Download `saturated` solution?

Dissolving and evaporating
Aseel Samaro
Introduction
 Gemstones, such as amethyst, are crystal that formed deep in the
Earth’s crust.
 Water dissolved salts and as this solution cooled over thousands of
years, the precious crystals formed.
Saturated solution
 A saturated solution is one that can simply hold no more solute.
 An unsaturated solution, then, CAN hold more solute, if you add more
 A supersaturated solution holds MORE than it can.
Temperature effects
 The mass of solute that dissolves in a solvent at a particular
temperature is called its solubility.
 One way to help things dissolve is to  increase the temperature of
the water.
 This is why we wash clothes in warm water.
 Any soluble stains in the clothes will dissolve better at a higher
temperature.
Dissolving sugar in water at different temperatures
Look at the data in this table. The results show the mass of
sugar (sucrose) that can dissolve in 100 cm3 of water.
Temperature of
water (°C)
0
Mass of sucrose 180
that can dissolve
(g)
20
40
80
200
240
600
What does the data in the previous table tell you about the solubility
of sucrose at different temperatures?
Making crystals
 Heat can also be used to separate soluble substances from their
solutions.
 When the solvent evaporates it leaves behind the solid solute – this is
called crystallisation.
 If this process happens quickly, small crystals of the solute will form.
 However, if the evaporation happens slowly, bigger crystals can grow.
 Crystallization:
process of formation of solid crystals precipitating from a solution
 Crystallisation happens most efficiently when the solution is saturated.
 This means that there is as much solute dissolved in a solvent as
possible.
 The solubility of substances depends on the temperature of the
solvent.
 If any more solute is added to a saturated solution it will not dissolve.
 As the solvent cools, the crystals start to form. (As the solution cools,
the solvent can no longer hold all of the solute molecules, and they
begin to leave the solution and form solid crystals)
You can grow your own crystals.
What is a ‘saturated’ solution?
One in which no more solute will dissolve.
Describe how you could grow salt crystals.
Stages should include preparation of saturated solution and evaporation of water to grow crystals.
Why do you think that the crystals start to form as the solvent cools?
Less solute can be dissolved at lower temperatures/less soluble
Using graphs
 Referring to the figure in your book:
 Substances dissolve more in hot water because the water molecules
have more energy and move faster.
 They can break down the crystals and separate the solute molecules
more quickly.
 Solubility also depends on the type of solute.
HOMEWORK