SEPARATING MIXTURES AND PURE SUBSTANCES

In Chemistry it is very important to be able to create pure chemical substances. These are some of the methods tha Chemists use to separate mixtures of chemicals and to create pure substances.

Methods of separating insoluble solids from liquids

• Decanting: For separating insoluble solids with heavy particles from liquids. e.g. mixture of sand and water.
  
  
• Filtration: For separating insoluble solids with fine particle from liquids e.g. mixture of chalk powder and water.
  
  
• Centrifugal method: For separating fluids of different densities (e.g. cream from milk) or liquids from solids in suspensions.

Methods of separating soluble solids from liquids

• Evaporation: (e.g. sodium chloride from water).
  
  
• Simple distillation: Used when both solid and liquid is to be recovered.
  
  
• Crystallisation: For separating soluble solids from liquids by allowing them to form crystals.
  
  
• Fractional crystallisation: For separating two or more solutes with different solubilities at the same temperature form a liquid.

Method of separating coloured substances in solution

• Chromatography: Uses the difference in rates of movement of solutes over a porous medium like filter paper by a moving solvent (e.g. ink, chlorophyll, dyes, and so on).

Methods of separating liquid – liquid mixtures

• Separating funnel: For immiscible liquids (e.g. paraffin and water).
  
  
• Simple distillation: For miscible liquids with boiling points widely far apart.
  
  
• Fractional distillation: For separating miscible liquids with closely related boiling points (e.g. petrol and paraffin, ethanol and water).  

Methods of separating solid-solid mixtures

• Sublimation: Used when one of the solids sublimes on heating (e.g. iodine and sand).
  
  
• Magnetisation: Used when the mixture consist of and any other substance.
  
  
• Hand picking and winnowing: Used when the solid particles of one of the solids are big (e.g. husks/chuff and rice).

Methods of drying solids

• Pressing between filter papers.
  
  
• Sun drying.
  
  
• Oven drying.
  
  
• Desiccators with drying agents.
  

EXPERIMENTS

Caution: You should be prepared to work in groups with peers under minimal supervision of your teacher (s). Ensure that you carefully follow instructions and guidelines provided. Attempt to observe ALL laboratory safety precautions.

These experiments should help you learn some basic concepts on uniform and non uniform mixtures, miscible and immiscible liquids, soluble and insoluble solids, solute and solvent. You are required to mix various substances and separate them:

Expt 1. Filtration

Apparatus: Mineral water bottle, knife or razor blade.

Other requirements: Toilet paper, cotton cloth, cotton wool, grass.

Procedure:

• Collect muddy water from a nearby well or tap drainage system.
  
  
• Cut a mineral water bottle into two parts, with the top part acting as a filter funnel. Turn the upper part upside down and stuff it with cotton wool. Place this on top of the other part of the mineral water bottle.
  
  
• Filter muddy water.

Questions

Were you successful in obtaining a clean filtrate?

Name an application of filtration at home.

Expt 2. Sodium chloride and water by evaporation.

Apparatus: Metal can, charcoal stove (sigiri), paraffin stove and spoon.

Other requirements: Common salt, water,

Procedure:

• Pour some water into the metal can and add three spoonful of common salt. Stir the mixture with the spoon until the salt dissolves.
  
  
• Place the can on a charcoal stove and heat the solution until it evaporates to dryness.
  
  
• Scrape out the salt crystals onto a paper and allow your students to taste it.

Expt 3. Parrafin and water

Apparatus: Separating funnel and beakers.

Other requirements: Cooking oil or paraffin and water.

Procedure:

• Pour some water into a separating funnel followed by some paraffin or cooking oil.
  
  
• Open the tap so that the water runs out.
  
  
• Close the tap when the water level is just at the top of the tap.
  
  
• Run the paraffin or cooking oil into a separate beaker.

Question

How can the water be purified?

Expt 4. Alcohol and water

Apparatus: Distillation kit, conical flasks or mineral water bottles, bumping stones, thermometer, tripod stand, wire gauze, rubber corks and rubber tubing.

Other requirements: Ethanol and water.

Procedure:

• Pour a mixture of ethanol and water into a round bottom flask and drop a few bumping stones into the flask.
  
  
• Set up the distillation apparatus.
  
  
• Heat the mixture until it boils.
  
  
• Collect the distillate in a conical flask.

Questions

What did you collect as the distillate?

What is the purpose of the bumping stones?

What was the temperature on the thermometer?

Name one economic activity that is carried out locally in the villages or town suburbs using ideas from this method.

Expt 5. Iron and sulphur.

Apparatus: Bar magnet

Other requirements: Iron filings and powdered sulphur.

Procedure:

• Take a little iron filings on a paper, add powdered sulphur to it and mix them.
  
  
• Pass a magnet over the mixture.

Questions

What did you observe? What substance was attracted by the magnet?

