenolphthalein to another small portion of the ash water.

Comparing what you saw at each stage with what your classmates saw gives the full list of the physical properties of a base.

• A base turns red litmus paper blue.
• Blue litmus paper stays blue in a base.
• Most bases taste bitter and astringent.
• A base feels slippery, like soap solution.
• A base turns methyl orange yellow.
• A base turns phenolphthalein pink.

(a) Ionisation in water. A base dissolving in water ionises and gives hydroxide ions. Sodium hydroxide dissolving in water separates into sodium ions and hydroxide ions:

\[ \mathrm{NaOH} \rightleftharpoons \mathrm{Na}^{+} + \mathrm{OH}^{-} \]

That hydroxide ion is the signature of a base. Every colour change listed above happens because these ions are present in the solution.

(b) Reaction with an ammonium salt. A base reacts with an ammonium salt and produces ammonia gas. When sodium hydroxide is added to ammonium sulphate, the products are sodium sulphate, ammonia and water:

\[ (\mathrm{NH_4})_2\mathrm{SO_4} + 2\mathrm{NaOH} \longrightarrow \]

\[ \mathrm{Na_2SO_4} + 2\mathrm{NH_3} + \mathrm{H_2O} \]

Whether ammonia has actually been made is settled with a strip of moist red litmus paper. Work through the steps below and test it yourself.

• Take a little dilute ammonium sulphate in a dry test tube.
• Add a little sodium hydroxide to it.
• Watch carefully what happens inside the test tube.
• Hold a moist red litmus paper close to the mouth of the test tube and look at its colour.

If the moist red litmus turns blue, ammonia gas is certainly coming out of the tube, because ammonia dissolving in the water on the paper makes an alkaline solution.

(c) Reaction with an acid. A base reacts with an acid and gives a salt and water. This reaction is also called a neutralisation reaction. When potassium hydroxide is added to sulphuric acid, potassium sulphate and water are formed:

\[ \mathrm{H_2SO_4} + \mathrm{KOH} \rightarrow \mathrm{K_2SO_4} + \mathrm{H_2O} \]

In a neutralisation both properties disappear at once: the acid loses its acidity and the base loses its alkalinity. Dipping litmus paper into the mixture afterwards shows no colour change at all, and that is the proof.

Bases are not shut inside a laboratory. The cement a house is built with, the soap used for washing, the paper written on and the fertiliser spread on a field are all made with the help of a base. The common bases and their uses are set out below.

Calcium hydroxide lowering the acidity of soil is itself a neutralisation. Lime spread over sour soil cancels the acid in it, and the crop then grows well. A dose of magnesium hydroxide taken for gastritis does exactly the same job inside the stomach, cancelling the extra acid that caused the burning.