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7.1.2 Bases, alkalis and their properties

Bases and alkalis

Definition

Base

A metal oxide or metal hydroxide that reacts with an acid to form a salt and water.

Definition

Alkali

A base that dissolves in water.

  1. A base is an oxide or hydroxide of a metal.
  2. Metal oxides such as copper(II) oxide, CuO\text{CuO}CuO, magnesium oxide, MgO\text{MgO}MgO, and calcium oxide, CaO\text{CaO}CaO, are bases.
  3. Metal hydroxides such as sodium hydroxide, NaOH\text{NaOH}NaOH, potassium hydroxide, KOH\text{KOH}KOH, and calcium hydroxide, Ca(OH)2\text{Ca(OH)}_2Ca(OH)2​, are also bases.
  4. Most bases are insoluble in water, so they stay as a solid when water is added.
  5. A base that does dissolve in water is called an alkali, and its solution is described as alkaline.
  6. Sodium hydroxide and potassium hydroxide dissolve readily, so they are alkalis.
  7. Calcium hydroxide dissolves only slightly, and its solution in water is known as limewater.
  8. Copper(II) oxide and magnesium oxide do not dissolve, so they are bases but not alkalis.
  9. Every alkali is therefore a base, but most bases are not alkalis.
Example

Sorting bases and alkalis

  • Copper(II) oxide is a metal oxide, so it is a base, and it is insoluble, so it is not an alkali.
  • Potassium hydroxide is a metal hydroxide, so it is a base, and it dissolves in water, so it is also an alkali.
  • Carbon dioxide is the oxide of a non-metal, so it is not a base at all.

Bases and acids

  1. A base reacts with an acid to form a salt and water.

    base+acid→salt+water \text{base} + \text{acid} \rightarrow \text{salt} + \text{water} base+acid→salt+water

  2. The metal in the base becomes the first part of the salt's name, while the acid decides the second part.

  3. An insoluble base such as copper(II) oxide reacts as a solid, so you see the solid disappear and the solution change colour.

  4. An alkali such as sodium hydroxide reacts in solution, and the mixture warms up because the reaction gives out thermal energy.

  5. Adding enough base uses up all of the acid, so the mixture stops behaving as an acid.

  6. This type of reaction between an acid and a base is called a neutralisation reaction.

Example

Three bases reacting with acids

  • Magnesium oxide and hydrochloric acid give magnesium chloride and water.
MgO(s)+2HCl(aq)→MgCl2(aq)+H2O(l) \text{MgO(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{O(l)} MgO(s)+2HCl(aq)→MgCl2​(aq)+H2​O(l)
  • Potassium hydroxide and nitric acid give potassium nitrate and water.
KOH(aq)+HNO3(aq)→KNO3(aq)+H2O(l) \text{KOH(aq)} + \text{HNO}_3\text{(aq)} \rightarrow \text{KNO}_3\text{(aq)} + \text{H}_2\text{O(l)} KOH(aq)+HNO3​(aq)→KNO3​(aq)+H2​O(l)
  • Calcium hydroxide and sulfuric acid give calcium sulfate and water, and two water molecules are needed to balance the two hydroxide groups.
Ca(OH)2(aq)+H2SO4(aq)→CaSO4(s)+2H2O(l) \text{Ca(OH)}_2\text{(aq)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{CaSO}_4\text{(s)} + 2\text{H}_2\text{O(l)} Ca(OH)2​(aq)+H2​SO4​(aq)→CaSO4​(s)+2H2​O(l)

Bases and ammonium salts

Definition

Ammonium salt

A salt in which the positive ion is the ammonium ion, NH4+\text{NH}_4^{+}NH4+​, instead of a metal ion.

  1. An ammonium salt contains the ammonium ion, NH4+\text{NH}_4^{+}NH4+​, in place of a metal ion.

  2. Common examples are ammonium chloride, NH4Cl\text{NH}_4\text{Cl}NH4​Cl, ammonium sulfate, (NH4)2SO4(\text{NH}_4)_2\text{SO}_4(NH4​)2​SO4​, and ammonium nitrate, NH4NO3\text{NH}_4\text{NO}_3NH4​NO3​.

  3. When an ammonium salt is warmed with a base, the products are a salt, ammonia and water.

    ammonium salt+base→salt+ammonia+water \text{ammonium salt} + \text{base} \rightarrow \text{salt} + \text{ammonia} + \text{water} ammonium salt+base→salt+ammonia+water

  4. Ammonia, NH3\text{NH}_3NH3​, is a colourless gas with a sharp, choking smell.

  5. Ammonia is very soluble in water, so warming is needed to drive the gas out of the mixture.

  6. Ammonia is an alkaline gas, so it turns damp red litmus paper blue.

  7. The litmus paper must be damp, because ammonia has to dissolve in water before it can act as an alkali.

  8. This reaction explains why farmers do not spread lime, which contains calcium hydroxide, at the same time as an ammonium fertiliser, because the nitrogen would be lost as ammonia gas.

