MIXTURES
MIXTURES, ELEMENTS AND THEIR ENVIRONMENT
A mixture is a combination of two or more substances put together resulting to a mass.
Mixtures are commonly used in daily life such as soil, coffee and air.
Soil is a mixture of sand, stone, clay, salts and living organisms.
Air is a mixture of different gases such as oxygen, nitrogen, carbon dioxide, water vapour and other gases.
The substances that are combined to form a mixture are called components.
Types of Mixtures
Homogeneous mixture.
It appears uniform to the eyes.
Homogenous mixtures form a uniform composition.
Example, cement is a homogenous mixture of different solids such as limestone, shells and chalk among others.
Other homogeneous mixtures are steel (iron and carbon) and bronze (copper and tin).
Heterogeneous mixture.
Heterogeneous mixture is a mixture made up of a non-uniform composition.
If you take two samples from different parts of the mixture they will not be identical.
For example, you can differentiate rice (solid) from salt(solid) in the mixture.
Other heterogeneous mixtures include salt and sand, sand -sugar, salt -gravel mixtures.
In some cases, two or more substances are mixed together to form a special kind of mixture.
For example, when you mix salt (solid) and water (liquid), the solid seems to disappear in the water. This process is called dissolving and it forms a solution.
When a solid dissolve in a liquid, the liquid is called a solvent and the solid that dissolves is called a solute.
A solution is a special type of mixture that is homogenous and so you cannot tell the difference between the components of a solution.
Water and sand forms a heterogeneous mixture. One can easily separate the sand from water.
Milk (liquid) mixes with water (liquid) completely to form a uniform mixture. Therefore, this mixture is homogeneous.
Oil (liquid) mixes with water (liquid) to form a new substance that is not uniform. Both substances of the mixture can be seen. Therefore, this mixture is heterogeneous.
A gas-gas mixture is comprised of various gases, for example, the air we breathe is a combination of oxygen, nitrogen, carbon dioxide, water vapour and other gases.
The air we breathe is therefore a homogeneous mixture.
Pure and Impure substances.
When two pure substances are mixed together they form a mixture.
A mixture is an impure substance. Therefore, a pure substance is any material that is not a mixture at all.
The melting and boiling points of pure and impure substances can be determined.
Melting point of pure substances (Ice).
It is the temperature at which a solid change into liquid state.
The melting point of ice is the temperature at which ice is converted from its solid state to its liquid state.
Heat from a heat source is used to melt the ice.
The thermometer helps to note the temperature at which the ice melts.
After the initial and final readings are taken, it will be observed that once the ice attained its melting temperature (0oC), the temperature remains the same until all the ice is converted into liquid state.
Pure solids have specific melting points. Therefore, it means that ice is a pure substance.
Melting point of Impure substances (Candle wax)
The melting point of candle wax is the temperature at which the solid materials turns into liquid (by heating it).
The melting point of candle wax ranges between 46oC to 68 oC. Therefore, it means that candle wax is an impure substance.
Impure substances do not have specific melting points. The presence of impurities affects the melting point of the substances.
Determining Boiling points of Pure and Impure substances.
Requirements for the experiment
Boiling tube.
Thermometer.
Heating apparatus.
Distilled water.
A spatula.
Salt and water.
Procedure for the experiment above.
Put about 10cm3 of distilled water on a boiling tube.
Close the tube with a stopper that has two holes.
Pass a thermometer through one hole. Immerse the thermometer bulb into the water.
Push and ‘L’ shaped tube through the other hole as shown above.
Heat the apparatus using a small flame.
Observe the changes in temperature and record your observations.
Cool the apparatus and remove the stopper.
Add a spoonful of salt to some water/ stir to dissolve all the salt to form a salty water solution.
Repeat the above experiment using a salty water solution in place of pure distilled water.
Observe changes in temperature, record your observation. What conclusion have you made.
At what temperature does water (pure water) boil?
At what temperature does water with dissolved salt (impure water) boil/
Compare your observations.
Observations and conclusion & Explanation.
When distilled water is heated, the temperature of the water rises to about 100oC.
After this, the temperature remains constant (not changing) for some time. The heat absorbed changes liquid water into water vapour or steam.
Pure water has a definite or specific boiling point.
Salty (impure) water has a range of boiling temperatures above 100oC.
