Mr. Robert Zammit M.Sc.

Unit 4 – Water - Lesson 3 of 4 Page 1 of 6

Unit 4: Lesson 3 of 4

Water of Crystallization; Reactions of water.

Water of crystallization.

When blue crystals of copper(II) sulfate are heated it turns grey-white and steam is released. This vapour can be condensed on a cold surface in order to show that it is pure water.

The blue copper(II) sulfate is said to be hydrated, because it contains water molecules attached to it.

The water molecules which are chemically bonded into the crystal structure are called water of crystallization.

When hydrated copper(II) sulfate is heated, it loses its water of crystallization to form anhydrous copper(II) sulfate.

CuSO4.5H2O(s) → CuSO4(s) + 5H2O(g) If anhydrous copper(II) sulfate is exposed to water, it will absorb water to

reform the hydrated salt. CuSO4(s) + 5H2O(l) → CuSO4.5H2O(s)

Water of crystallization can be removed by heating the hydrated salt. The action of heat on such salts is a reversible reaction.

CuSO4(s) + 5H2O(g) ⇌ CuSO4.5H2O(s)

Another example of a hydrated salt are cobalt(II) choride, which is pink when hydrated (CoCl2.4H2O) and blue when anhydrous.

Both anhydrous copper(II) sulfate and anhydrous cobalt(II) choride are used to test for the presence of water in any substance. If they change colour and form the hydrated salts when exposed to a particular substance, then that substance must contain water.

hydrated means that the solid crystals contain water of crystallisation.

dehydration is the removal of water of crystallisation.

an anhydrous substance contains no water of crystallisation.

the degree of hydration is the number of moles of water of crystallisation chemically bonded in 1 mole of the compound. The degree of hydration in the example above is 5.

Mr. Robert Zammit M.Sc.

Unit 4 – Water - Lesson 3 of 4 Page 2 of 6

Types of hydrated compounds .

HYGROSCPIC: Substances that readily absorb water from the atmosphere and remain in the same physical state

are called hygroscopic. Substances like anhydrous copper(II) sulfate and anhydrous cobalt(II) chloride readily absorb

water vapour from the atmosphere to form the hydrated crystals and remain solids. Silica gel (found in packets used in packaging of shoes and clothes) is another example.

Substances such as concentrated sulfuric acid also absorbs water readily and remains a liquid.

DELIQUESCENT: Some solid substances absorb so much water from the atmosphere that they form a solution.

These are called deliquescent. Examples are: calcium chloride, sodium hydroxide and potassium hydroxide

EFFLORESCENT: Other substances tend to lose their water of crystallization to form the anhydrous salt. Such

compounds are called efflorescent. Examples are: hydrated calcium sulfate (CaSO4.2H2O) and hydrated sodium carbonate

(Na2CO3.10H2O).

Exercise 1:

1. Hydrated iron(II) sulfate has a blue-green colour. It has the formula FeSO4.7H2O and is also called iron(II) sulfate heptahydrate. When heated, the white anhydrous salt is produced. Write a balanced equation to represent this change.

2. Look at the following reactions, and classify them as Deliquescence, Efflorescence or Hygroscopy.

a. On exposure to air, hydrated calcium sulfate slowly undergoes the following change: CaSO4.2H2O(s) → CaSO4(s) + 2H2O(g)

b. On exposure to air, anhydrous cobalt(II) chloride slowly undergoes the following change: CoCl2(s) + 4H2O(g) → CoCl2.4H2O(s)

c. On exposure to air, anhydrous calcium chloride slowly forms an aqueous solution: CaCl2(s) + nH2O(g) → CaCl2.nH2O(aq)

d. On exposure to air, sodium carbonate decahydrate slowly undergoes the following change: Na2CO3.10H2O(s) → Na2CO3.H2O(s) + 9H2O(g)

Mr. Robert Zammit M.Sc.

Unit 4 – Water - Lesson 3 of 4 Page 3 of 6

Reactions of water with metals.

Potassium is a soft, silvery metal. It quickly tarnishes on exposure to air and is kept under oil. When placed in water, a piece of potassium catches fire and sometimes explodes. It forms hydrogen gas and potassium hydroxide in solution.

2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)

Sodium looks like potassium, but is a bit harder. It can still be cut with a knife and is kept under oil. When put in water it catches fire and gives hydrogen and aqueous sodium hydroxide.

Calcium is not soft like potassium and sodium, however it also tarnishes on exposure to air. It is not kept under oil. Calcium sinks in water at first, then floats after some time and produces hydrogen and calcium hydroxide. Unlike the hydroxides of sodium and potassium, calcium hydroxide is only slightly soluble in water.

Ca(s) + 2H2O(l) → Ca(OH)2(s) + H2(g)

Magnesium looks like calcium. It reacts very slowly with cold water to give hydrogen and magnesium hydroxide which is only slightly soluble. However it burns in steam to give hydrogen gas and ashes of magnesium oxide.

Mg(s) + H2O(g) → MgO(s) + H2(g)

Aluminium seems to be quite unreactive as used in everyday life. This is because aluminium reacts with oxygen to form aluminium oxide. This oxide layer covers aluminium rendering it very unreactive. If this oxide layer is removed (by scratching), aluminium would react with steam to give aluminium oxide and hydrogen.

