A pure substance

Fig.3 Some kitchen accessories for mixing.

Some Common Mixtures.

Main Differences between  Pure and Impure substances

 Let us consider these one by one.

1.    In soil sand particles can easily be seen. In sugar or salt all particles are the
same.If you keep muddy water in a bottle for some time sand and clay will settle down in layers..

2.    Some of the physical methods of separating mixtures are, Sedimentation, Filtering,
Distillation, Chromatography , using Magnets, and Centrifuges. (Water can be separated into Hydrogen and Oxygen by electrolysis, but this is a chemical process)

3.    If you take a soil sample from one garden and compare with another sample they will show a number of differences. On the other hand salt from where ever you get will only have the same taste and other similar properties. ( Some salts may have Iodine added, just as Chlorine is added to tap water).

4.    One may have observed that Coconut oil freezes on a cold day. But the entire
contents of a bottle will not melt. Even the purest coconut oil , would never be
chemically pure.  It would contain different oils naturally mixed. Their melting points
and Boiling points differ. It would be the same with all other oils including kerosene
oil and petrol.

We may say , “All mixtures are not pure substances” but the word ‘pure’ does not have a clear meaning. Drinking water or country air may be considered as ‘pure’ but they will contain impurities. Even the cleanest water will contain dissolved gases and salts. Air is a mixture of Nitrogen, Oxygen, Argon and water vapour with various suspended particles.

Even supposed to be pure coconut oil or kerosene oil are not pure but mixtures of similar compounds. ( Compounds are not mixtures but pure substances)

The ingredients of a mixture can generally be separated by physical methods whereas the elements in a compound can only be separated by chemical methods..

        Homogeneous  Mixtures 

The composition of ingredients in a homogenous mixture will be the same in two or mare samples taken.

    Eg : -  All true solutions,  Air, clear tap water, sea water , blood , brass (an alloy of zinc and copper)  18 carat gold are examples. Ingredients cannot be separately seen even through a microscope.

        Heterogeneous mixtures

Often the separate substances can be seen with the naked eye or through a microscope. Eg :- Soil, mortar, muddy water, rocks, cloud

         Colloids

Dispersed solid or liquid particles are so small and cannot be seen separately. If a beam of light is passed through the medium the light will scatter.  When a colloid is formed by two liquids, oil and water, it would be an emulsion.

•  Types of Mixtures

                                                                B.    Solid and liquid
         i.      Amalgams
                           Here a metal is dissolved in another metal which is a liquid. Mercury is the only metal which is a liquid under normal conditions.  All metals except iron form homogenous mixtures with mercury.
      Ii.        Solutions
                          Many solid substances dissolve  (disappear) in a liquid without undergoing a chemical change. The homogenous mixture called a solution will not indicate the presence of a solid in the mixture.        Solute    +  Solvent  =  Solution
                         ( Dissolving of egg shells in vinegar cannot be taken here as it is a chemical reaction. )

A solution has the following properties.

•    A solution has to pass through a filter paper without leaving a residue.
•    The solute cannot settle down even after a long time, (unless the solvent is evaporated)
•    May be colourless ( if the solute is white) or coloured (depending on the solvent and solute)  but it can never be white.
•    A solution has to pass the light without spreading. (Tyndall effect)
•    The solute can be recovered by the removal of the solvent by physical methods.

ii.     Suspensions
After the rains, rivers appear brown. This is due to the un-dissolved particles present. Sand and clay settle down as the speed of the current reduces.
iii.    Colloids.
When the particles are too small either to  be seen or settle down but scatters the light the mixture is a colloid. Eg Paint.

                                                       C.    Liquid and liquid.

i.    Miscible and immiscible.
One liquid can dissolve in another or remain un dissolved.  The liquids that dissolve, like water and alcohol, are called miscible liquids and the others, like oil and water, are immiscible.
ii.    Emulsion
An emulsion is a mixture of two immiscible liquids thoroughly mixed to form a colloid like phase. Milk is an example where fat globules are in watery medium.

                                           D.    Liquid and Gas
                    Many gases in the air can dissolve in water.
Mass of gas soluble in 1kg of water at atmospheric pressure and 30 C   

The solubility may be increased by increasing the pressure or by decreasing the temperature.

 Some methods employed to separate Mixtures

1. Settling and Decanting
   When a solid mixture, such as soil is mixed with water and left undisturbed, rocks, sand and clay settle down in different layers. Water can be decanted off leaving the settled matter. Water will take the soluble matter.

2. Filtration.
Solid particles mixed in a liquid can be quickly separated by filtering. There are various filters but the one often used in school laboratories is the paper filtering process. 

Fig,  Filtering using a filter paper.

There are more advanced filtering systems for homes and industry.

3. Using a separating funnel
This is used to separate two immiscible liquids.
4.  Paper Chromatography
A plant extract can be analysed by this method.

5. Evaporation.   
This method is used to separate a solute from a solvent from a given solution. When the solvent is removed by slow heating or by reducing the pressure by suction the solid matter dissolved will remain as a residue. Heating  cannot be used if the solute such as sugar,  gets decomposed due to heat.

.6.   Distillation
This is used to separate two miscible liquids, such as alcohol and water whose boiling points differ. Fractional distillation is a more advanced system used in petroleum refineries.

This method is employed to separate the pigments in ink, paint or chlorophyll.

 Part-1     Multiple Choice Questions.

1 -5  Answers:-  A- Pure substance,  B- Homogenous mixture,  C -Heterogenous mixture, D- Colloid.

Name the type of substance for the following?

1.Sea water,  2. Mortar, 3. White sugar, 4.Coconut oil, 5.Blood

6.-10 Answers:-

A-Filtering,    B-Distillation,.    C- Evaporation.  D-Chromatography.

Select the most suitable method for the following.

6. To obtain copper sulphate crystals from a solution.

7.Separating pigments in a sample of water colours.

8.To obtain fresh water from a sea water.

9.To get flour from a mixture of flour and water.

10.Which one could not be a property of a pure solution?

A-Transparent.    B-White colour,    C- Passing through a filter paper,   D- Blue colour.

marks 3×10=30.

Part- 2.

1. Name the numbered parts.

   A.                              B.

ලකුණු 5+5=10.

        1.0 A 1-filtering. 2.funnel, 3,Residue, 4-filter paper, 5-Filtrate.

B 1-distillation. 2-thermometer. 3-water out. 4-Water in,5- distillate.

2.0

Right:- 1, 4, 6, 8,

Wrong:-2,3,5,7,9, 10.

2.0 Right or wrong?

1.A laser ray passing through a solution cannot be seen.

2.Water consists of Hydrogen and oxygen and as such it is a mixture.

3.Mountain air is a pure substance.

4.Suspended solids in a colloid will not settle to the bottom.

5.The fish can extract oxygen from pure water.

6. A pure substance should have a definite boiling point.

7.The label in a bottle in the super-market reads as “Pure coconut oil”. Therefore that oil must be a pure substance.

8. A pure substance should have only one kind of atoms or molecules.

9. As salt is a pure substance it has only one kind of atoms.

10. As a copper sulphate solution has a blue colour it must be a heterogenous mixture.

marks 3×10=30

        1.0 A 1-filtering. 2.funnel, 3,Residue, 4-filter paper, 5-Filtrate.

B 1-distillation. 2-thermometer. 3-water out. 4-Water in,5- distillate.

2.0

Right:- 1, 4, 6, 8,

Wrong:-2,3,5,7,9, 10.