Impurities cause a rise in temperatures of the boiling point of liquids.
The greater the impurities in the given solution, the higher the boiling point. Therefore, we can use the boiling point to determine the purity of a liquid.
Separating Mixtures using different methods.
Different methods can be used to separate various mixtures.
Each method is suitable for separating a particular mixture.
Separating mixtures through distillation.
Requirements.
A spatula.
Water.
A measuring jar.
A thermometer.
Common salt.
Distillation flask
Condenser.
Procedure.
Add about 25cm3 of water to common salt in a distillation flask.
Shake the mixture carefully to dissolve the salt.
Set up the apparatus as shown above.
Heat the mixture until only a little solution remains in the distillation flask.
Questions from the experiment.
Through which process does the steam change to liquid in the collecting jar?
What is the term used to refer to the water that is collected in the receiving flask?
Why is cold water used in the condenser?
What is the substance that is left in the boiling tube after the water has evaporated?
Observation and explanation.
Simple distillation is successful when the substances in the mixture have different boiling points such as water and salt.
The process of distillation is used to get salt and water from the salt-water mixture.
It involves heating a liquid to form vapour which is then cooled back to the liquid.
The liquid is obtained by condensing the vapour.
Separating mixtures through filtration
Requirements.
A filter funnel.
Conical flask.
Filter paper.
Water.
Fine sand particles.
Procedure.
Get a filter paper and fold it into a cone.
Clamp the filter funnel.
Add sand into a conical flask that contains warm water and stir. Pour the mixture into the filter funnel and observe. Record your observations.
Question from the experiment.
What observation did you make?
What made it possible for the mixture to separate?
What is the purpose of the filter paper?
What is the name of the liquid that passes through the filter paper?
Give the name of the solid particles trapped by the filter paper.
Observations and explanation
Filter paper is made up of porous materials that allow liquids to pass through but traps solid particles during filtration.
The liquid obtained through the filter paper is called filtrate.
In the experiment water is the filtrate.
The filter paper traps solid particles.
The solid particles trapped is called residue.
In the experiment sand particles form the residue.
Sand is insoluble in water and therefore it does not dissolve in water.
Separating mixtures through decantation.
Requirements.
Sand particles.
Water.
Two beakers.
Procedure.
Mix sand and water in a beaker.
Stir the mixture.
Leave the mixture to stand for some time until the sand settles at the bottom of the beaker.
Carefully, pour the water (liquid) into the other beaker.
Pour the water in such a manner that the insoluble material (sand) remains in the fisrt beaker.
Discuss and record your observations.
Observation and explanation.
Decantation is the process of separating insoluble solid particles from a liquid, for example, sand from water.
After leaving the mixture to stand for some time, the heavy particles of sand settle at the bottom of the beaker, however the water collected is not clear as it contains some solids and dirt.
Decantation can also separate two insoluble liquids such as paraffin and water. The liquid at the top is poured off in a different container.
Separating mixtures using a magnet.
Requirements.
Iron fillings
Sand
A magnet
A bowl.
Procedure
Put the iron fillings in a bowl.
Add some sand into the bowl.
Mix the two substances. Describe the mixture.
Pick the magnet and hold it slightly above the mixture.
Observe what happens. Record your observations
Questions from the experiment.
What do you notice on the surface of the magnet?
Why did the magnet pick the iron filings and not the sand?
What kind of mixture is a magnet used to separate?
Observation and Explanation
A magnet is used to separate materials attracted to it from those not attracted to it.
Materials that are attracted to a magnet are called magnetic materials.
In the experiment above, a magnet attracts iron filings because iron filings are magnetic.
A magnet does not attract sand as it is nonmagnetic.
Therefore, the magnet will attract iron filings and separate them from the sand.
Separating mixture using sublimation
Requirements.
Sodium chloride.
Ammonium chloride.
Bunsen burner.
Wire gauze.
Watch glass.
Beaker.
Cold water in a round bottomed flask.
Procedure.
Mix sodium chloride (common salt) and ammonium chloride in a glass beaker.
Cover the beaker with a round-bottomed flask containing cold water.
Heat the mixture gently and observe what happens. Record your observation.
Questions from the experiment.
What happens to the mixture when heated?
What happens to the surface of the round-bottomed flask?
