Common Bases and Their Formulae

Introduction

If acids are defined by the hydrogen ions they release, bases are defined by what they do to those ions: they neutralise them. Bases come in several families — metal oxides, metal hydroxides, metal carbonates and ammonia. Some dissolve in water and some do not. Learning their formulae, and which of them are alkalis, lets you predict the products of almost any neutralisation you meet.

Core explanation

Base or alkali? A base is any substance that reacts with an acid to form a salt and water. An alkali is a base that is soluble in water and produces hydroxide ions, OH⁻(aq). So all alkalis are bases, but not all bases are alkalis. Copper(II) oxide neutralises acids, so it is a base, but it is insoluble, so it is not an alkali.

Metal oxides. Oxides of metals are typically basic. Examples:

- Copper(II) oxide, CuO — black, insoluble - Magnesium oxide, MgO — white, only very slightly soluble - Calcium oxide, CaO — "quicklime", reacts with water to form calcium hydroxide - Iron(III) oxide, Fe₂O₃ — the main component of rust

The oxide ion, O²⁻, combines with two H⁺ ions to form water.

Metal hydroxides. These contain OH⁻ ions.

- Sodium hydroxide, NaOH, and potassium hydroxide, KOH — very soluble, strong alkalis - Calcium hydroxide, Ca(OH)₂ — slightly soluble; its solution is limewater - Copper(II) hydroxide, Cu(OH)₂, and iron(III) hydroxide, Fe(OH)₃ — insoluble bases

The Group 1 hydroxides are the classic laboratory alkalis.

Metal carbonates. These contain the CO₃²⁻ ion and neutralise acids while releasing carbon dioxide:

- Calcium carbonate, CaCO₃ — limestone, chalk and marble; insoluble - Sodium carbonate, Na₂CO₃ — "washing soda"; soluble and alkaline in solution - Copper(II) carbonate, CuCO₃ — green, insoluble

Hydrogencarbonates such as sodium hydrogencarbonate, NaHCO₃, behave similarly.

Ammonia, NH₃. Ammonia is a gas that is very soluble in water. It is not a hydroxide, but it reacts with water to produce some OH⁻ ions:

NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)

Only a small fraction reacts, so ammonia solution is a weak alkali , with a pH of about 11 for a typical laboratory solution. Its salts contain the ammonium ion, NH₄⁺.

Base Formula Soluble? Alkali? --- --- --- --- Sodium hydroxide NaOH Yes Yes (strong) Calcium hydroxide Ca(OH)₂ Slightly Yes Copper(II) oxide CuO No No Calcium carbonate CaCO₃ No No Ammonia NH₃ Yes Yes (weak)

Step-by-step reasoning

To decide whether a substance is an alkali:

1. Check whether it neutralises acids. If not, it is not a base at all. 2. Check whether it is soluble in water. 3. If it is soluble and gives OH⁻(aq) ions, it is an alkali. 4. If it is insoluble, it is a base but not an alkali.

Visual explanation

Draw a large circle labelled "bases" and a smaller circle inside it labelled "alkalis". CuO, CaCO₃ and Cu(OH)₂ sit in the large circle only; NaOH, KOH, Ca(OH)₂ and NH₃ sit inside the smaller circle. The picture shows that every alkali is a base.

Real-world analogy

"Base" and "alkali" are like "vehicle" and "bicycle". Every bicycle is a vehicle, but a lorry is a vehicle that is not a bicycle. Every alkali is a base, but copper(II) oxide is a base that is not an alkali.

Real-world example

Farmers spread powdered limestone (CaCO₃) or slaked lime (Ca(OH)₂) on acidic fields; indigestion tablets contain magnesium hydroxide or calcium carbonate; household cleaners often contain ammonia or sodium hydroxide. Each uses a different base suited to the job.

Why?

Why are most metal oxides basic? The oxide ion, O²⁻, has a strong attraction for H⁺ ions. When an oxide meets an acid, O²⁻ picks up two H⁺ ions to form water, removing acidity. Any substance that removes H⁺ in this way is acting as a base.

Common misconception

"Alkali and base mean exactly the same thing." An alkali is only the soluble kind of base. Insoluble oxides, hydroxides and carbonates still neutralise acids, so they are bases, but they cannot form alkaline solutions.

Worked example

Question: Classify magnesium hydroxide, Mg(OH)₂, which neutralises stomach acid but is almost insoluble in water.

Reasoning: It neutralises an acid, so it is a base. It barely dissolves, so it produces very few OH⁻(aq) ions in water.

Answer: It is a base; it is not usually classed as an alkali because it is almost insoluble.

Quick check

1. Why is ammonia solution called a weak alkali? Answer: Only a small fraction of NH₃ molecules react with water to form OH⁻ ions.

Exam focus

Be ready to define both "base" and "alkali" precisely and to give an example of a base that is not an alkali. Learn the formulae with brackets: Ca(OH)₂, not CaOH₂. Remember that ammonium, NH₄⁺, is the ion in ammonia's salts.

Advanced insight

A broader definition, due to Brønsted and Lowry, calls any proton (H⁺) acceptor a base. On this view, ammonia is a base because it accepts H⁺ from water to become NH₄⁺, and the oxide, hydroxide and carbonate ions are all bases because each accepts H⁺. This definition works even when no water is present.

Summary

Bases neutralise acids to form salts and water; alkalis are soluble bases that release OH⁻(aq). Common bases include metal oxides (CuO, MgO, CaO), metal hydroxides (NaOH, KOH, Ca(OH)₂), metal carbonates (CaCO₃, Na₂CO₃) and ammonia (NH₃), a weak alkali whose salts contain NH₄⁺.

Practice questions

1. Give one example of a base that is not an alkali, and explain why. Answer: Copper(II) oxide, CuO; it neutralises acids but is insoluble in water, so it cannot form OH⁻(aq) ions in solution. 2. Write the formula of calcium hydroxide. Answer: Ca(OH)₂. 3. Write the equation that shows why ammonia solution is alkaline. Answer: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq). 4. Which ion is present in all carbonates, and what gas do carbonates release with acids? Answer: The carbonate ion, CO₃²⁻; they release carbon dioxide.