4. Gas and salt solubility in water
Gas and salt solubility and solubility curves
are considered - variation of solubility with temperature
Doc Brown's GCSE level school chemistry notes
Water chemistry notes index
5a.
How
well do different
gases and solids dissolve in water?
-
First, some definitions
of words you may encounter in talking about solubility and
other water related situations:
-
solute: the
material which is to be dissolved in a solvent.
-
solvent: the
liquid which dissolves the material (the solute). You will come
across water more than any other liquid solvent BUT lots of
important organic solvents like hexane (petrol like), ethanol
(alcohol) and propanone (acetone) are in common laboratory use.
-
solution: the
result of dissolving something in a liquid (solute + solvent =>
solution).
-
solubility: to
what extent a solute material will dissolve.
-
soluble: the
material will dissolve in a particular liquid solvent.
-
saturated:
means that no more of a substance (the solute) will dissolve in its
solution i.e. maximum solubility achieved at a particular
temperature.
-
Therefore a saturated solution is one in which no
more solute will dissolve at that temperature giving the maximum
solubility at that particular temperature.
-
insoluble: not
soluble, will not dissolve in a particular liquid (don't assume it
means will not dissolve in anything).
-
hydration:
means the addition of water to a material.
-
dehydration:
means to remove water from a substance.
-
Factors affecting
rates of dissolving.
-
heat: heating
the mixture to raise the temperature will increase the rate of a
substance dissolving - the energy of all the particles involved is
increased - increased rate of more energetic collisions between solute
and solvent particles speeding up the dissolving process.
-
surface area:
if a solid is broken up and crushed into smaller pieces or a powder it
will dissolve faster. This breaking down of a solid increases the
surface are for the solvent to 'attack' and dissolve the solid.
-
stirring:
this increases the rate of dissolving because it prevents 'local'
saturation of the solution which will inhibit dissolving.
-
volume of solvent:
adding more solvent increases the speed of dissolving, the less
concentrated
-
These factors are
similar with those affecting the rates of chemical reactions except
there is no catalyst that speed up dissolving as far as I know?
Also, increasing the volume of the solvent will decrease the rate of
reaction because concentrations are reduced.
-
Some gases and solid substances are more
soluble in water than others and some are hardly
soluble at all.
-
The solubility of gases and solids in water also depends on the
temperature of the water:
-
Many gases are soluble in water and the
solubility increases as the temperature decreases and as the pressure
increases.
-
Carbonated water
is produced by dissolving
carbon dioxide under high pressure. When the pressure is released the gas
bubbles out of the solution. Carbonated water is used to give fizzy drinks
a 'tang' to the taste.
-
Thermal Pollution:
Dissolved oxygen is essential for aquatic
life and the colder the water, the more of it dissolves. Hot water from power stations may be discharged into rivers or
lakes. This discharge reduces the amount of oxygen dissolved in the water
and this can damage aquatic life and disrupt the natural eco-systems.
-
Chlorine water is made by dissolving Chlorine
gas in water and can be a useful chemical reagent, both in the laboratory
and industry (e.g. displaces iodine from sea water).
-
Chlorine water is used to bleach materials
and kill bacteria.
-
Many ionic compounds are soluble in water
and many covalent compounds are insoluble in water (but don't make
assumptions!).
-
The
solubility of a solute
in water, or any other solvent, is usually given in grams of solute per
100 grams of solvent (e.g. water) at that temperature.
-
The solubility of most solid solutes increases as
the temperature increases (opposite of gases, but the ambient air
pressure has no effect).
-
A saturated solution is one in which no
more solute will dissolve at that temperature giving the maximum
solubility at that particular temperature.
-
When a hot saturated
solution cools some of the solute will separate from the solution
(crystallisation). The
crystals form because the solubility is lower at the lower temperature.
-
From solubility graphs-data you can calculate how much will dissolve at a
given temperature and how much will crystallise out on cooling.
-
Solubility curves:
-
General rules
which describe the solubility of common types of compounds in water:
-
All common sodium, potassium and
ammonium salts are soluble e.g. NaCl, K2SO4, NH4NO3
-
All nitrate
salts are soluble
e.g. NaNO3,
Mg(NO3)2, Al(NO3)3, NH4NO3
-
Some ethanoate
salts are soluble
e.g. CH3COONa
-
Common chloride
salts are soluble except
those of silver and lead e.g.
