MIXTURES, SOLUTIONS AND SOLUBILITY
A mixture is two or more substances (elements and /or compounds) combined in varying
proportions.
Each component retains its own independent properties and has undergone no chemical
reaction with any other substance in the mixture.
A solution is a mixture of two or more substances (one of which is usually a liquid),
which is the same throughout.
The substance which does the dissolving is called the solvent. It is present in the highest
concentration. That is to say, the solvent contributes the most particles to the mixture.
The solute(s) is/are the substances which dissolve and are present in lower
concentrations.
Types of Solutions
Solute
Solvent
Examples
Gas
Gas
Air
Liquid
Liquid
Vinegar
Solid
Solid
Metal alloys, e.g. brass,
bronze, steel, alnico.
Gas
Liquid
Soda water, oxygen in tap
water
Solid
Liquid
Sea water
Characteristics of solutions
Solute and solvent are thoroughly mixed: all parts of the solution have the
same composition, colour, density and appearance
Solute and solvent do not separate when the solution is allowed to stand
Particles of solute are not visible.
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Even if the solution is coloured, it is usually transparent if the solution is a
liquid
Solute may be separated from the solvent by purely physical means.
Solutions can be described as ‘saturated’ and ‘unsaturated’
A saturated solution at a specified temperature cannot take anymore solute. The excess
solute simply does not dissolve.
On the other hand, an unsaturated solution can still dissolve any solute added to it.
Solubility
Solubility is the maximum amount of a solute which would dissolve in a given amount of
solvent at a specified temperature. Solubility of a solute is also defined as the mass of
solute which will saturate 100g of solvent at a given temperature.
A soluble substance dissolves in a solvent.
The solubility of a solute depends on four main factors:
The nature of the solute
The nature of the solvent
The temperature of the solution: higher temperatures make substances more
soluble
The surface area of the solute in contact with the solvent.
Pressure (for gases only): Gases dissolve under high pressure.
Solubility Experiments
A. Investigating the solubility of solutes in solvents at different temperatures
The graph showing the way solubility changes with temperature is known as a
solubility curve.
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Instructions for reading a solubility curve
1.
Find the given temperature on the x-axis. For example, to determine how much
sugar dissolves in 100 grams of water at 20˚C, look on the x-axis for the 20˚C
point.
2. Trace a vertical line from the point on the x-axis to the solubility curve of the
solute. In this case, trace the line to the solubility curve of sugar.
3. Trace a horizontal line from the intersection with the solubility curve to the y-axis.
The value on the y-axis indicates the amount of solute that will dissolve in the
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solvent at a given temperature. For example, 200 grams of sodium chloride
dissolves in 100 grams of water at 20˚C.
In general:
The solubility of solids in liquids increases as temperature increases. (Think of
dissolving sugar in hot water when making syrup or fudge.)
The solubility of gases in liquid solvents decreases as temperature increases.
(Think of warming a cup of a carbonated beverage. Does it go ‘flat’ quickly?
Why does this happen?)
Solubility of gases in water
In industry, large amounts of carbon dioxide dissolve in water to make carbonated
beverages such as soda and sparkling wine.
Dissolved gases are important to:
1.
Aquatic organisms because they need the oxygen in the water for
respiration. The hotter the environment gets, the less oxygen gas dissolves
in water and aquatic organisms will die from lack of oxygen.
2.
Deep Sea Divers because if they don’t control how fast they ascend from
the bottom of the sea, they can develop the ‘Bends”. The Bends occurs
when Nitrogen gas dissolved in their blood starts to bubble/boil. It can be
extremely painful or even fatal.
3.
The process of gaseous exchange because oxygen can only diffuse into the
blood if it dissolves in water on the air sacs.
Questions:
1. Identify three situations in daily life in which knowledge of solubility is put to use.
2. Are gases important to the survival of aquatic life? Name the gas(es). If there
is/are important gas(es), how does it get to living things in the water?
3. Identify two industrial processes which depend on the solubility of gases in water.
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www.dspace.dial.pipex.com/town/parade/rbd41/water.htm (dissolved gases)
www.wpbschoolhouse.btinternet.co.uk/page01/AqueousChem/AqueousChem.htm (go to
Gas and salt solubility)
How does the solute affect the boiling/ melting point of the solvent?
Pure solids have a single temperature at which they melt once pressure is kept constant.
This is the melting point. Adding a solute lowers the melting point of a solid. This
knowledge is utilized when making home made ice-cream. Pure ice melts at 0ºC. Adding
table salt to the ice in the bucket lowers the melting temperature of the ice to -2ºC or less.
Pure liquids boil at a single fixed temperature provided that the pressure is kept constant.
This is the boiling point of the liquid. The presence of impurities raises the boiling
temperature.
When impurities are present, melting or boiling will take place over a range of
temperatures instead of just one temperature.
What’s the relationship between the amount of solute and a solution’s colour?
Coloured solutes directly affect the intensity of colour of a solute. A very small amount
of solute makes a lightly coloured solution. A large amount of solute makes the solution
very deeply coloured.
