Page 651
- 33 -
W A T E R
78. OCCURRENCE. Water is present in nature, and it occupies about 3/4 of
the earth's surface. It is also present in many salts and animal and
vegetable bodies. For example, lean meat is about 60% water, while
tomatoes are about 95% water. The human body contains about 70%
water.
79. FORMATION. Being abundant in nature, water is not usually prepared
from other materials. However, it is a product of many chemical re-
actions some of which are:
1. Direct union of oxygen and hydrogen by an electric spark.
2 H₂ + O ⟦line⟧> 2 H₂O.
2. Oxidation of a compound of hydrogen.
CH₄ + 2O₂ ⟦line⟧> CO₂ + 2 H₂O
3. Reduction of an oxide by hydrogen.
CuO + H₂ ⟦line⟧> Cu + H₂O
4. Neutralization of acids and bases.
NaOH + HCl ⟦line⟧> Na Cl + H₂O
80. PROPERTIES.
a- Physical Properties:
1. Water is liquid at ordinary temperature. It is tasteless and
odourless. It has no colour when in thin layers, but it acquires
a bluish tinge when it is observed through great thickness.
2. Water freezes at 0°C (32°F), and boils at 100°C (212°F).
3. Its greatest density, namely 1, (i.e. 1cc weighs 1 gm), is reached
at 4 degrees centigrade.
4. It dissolves almost all substances to some extent, and may there-
fore be considered as a universal solvent. The pleasant taste of
drinking water is due to dissolved air and minerals.
5. Water when pure, does not conduct electricity.
b- Chemical Properties:
1. Water is extremely stable.
2. It reacts with various active metals such as Na, Ca, Fe, with
formation of hydrogen.
3. It is decomposed by an electric current into hydrogen and oxygen.
4. It combines with certain metallic oxides forming bases:
Ca O + H₂O ⟦line⟧> Ca (OH)₂
Calcium oxide + water Calcium hydroxide
Page 652
- 34 -
5. It combines with certain non-metallic oxides forming acids.
C O₂ + H₂O ⟦line⟧> H₂C O₃
Carbon dioxide + water Carbonic acid
6. Water reacts with certain salts, forming both an acid and a
base. This process is called "hydrolysis".
Al₂(SO₄)₃ + 6 H₂O ⟦line⟧> 2 Al(OH)₃ + 3 H₂SO₄
Aluminium sulphate + water Aluminium + Sulphuric
hydroxide acid
7. Water combines with some compounds when they crystallize from
solution, forming hydrates, and this water is called water of
crystallization. For example:
Cu SO₄ + 5 H₂O ⟦line⟧> Cu SO₄.5H₂O
unhydrous copper sulphate Hydrated crystallized
plus water copper sulphate
8. Water acts as a catalyst. Many chemical reactions cannot
take place unless there is some moisture present, as for
example, rusting of iron.
81. WATER IS A COMPOUND. Water is a compound of oxygen and hydrogen.
in the ratio of 8:1 by weight and 1:2 by volume. One of the first
steps in the study of a chemical compound is to learn its composi-
tion, that is what elements it contains, and the exact amount of
each. There are two methods of obtaining this information.
1. Analysis. On passing an electric current in an acid water in
a voltametre (Hoffman apparatus), we get hydrogen at the Cathode
and oxygen at the anode. See pages 18 and 19.
2. Synthesis. On passing a spark through a mixture of definite
quantities of oxygen and hydrogen in an eudiometer, an explosion
occurs and drops of water form on the inner surface of the tube.
Spark wires connected to
gap induction coil
Mercury
Eudiometer
Page 653
- 35 -
82. WATER CYCLE IN NATURE Water has the following cycle in nature:
1. Water evaporates from oceans and seas.
2. After evaporation it condenses and forms clouds.
3. Clouds change into rain.
4. Rain falls on the earth's surface dissolving some salts found
on its crust. Rain water then passes down through the soil
until it meets a solid layer where it collects. When sufficiently
large quantities of water are collected in this way, water is
again forced up to the earth's surface forming springs.
5. Waters from Springs and Brooks flow and meet together to form a
river. This river flows into the sea and thus the cycle is
repeated.
83. HARDNESS OF WATER. Hard water is that water which does not lather
with soap, because of the presence of SOLUBLE calcium or magnesium
salt in it which form an insoluble precipitate with soap.
Ca Cl₂ + 2 NaC₁₇H₃₅CO₂ ⟦line⟧→ Ca(C₁₇H₃₅CO₂)₂ + 2 NaCl
Sod. Stearate Calcium Stearate
soluble soap insoluble soap
Hardness is of two kinds:
a- Temporary Hardness: Is the hardness caused by the presence of
the bicarbonate of either Calcium or Magnesium (Ca(HCO₃)₂ or
Mg(HCO₃)₂). These are unstable salts, i.e. they easily
decompose on heating forming the insoluble carbonates. There-
fore this hardness can be got rid of by boiling the hard water,
as is represented by the following equations:
θ
Ca(HCO₃)₂ ⟦line⟧→ CaCO₃↓ + CO₂↑ + H₂O
Ca. bicarbonate Calcium Carbon Water
Carbonate dioxide
θ
Mg(HCO₃)₂ ⟦line⟧→ MgCO₃↓ + CO₂↑ + H₂O
Mg. bicarbonate Mg. carbonate Carbon Water
dioxide
We can also get rid of temporary hardness by adding lime water
that is calcium hydroxide (Ca(OH)₂), or washing soda, i.e., sodium
carbonate (Na₂CO₃) as follows:
Ca(HCO₃)₂ + Ca(OH)₂ ⟦line⟧→ 2 CaCO₃↓ + 2 H₂O
Ca. Bicarbonate Ca. Hydroxide Ca. Carbonate Water
Ca(HCO₃)₂ + Na₂CO₃ ⟦line⟧→ CaCO₃↓ + 2 NaHCO₃
Ca. Bicarbonate Sod. Carbonate Ca. Carbonate Sod. Bicarbonate
The precipitate of calcium carbonate or Magnesium carbonate can
be removed by filtration or decantation.