Example

Ammonium salts warmed with bases

  • Ammonium chloride and sodium hydroxide give sodium chloride, ammonia and water.
NH4Cl(s)+NaOH(aq)→NaCl(aq)+NH3(g)+H2O(l) \text{NH}_4\text{Cl(s)} + \text{NaOH(aq)} \rightarrow \text{NaCl(aq)} + \text{NH}_3\text{(g)} + \text{H}_2\text{O(l)} NH4​Cl(s)+NaOH(aq)→NaCl(aq)+NH3​(g)+H2​O(l)
  • Ammonium sulfate and calcium hydroxide give calcium sulfate, ammonia and water.
(NH4)2SO4(s)+Ca(OH)2(s)→CaSO4(s)+2NH3(g)+2H2O(l) (\text{NH}_4)_2\text{SO}_4\text{(s)} + \text{Ca(OH)}_2\text{(s)} \rightarrow \text{CaSO}_4\text{(s)} + 2\text{NH}_3\text{(g)} + 2\text{H}_2\text{O(l)} (NH4​)2​SO4​(s)+Ca(OH)2​(s)→CaSO4​(s)+2NH3​(g)+2H2​O(l)
  • The ammonium salt has two NH4+\text{NH}_4^{+}NH4+​ ions, so two NH3\text{NH}_3NH3​ molecules and two water molecules form.
Practical

Warming an ammonium salt with an alkali

  • Aim: to show that ammonia gas is given off when an ammonium salt is warmed with an alkali.
  • Apparatus: test tube, test-tube holder, spatula, ammonium chloride, dilute sodium hydroxide solution, 10 cm310\ \text{cm}^310 cm3 measuring cylinder, red litmus paper, wash bottle of distilled water, Bunsen burner, heat-resistant mat and eye protection.
  • Method:
    • Place one small spatula measure of ammonium chloride in a dry test tube.
    • Add 2 cm32\ \text{cm}^32 cm3 of dilute sodium hydroxide solution.
    • Wet a strip of red litmus paper with a few drops of distilled water.
    • Hold the tube in a test-tube holder at an angle, with the mouth pointing away from everyone, and warm it gently in a small Bunsen flame.
    • Hold the damp litmus paper just above the mouth of the tube without touching the glass, because alkali on the rim would turn it blue even without any gas.
  • Results: the damp red litmus paper turns blue and a sharp smell may be noticed near the tube.
  • Conclusion: ammonia gas is released, which shows that a base reacts with an ammonium salt on warming.
  • Safety: wear eye protection, warm the mixture gently so it does not spit, work in a well-ventilated room, and do not sniff the gas directly from the tube.

Alkalis and indicators

  1. Alkalis change indicator colours in the opposite direction to acids.
  2. Litmus
    1. An alkali turns red litmus paper blue.
    2. Blue litmus paper stays blue in an alkali.
  3. Methyl orange
    1. Methyl orange is yellow in an alkaline solution.
    2. A few drops added to an alkali turn the whole solution yellow.
  4. Thymolphthalein
    1. Thymolphthalein is blue in an alkaline solution.
    2. It is colourless in acidic and neutral solutions, so a blue colour is a clear sign that the solution is alkaline.
  5. An insoluble base such as copper(II) oxide does not change the colour of an indicator, because it does not dissolve to form an alkaline solution.
Common Mistake
  • Do not call every base an alkali, because only bases that dissolve in water are alkalis.
  • Do not say that the gas from an ammonium salt and a base is hydrogen, because the gas is ammonia.
  • Do not test for ammonia with dry litmus paper, because the gas must dissolve in water on the paper first.
Exam technique

Answering indicator questions

  • Give the final colour of the indicator, such as "thymolphthalein turns blue", not just "it changes colour".
  • For litmus, state both colours, such as "red litmus turns blue".
  • Remember that methyl orange is yellow in an alkali, not orange.
Self review
  • What is the difference between a base and an alkali?
  • Name one base that is not an alkali.
  • What are the products when an ammonium salt is warmed with a base?
  • Why must litmus paper be damp when testing for ammonia?
  • What colours are red litmus, methyl orange and thymolphthalein in an alkaline solution?
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7.1.2 Bases, alkalis and their properties Revision Guide

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