-
Common sulfates are soluble except
those of lead, barium and calcium: soluble e.g.
-
soluble:
Na2SO4,
MgSO4, Al2(SO4)3
-
insoluble: PbSO4, BaSO4, CaSO4
is slightly soluble.
-
Common oxides,
hydroxides and carbonates are usually insoluble (e.g. Group 2 and Transition Metals)
except those of the Group
1 Alkali Metals sodium, potassium etc. and ammonium:
-
soluble:
K2O,
KOH, NaOH, NH4OH actually NH3(aq), Na2CO3,
(NH4)2CO3
-
insoluble:
MgO, CuO, ZnO, Mg(OH)2, Fe(OH)2,
Cu(OH)2, CuCO3, ZnCO3, CaCO3
-
Knowledge of salt solubility is
important in deciding which method of salt preparation is employed.
-
See
methods of making salts and details of
various methods.
TOP OF PAGE
and sub-index
4b.
Solubility curves for selected salts

-
Interpretation of graph e.g.
-
Reading graph: at 38oC the
solubility of copper sulphate, CuSO4, is 28g of anhydrous
salt per 100g of water.
-
Reading graph: at 84oC the
solubility of potassium sulphate, K2SO4, is 22g
per 100g of water.
-
Ex Q1: How much potassium
nitrate will dissolve in 20g of water at 34oC?
-
At 34oC the
solubility is 52g per 100g of water,
-
so scaling down, 52 x
20 / 100 = 10.4g will dissolve in 20g of water
-
Ex Q2: At 25oC
6.9g of copper sulphate dissolved in 30g of water, what is its
solubility in g/100cm3 of water?
-
Scaling up, 6.9 x 100
/30 = 23g/100g of water
-
(check on graph, just less than 23g/100g
water).
-
Ex Q3: 200 cm3
of saturated copper solution was prepared at a temperature of 90oC.
What mass of copper sulphate crystals form if the solution was cooled to
20oC?
-
Solubility of copper
sulphate at 90oC is 67g/100g water, and 21g/100g water at 20oC.
-
Therefore mass
of crystals formed = 67 - 21 = 46g (for 100 cm3 of solution).
-
However, 200 cm3 of
solution was prepared,
-
so total mass
of copper sulphate crystallised = 2 x 46
= 92g
Note: The density of
water is close to 1.0g/cm3 or ml, so for approximate purposes.
the volume in cm3 or ml of just the water is numerically close to
the value in g, i.e. 100 cm3 of water or solution is about 100g
of water.
|
Examples of |
SALT SOLUBILITY DATA |
SOLUBILITY |
g salt / 100g water |
Salt name |
potassium
nitrate |
potassium
sulphate |
sodium
chloride |
hydrated
copper(II) sulphate |
and formula |
Temp. oC |
KNO3 |
K2SO4 |
NaCl |
CuSO4
(anhydrous *) |
0 |
13.9 |
7.4 |
35.7 |
14.3 |
10 |
21.2 |
9.3 |
35.8 |
17.4 |
20 |
31.6 |
11.1 |
36.0 |
20.7 |
30 |
45.3 |
13.0 |
36.2 |
24.2 |
40 |
61.4 |
14.8 |
36.5 |
28.7 |
50 |
83.5 |
16.5 |
36.8 |
33.8 |
60 |
106.0 |
18.2 |
37.3 |
40.0 |
70 |
|
19.8 |
37.6 |
47.0 |
80 |
|
21.4 |
38.1 |
56.0 |
90 |
|
22.9 |
38.6 |
67.5 |
100 |
|
24.1 |
39.2 |
80.0 |
|
|
|
|
* multiply by 1.562 for hydrated crystals CuSO4.5H2O |
WHERE NEXT?
Extra Aqueous Chemistry
Index:
1. Water
cycle, treatment, pollution
2.
Colloids - sols, foam and emulsions
3.
Hard
and soft water - causes and treatment
4.
Gas and salt solubility
in water and solubility curves (this page)
5.
Calculation of water of crystallisation
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