Thus we see that:
A concentrated solution is one in which the solute is present in high concentration.
A dilute solution is one in which the solute is present in low concentration.
The terms concentrated and dilute are not precise ways of indicating how much solute is
in a solvent.
Refer: www.geocities.com/barnhardtge/solutions3.html
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Students are asked to :
1. Write the following paragraph:
A colloid is a mixture in which the particles of one substance are dispersed in
another substance. The particles do not dissolve and they do not settle. Colloids
are in between solutions and suspensions in particle size and behaviour. Thus they
are able to pass through filter paper but do not allow light to pass through
unobstructed. The light is scattered to give a cloudy appearance. This is called the
Tyndall effect. Colloids can adsorb electric charges and become electrically
charged. There are different types of colloids depending on the states of the
colloidal particles and the dispersing medium. Examples are: sol, gel, foam,
aerosol and emulsion. Colloids are widely used at home, in industries and in
medicine. Many of the chemical reactions and life processes in living systems
involve colloids (The cytoplasm of the cell is a colloid).
2. Copy the following tables into their notebooks. The General Topic is Types of
Mixtures. The Sub-heading for the first table is Types of Colloids According to
State of Matter.
3. Differentiating Solutions, Colloids and Suspensions
Solutions, colloids and suspensions can be differentiated by
a) Observing Their appearance
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b) Shining a light on them
c) Whether the substances they are made of can be separated by filtration
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Separating a solution into its components
As mentioned before the solute may, in many cases, be separated from the solvent by
purely physical means. Methods include simple distillation, crystallization and
evaporation.
Separating soluble solids from liquids
Evaporation works when the solute has a substantially higher boiling point than the
solvent. Also the solute must not decompose on heating or contain water essential for the
formation of the crystals.
Crystallization is used to obtain the soluble solute only from a solution. Solid crystals are
formed from a saturated solution. Such a solution would contain more dissolved solids
(solute than can ordinarily accommodated at that temperature. The process is based on
the fact that there is a maximum amount of solid that can dissolve in a solvent at a
specified temperature.
Simple distillation is used to obtain the pure solute and solvent separately. The process
works because of differences in the boiling points of the solutes/ solvent. The mixture is
boiled and the components with the lower boiling points come off first. This vapour is
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cooled and condenses to form the distillate. The original mixture now has more
components with a higher boiling point. A more advanced distillation method is used in
our oil refinery. www.science.howstuffworks.com/oil-refining4.htm .
Separating Immiscible Liquids
Separation funnel is used to separate liquids that are immiscible (they do not mix) e.g. oil
and water. The liquids in such a mixture separate into two distinct layers. The less dense
layer is on top and the denser layer is below. When the tap is opened, the denser liquid is
collected first.
Immiscible liquids do not mix.
Separating Miscible Liquids:
Fractional Distillation is used to separate liquids with similar boiling points. It uses a
process of repeated vaporization and condensation. As the vapour rises up the
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fractionating column it becomes richer in the more volatile component (the one with the
lower boiling point). In fractional distillation, the component with the lowest boiling
point comes off first.
Fractional Distillation is used to separate air and crude oil into their constituents.
Separating Insoluble solids in liquids:
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Filtration
Separating Magnetic Substances
Magnetism
Use magnets to separate magnetic materials (nickel, iron and cobalt) from mixtures.
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SEPARATING COMPOUNDS
Compounds are the product of chemical reactions. In chemical reactions, substances
called reactants are combined or rearranged to form new substances called products.
Chemical reactions are brought about by heat, light, electricity or the process of mixing to
produce new substances.
How are compounds created?
In chemical reactions no atoms are created or destroyed.
1. Some substances decompose (break down) to produce new substances
Type of reaction: Decomposition
e.g. thermal decomposition of Lead (II) nitrate
Word equation: Lead (II) nitrate  Lead (II) oxide + Nitrogen dioxide + Oxygen
2.
Some substances produce heat
E.g. Combustion of magnesium ribbon in air
Type of reaction: Combustion, Combination, Exothermic
Magnesium burns in air with a bright light. When magnesium burns in air, it
combines with the oxygen in the air to form a white solid – magnesium oxide.
The word equation for the combustion of magnesium is:
Magnesium + Oxygen  Magnesium oxide
3.
Some chemical reactions absorb heat (Endothermic):
E.g. photosynthesis
Word equation: Carbon dioxide + Water  Glucose + Oxygen
How are compounds separated?
1. Some compounds can be separated by passing electricity through a molten or
aqueous (dissolved in water) form of the compound. This is called electrolysis.
Electrolysis is defined as a chemical reaction brought about by the passing of an
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electric current through an electrolyte, which is a liquid that can conduct
electricity.
Water can be separated into hydrogen and oxygen by this process.
2. Other compounds like iron can be separated from its compound, iron oxide by
chemical reduction. Chemical reduction of iron oxide is done by heating the iron
oxide with Carbon. Pure iron is the product.
Next: Colloids and Suspensions
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