Describe the work of the cold water in the round-bottomed flask.
Observation and explanation.
Ammonium chloride form white fumes which condense into a white solid on the cooler surface of the round-bottomed flask and the beaker.
Sodium chloride (salt) remains in the beaker.
Sublimation is the direct change of a solid into gas after heating and vapour into solid on cooling without passing through the liquid state.
A sublimate is the solid formed when a vapour cools.
Separating mixture through paper Chromatography
Requirements.
Chromatography paper.
Ink.
Dyes.
Colouring agents.
Water and small plate or bowl with a flat bottom.
Procedure
Draw a line with a pencil on the chromatography paper and place spots of ink or dye on the pencil line.
Place the paper uprightly in water (solvent) in the small plate or bowl with flat bottom.
As the paper is lowered into the solvent, some dye spread on the paper. As the solvent soaks up the paper, it carries the mixture with it. Different components of the mixture will move at different rates. This separates the mixture.
The paper continues to absorb the solvent and the dye spreads further up the paper.
Compare your observations with those of others, make your conclusion.
Observation and explanation.
Paper chromatography is a method used to separate Coloured mixtures into their different parts.
Paper chromatography is commonly used to separate pigments, dyes and ink.
Paper chromatography works because ink contains different colours.
Separating mixture through solvent extraction.
Requirements
Piece of plain paper.
Evaporating dish.
Mortar and pestle.
Nuts.
Procedure.
Remove outside covering of the grounds.
Put the nuts in a mortar and grind them using pestle.
Add propanone and continue grinding for a while.
Leave the mixture to settle, and then decant the resulting solution into an evaporating dish.
Leave the solution in the sun for some time.
Smear a drop of the remaining solution on a piece of paper.
Hold the paper towards light and try to look through it. Discus and record your observation.
Questions from the experiment.
Explain why the solvent is used in the extraction of oil from nuts.
Name other seeds that can be used in place of nuts.
Observation and explanation.
Solvent extraction is the use of a solvent to dissolve a substance from a mixture to separate the components of the mixture.
A solvent can selectively dissolve one or more of the components of a mixture.
Propanone is a solvent for oil and therefore dissolves oil from the nuts.
When you put propanone solution in the sun, it quickly evaporates, leaving oil behind.
Oil seed like corn, sunflower, castor oil and cotton seed can be used in place of nuts.
h) Separating mixtures through crystallization
Requirements.
Water in a beaker.
Some salt.
A heat source.
Filter paper.
Glass bowl.
Procedure.
Place 50ml water in a beaker.
Add salt to it and stir.
Heat the solution.
Continue adding salt to the solution.
After some time, there will be a point at which no more salt can dissolve in water. This stage is called saturation point and the solution in known as a saturated solution.
Therefore, a saturated solution is a solution that can no longer dissolve any more solute.
Filter the salt with the help of a filter paper.
Collect the filtrate in a glass bowl and cool it.
Record your observation.
Observation and explanation.
In the experiment, some fine crystals are formed in the bowl.
Crystallization is a separation method used to separate a solid that has dissolved in a liquid.
A solid, for example salt(solute), is dissolved in a liquid, for example water, (solvent)
making a solution.
The solute is added to the solvent until no more of it dissolved. This makes the solution saturated.
As the solvent evaporates from the saturated solution, the solid will come out of the solution and crystals will start to form
Collect the crystals and allow them to dry.
Applications of Separating Mixtures in day to day life.
We filter dirty water through a piece of cotton cloth to obtain clean water at home. However, we should always boil such water to kill germs before drinking it.
We filter tea leaves from the tea before taking the tea.
Decantation separates the cream from milk. Cream rises to the top of the liquid and is easily skimmed off.
Fractional distillation is used in some industries to separate components of mixtures that have different boiling points. Example is crude oil refining. Crude oil consists of products with different boiling points. The components of crude oil include: Petrol- a fuel for petrol engines. Kerosene-used in cooking stoves and lamps.
Gas oil, lubricating oil, paraffin wax and bitumen or tar used for tarmacking roads.
Fractional distillation is used to separate liquid air into its major components of nitrogen and oxygen.
Chromatography can be used to find out which flavouring has been added to food.
Extraction of oil from nuts uses the solvent extraction method.
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