Zinc and iron also react with steam at very high temperatures in similar ways as aluminium.

Lead, copper, silver, gold and platinum, do not react with water at all.

NB.: It is important that prior to investigating the reactivity of metals with water, any oxide layer that may have formed on the surface of the metal is removed.

Mr. Robert Zammit M.Sc.

Unit 4 – Water - Lesson 3 of 4 Page 4 of 6

Exercise 2:

1. From the following list of substances choose, giving relevant balanced chemical equations where possible, one which: (N.B.: Each substance can be used once, more than once or not at all.)

copper (II) sulfate sodium hydroxide copper calcium

hydrogen aluminium magnesium silver a) burns to produce pure water in the lab. b) is deliquescent. (Reaction not required.) c) is hygroscopic. d) reacts readily with cold water. e) reacts with steam if scratched but not at all with cold water. f) is a metal that does not react with water or steam. (Reaction not required.) g) is formed by the reaction of sodium with water. h) is a metal that reacts very slowly with cold water to form a hydroxide. i) is the metal in (h) and which reacts more readily with steam to give an oxide. j) is a substance that can be used to test for the presence of water. k) is less soluble at higher temperatures.

2. In an experiment, known masses of some compounds are left in air for a period of time. The following changes were observed. Deduce, giving reasons, which of the substances is most likely to be:

Substance Mass Before Mass After Observations

A 10.00g 13.20g Substance turned from white powder to blue crystals

B 10.00g 16.30g Substance turned from white solid to colourless solution

C 10.00g 10.00g n/a

E 10.00g 8.02g n/a a) Calcium chloride b) Calcium sulfate c) Copper(II) sulfate d) Sodium chloride

Mr. Robert Zammit M.Sc.

Unit 4 – Water - Lesson 3 of 4 Page 5 of 6

Reactions of water with non-metals.

Chlorine acts on water to form a mixture of hydrochloric acid and chloric acid (HOCl). The latter has bleaching properties and can bleach a litmus paper to white.

Cl2(aq) + H2O(l) → HCl(aq) + HOCl(aq) Carbon reacts with superheated steam at high temperatures, forming carbon monoxide and hydrogen.

The reaction is reversible. C(s) + H2O(g) ⇌ CO(g) + H2(g)

Reactions of water with oxides.

Some metal basic oxides such as K2O, Na2O and CaO form hydroxides which are soluble in water (KOH, NaOH & Ca(OH)2 – the latter is only slightly soluble), and together with ammonium hydroxide (NH4OH) constitute the common alkalis. Example:

K2O(s) + H2O(l) → 2KOH(aq) Some non-metal acidic oxides, mostly gases, dissolve in water to give acidic solutions. Examples:

SO2(g) + H2O(l) → H2SO3(aq) Sulphurous acid CO2(g) + H2O(l) ⇌ H2CO3(aq) Carbonic acid

By the end of this lesson you should be able to:

Understand that, for some salts, the crystalline shape (and in some cases even the colour), is dependent on water combined in the crystals.

Distinguish between the terms deliquescence, hygroscopy and efflorescence, using suitable examples.

Recall a chemical test for the presence of water. Describe the reactions of specific metals and non-metals with water or steam. Recall that metal oxides which dissolve in water produce alkaline solutions. Recall that non-metal oxides which dissolve in water produce acid solutions. Describe the reactions of common basic oxides and acidic oxides with water.

Mr. Robert Zammit M.Sc.

Unit 4 – Water - Lesson 3 of 4 Page 6 of 6

Model answers to Exercise 1:

1. FeSO4.7H2O(s) → FeSO4(s) + 7H2O(g)

2. a) Efflorescence b) Hygroscopy c) Deliquescence d) Efflorescence

Model answers to Exercise 2:

1. a) hydrogen 2H2(g) + O2(g) → 2H2O(l)

b) sodium hydroxide

c) copper(II) sulfate CuSO4(s) + 5H2O(g) ⇌ CuSO4.5H2O(s)

d) calcium Ca(s) + 2H2O(l) → Ca(OH)2(s) + H2(g)

e) aluminium 2Al(s) + 3H2O(g) → Al2O3(s) + 3H2(g)

f) copper or silver

g) sodium hydroxide 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)

h) magnesium Mg(s) + 2H2O(l) → Mg(OH)2(s) + H2(g)

i) magnesium Mg(s) + H2O(g) → MgO(s) + H2(g)

j) copper(II) sulfate CuSO4(s) + 5H2O(g) → CuSO4.5H2O(s)

k) hydrogen

2. a) Calcium chloride – substance B.

Reason: It gains mass by absorbing water from air and dissolves in it - deliquescent. b) Calcium sulfate – substance E. Reason: It loses mass on exposure to air (loses water of crystallization) - efflorescent. c) Copper(II) sulfate – substance A. Reason: It gains mass by absorbing water from air and stays solid - hygroscopic. It changes from white (anhydrous) to blue (hydrated). d) Sodium chloride – substance C. Reason: It is not a hydrate – does not have water of crystallization.