Page 654
- 36 -
b- Permanent Hardness: Is that hardness due to the presence of
soluble salts of Mg and Ca in water. These salts do not decompose
on heating. They are: Calcium sulphate (CaSO₄), Calcium chloride
(CaCl₂), Magnesium sulphate (MgSO₄) and Magnesium Chloride (MgCl₂).
In other words, they are Sulphate and Chloride salts of Magnesium
and Calcium.
Permanent hardness is removed by adding washing soda (Na₂CO₃)
as follows:
CaSO₄ + Na₂CO₃ ⟦line⟧> Na₂SO₄ + CaCO₃
Calcium Sod. Sod. Cal.
sulphate Carbonate Sulphate Carbonate
MgCl₂ + Na₂CO₃ ⟦line⟧> 2 NaCl + Mg CO₃
Magnesium Sodium Sodium Magnesium
chloride Carbonate Chloride Carbonate
Sodium Chloride and Sodium sulphate remain soluble and have no
action on soap lather.
⟦line⟧
k.
Page 655
- 37 -
THE HALOGENS
84 NOMEN⟦C⟧LATURE
The halogens refer to the four elements fluorine, chlorine, bromine,
and iodine. These four elements can combine with metals to form salts
such as common salt (NaCl) and hence the word halogen (meaning salt-
former) is given to this group of elements. These four elements are
grouped as one family because they resemble each other in properties
and in their chemical compounds. We shall choose the chief member of
this family, namely, Chlorine for our study as representative for the
others.
CHLORINE
85. OCCURRENCE: This element is not present in nature in the free state
due to its great chemical activity. Therefore it exists in nature in
the combined form. The common compound of it is sodium chloride (NaCl)
which is common salt. Other Chlorides exist in nature such as those of
K, Mg and other elements.
86. PREPARATION:
Laboratory Methods
By oxidation of hydrochloric acid with an oxidizing agent such
as manganes dioxide:
4HCl + MnO2 ⟦line⟧→ MnCl2 + 2H2O + Cl2
The apparatus commonly used for this preparation is illustrated
in the following diagram.
HCl ↘ A B
C D
MnO2
↓ Cl2
Water Sulphuric Acid
Fig. 7
Page 656
- 38 -
Flask A contains MnO2 to which is added concentrated HCl through a
thistle tube.
The Chlorine gas which is librated in flask A is passed through the
connecting tube to bottle B. This bottle contains water to absorb any
gaseous (HCl) which might escape mixed with the librated chlorine.
Chlorine gas then passes through a 2nd connecting tube to bottle C which
contains concentrated H2SO4 (to absorb moisture). The dry chlorine gas is
lastly collected by the downward displacement of air. Chlorine gas, unlike
oxygen and hydrogen, cannot be collected over water because it is soluble
in it.
87. PROPERTIES:
(a) Physical properties:-
1- It is a greenish yellow gas.
2- It has a strong irritating odour and when inhaled it attacks
the delicate membranes of the nose; thorat & lungs.
3- It is extremely poisonous.
4- It is about 2 1/2 times as heavy as air.
5- It is somewhat soluble in water.
6- It is liquified easily by pr⟦ess⟧ure alone at room temperature.
(b) Chemical properties:-
1- It is extremely active.
2- It combines directly with many elements forming chlorides:
a) It combines with hydrogen:
Cl2 + H2 sun light -> 2HCl
b) It combines with metals such as heated Sodium:
Cl2 + 2Na -> 2Na Cl
c) It combines with phosphorous or sulphur
Cl2 + 2P ⟦line⟧ 2P Cl5 Phosphorus Penta chloride
Cl2 + 2S ⟦line⟧ S2 Cl4 Sulphur chloride
3- It reacts with hydrocarbons (compounds of carbon & hydrogen),
replacing part or all of the hydrogen
CH4 + 3Cl2 ⟦line⟧ Ch Cl3 + 3H Cl
Methane Chloroform
CH4 + 4Cl2 ⟦line⟧ C Cl4 + 4HCl
Carbon tetrachloride
4- It acts as a bleac⟦hing ag⟧ent in the presence of moisture.
This property is ex⟦plained b⟧y the following equation:
Cl2 + H2O ⟦line⟧ 2HCl + O
The nascent oxygen whi⟦ch is⟧ thus liberated oxidizes the colouring
materials & bleaches it.
Chlorine is also used according to the same principle for killing
Bacteria in the process of the purification of water.
Page 657
- 39 -
5- Action of Chlorine on alkalies
a) with diluted & cold alkali it forms hypochlorite &
chloride & water:-
Cl₂ + 2NaOH ⟦line⟧ NaCl + NaOCl + H₂O
b) with concentrated & hot alkali it forms chlorate & chloride
& water:-
3Cl₂ + 6NaOH ⟦line⟧ 5NaCl + NaClO₃ + 3H₂O
88. USES:
1- It was used in the 1st world war as a poisonous gas.
2- It is used in the purification of water.
3- It is used in the bleaching of coloured materials.
4- It is used for the preparation of bleaching powder, carbon
tetra-chloride, chloroform and other compounds.
89. TEST:
It is tested by exposing to it a filter paper moistured with a
solution of potassium iodide and starch. In such a case a blue
colouration indicates the presence of chlorine
Cl₂ + 2KI ⟦line⟧ 2KCl + I₂
I₂ + starch ⟦line⟧ blue colour.
⟦line⟧