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Mixtures.

posted Jan 28, 2015, 8:07 PM by Ranmini Perera   [ updated Jul 27, 2017, 5:22 PM by Upali Salpadoru ]














Fig. 1. This one mixes cement and sand with water to produce a mixture for conccrete.



Do you know ,what a mixture is? What a mixer makes is a mixture. 
There are food mixers, Concrete mixers and milk powder mixers. So Milk is a mixture?

Most mixtures can easily be recognized but It isn’t easy to distinguish a pure substance. ‘Drinking water’ and ‘fresh air’ are not sufficiently pure for chemistry. Generally, substances like distilled water, metals like gold nuggets,  aluminum, salt and sugar from the kitchen cupboard are considered as pure substances.


A pure substance

These only have one kind of atoms or molecules.  
Examples:- 1.Oxygen has O2 molecules,  2. Carbon has C atoms,  3. Water has H2O molecules.






Fig.2  Table salt (Sodium chloride) is a pure substance.  Iodised salt is not a pure substance.

Mixtures have more than one kind of atoms or molecules.

Fig.3 Some kitchen accessories for mixing.


 












Some Common Mixtures.


Example

Appearance

Remarks

Type of Mixture

Soil

Different particles visible

Most particles can be separated easily.

Heterogeneous mixture

 

Mortar

Salad

Vinegar

Looks a pure substance

Taste and smell differs between samples.

Homogeneous  mixture

 (solution )

 

 

Olive oil (pure)

Clear

Colours vary slightly. Contains many oils. No fixed BP.

Brine (a solution of salt and water)

Looks pure

Clear liquid.   A residue is left on evaporation.

Petrol

Clear

May vary slightly.  No fixed BP.

Brass

No particles.

Contains mainly zinc and copper.

Percentages differ.

Homogeneous.

(Alloy)

Pure dry air.

No particles

Mainly Nitrogen and oxygen. Invisible.

Homogeneous.

Paint

No particles

Opaque.   Solid suspended in liquid.

Colloid

(Emulsion/sol)

Milk

No particles.

White liquid 87.5% Water. 3.9% Fat. 3.4% Proteins. 4.8% Lactose. 0.8% Mineral ssalts

Colloid

(Emulsion)

Muddy water.

No particles.

Opaque

Samples differ.

Colloid

(Suspension)

Lather

White

Tiny bubbles

translucent

Air and water with soap.

Foam.

Smoke

White/grey/black

Opaque.

Aerosol

 
Air has, Nitrogen molecules,   O2 molecules,   H2O  molecules Argon atoms and other complex organic and in organic molecules as vapours and suspensions.

In nature most of the substances that we come across are mixtures.  A few examples of pure and impure aggregates of matter (mixtures) are tabled below.

Main Differences between  Pure and Impure substances

                 Impure substance (Mixtures)

Pure Substance

1. Different particles can be easily seen.

No differences can be seen

2. Different substances can be separated by physical methods.

As there is only one substance , this does not apply. ( Pure compounds can be split up into one or more substances using chemical changes)

3.Samples taken from different places may not be the same. 

All samples will show the same physical and chemical properties under same conditions.

4.There will be no fixed Boiling point and a Melting point.

There will be a fixed Boiling point and a Melting point.

 

 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

    Mixtures may be categorised in many ways.  

    1.    According to phase or state.
                                                       A.    Solid and solid:-  

                      Alloys      Homogeneous mixtures of metals

    Name of alloy

    Principal contents

    Uses

    Gold 

    (18 carat)

      18 :6       Au : Cu

    In making jewellery

    Brass

    35% Cu and 65% Zn

    Making Brassware

    Stainless steel

    18% Cr, 80.6% Fe, 1% Ni and 0.4% C

    Structural use

    Bronze

    87.5% Cu and 12.5% Sn

    Statues

    Alnico

    Al, Ni and Co.

    used to make permanent magnets

    Duralumin

    Cu, Mn, Fe, and Si.

    Aircraft manufacture.

     

                                                                    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. )

                     Solute

    Solvent

    Solution

    Name

    Colour

    Name

    Colour

    Name

    Colour

    Salt (powder)

    White

    Water

    No

    Saline, or Brine.

    No

    Sugar (powder)

    White

    Water

      No

    Syrup

    No

    Iodine

    Dark purple

    Alcohol

      No

    Tincture of Iodine

    Dark purple

    Rubber

    Brown

    Petrol

     No

    Patch solution

    Brown

     

    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   

    Symbol of gas

    Oxygen

    Carbon dioxide CO2

    Chlorine.

     Cl2

    Hydrogen

    H2

    Nitrogen.

    N2

    Ammonia,

    NH3

    Sulphur dioxide

    SO2

    Amount soluble

    O .04g

    1.5g  

    5.5g

    0.001g

    .015g

    400g

    75g



    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.


      7.   Paper chromatography.

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

    Fig. Separating the pigments in a plant leaf using an alcohol extract. 

     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.

    ANSWERS
    1-B. 2-C, 3-A, 4-B, 5-D,  6- C, 7-D, 8-B, 9-A.  10-B,

    Part- 2.


    1. Name the numbered parts.


                     

       A.                              B.

    ලකුණු 5+5=10.

    ANSWERS
    1-B. 2-C, 3-A, 4-B, 5-D,  6- C, 7-D, 8-B, 9-A.  10-B,
    Part -2

            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


    3.0
    Name the contents in addition to water in the mixtures, selecting from these:-
    Carbon,   Soil, Potassium permanganate, Soda, Salt, Lime, Sulphur, Copper sulphate.

    Mixture.

    A

    Residue on filtering.

    B.

    Filtrate.

    C.

    Residue on evaporating filtrate..

    1.clear/ colourless.

    absent.­­­­

    Clear colourless..

    White powder.

    2.black opaque.

    Black powder.,

    Clear liquid.

    absent.

    3.white opaque.

    White powder.

    Clear, colourless.

    White powder.

    4.Brown/cloudy.

    Particles and powder.

    Clear colourless.

    Off white powder.

    5.yellow cloudy..

    Yellow powder,

    clear,colourless.

    absent.

    6.dark purple, clear when dilute.

    absent.

    purple.

    Dark substance.

    7.clear, blue.

    absent.

    Clear blue.

    Blue crystals..

    8.colourless bubbling.

    absent.

    Clear colurless.

    absent.


    ANSWERS   Part - 1
    1-B. 2-C, 3-A, 4-B, 5-D,  6- C, 7-D, 8-B, 9-A.  10-B,

                     Part -2

            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.

             3.0
              1- salt. 2-carbon,3-lime,4-soil, 5-sulphur, 6-potassium permanganate, 7-copper sulphate, 8-soda. CO2.


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