Expt 6. Ammonium chloride and sand (Sublimation)

Apparatus: Glass beaker, conical flask, wire gauze, tripod stand, spoon or spatula and heating source.

Other requirements: Water, ammonium chloride and sand.

Procedure:

• Place a mixture of ammonium chloride and sand in the glass beaker.
  
  
• Pour some cold water into the conical flask and place the flask on top of the beaker.
  
  
• Heat the mixture in the beaker and observe what collects at the bottom of the conical flask.

Questions

What substances collected at the bottom of the flask?

Name one other substance that could be used in place of the one collected.

Expt 7. Copper(II) sulphate solution  (Evaporation and crystallisation)

Apparatus: Glass beaker, heating source, wire gauze,

Other requirements: Copper(II) sulphate solution.

Procedure:

• Pour some copper(II) sulphate solution into the glass beaker.
  
  
• Place the beaker and solution onto the wire gauze and heat it till crystals begin to form.
  
  
• Leave the contents in the beaker to cool.
  
  
• Decant off the crystal or filter them off, wash with a little water and dry.

Questions

What is the colour of the substance that crystallized out?

What did you obtain?

Name three different methods of drying salts.

 Expt 8. Separation of the components of chlorophyll using ethanol as solvent (Chromatography)

Apparatus: Mortar, pestle, glass rod, beaker, office pin or wire, filter paper, ruler and razor blade.

Other requirements: Leaves and ethanol

 Procedure:

• Place some leaves in a mortar and grind them. Add some ethanol to the leaves and grind further.
  
  
• Using a glass rod place a drop or two of the green colouring on a rectangular piece of filter paper. Leave the drop to dry.
  
  
• Pour a little ethanol in a glass beaker and suspend the filter paper in the beaker. The drop must not be submerged in the ethanol in the beaker.
  
  
• Leave the set up to stand for some (15) minutes.

Questions

What did you observe? How many coloured spots do you see?

Name the method of paper chromatography used in this experiment?

ADDITIONAL  EXPERIMENTS

Filtration: Filter muddy water using filter paper and various materials as filter paper( e.g. cotton wool, cotton cloth, grass etc ).

• Distillation: Distillation of local alcohol.
  
  
• Separating funnel: Separation of a mixture of cooking oil and water.
  
  
• Sublimation: Separation of  a mixture ammonium chloride and sand.
  
  
• Evaporation:  copper(II) sulphate solution.
  
  
• Crystallisation: Copper(II) sulphate crystals from copper(II) sulphate solution.
  
  
• Chromatography: Chlorophyll using ethanol as solvent.
  
  
• Magnetisation:  Separation of a mixture of iron filing and copper turnings.

TASK 1

1. Name one method that can be used to separate the mixture of the following substances;

• Sodium chloride and chalk powder
  
  
• Coloured dyes in sun –sip straw berry drink.
  
  
• Sodium chloride and aluminum chloride
  
  
• Petrol and water
  
  
• Paraffin and diesel
  
  
• Iron filing and ash.
  
  
• Copper oxide and copper(II) sulphate
  
  
• Copper(II) sulphate and water.

2. Name two drying agents?

3. Explain how you can separate a mixture  containing sodium chloride, ammonium chloride and chalk powder.

4. State the difference between the following;

• Solute and solvent
  
  
• Miscible and immiscible liquids
  
  
• Filtrate and distillate
  
  
• Melting and boiling point

5. The picture below shows a local distillation  set up.

[

• What is the role of the copper tube?
  
  
• Why is the tube made of   copper and not plastic?
  
  
• Why is the tube coiled in the middle?

6. Name separation apparatus you can see in the pictures below.

7. What is the effect of impurity on melting and boiling points of substances?

ANSWERS FOR TASK 1

1.         (i)         Filtration then evaporation of filtrate

            (ii)        Chromatography

            (iii)       Sublimation

            (iv)       Use of Separating funnel

            (v)        Fractional distillation

            (vi)       Magnetism

            (vii)      Dissolution in water and filtration and later crystallization

            (viii)      Crystallization

2. Concentrated sulphuric acidAnhydrous calcium chloride

3.The mixture is first heated where the ammonium chloride escapes off as vapour (sublimes). 

The remaining solid mixture of sodium chloride and chalk is now then shaken with water to dissolve all the sodium chloride.  This mixture is then filtered. 

The filtrate which is sodium chloride solution is then evaporated carefully in an evaporating dish.  The residue is chalk.

4.     4. (i)  Solute is a substance that dissolves in a solvent

           (ii) Miscible liquids have a homogenous layer while immiscible liquids are separated by a     clearly defined boundary.

            (iii) Filtrate is obtained by filtration process while a distillate is obtained by distillation process.

• Melting point occurs at lower temperature than boiling point.  Melting point is defined as the temperature at which a solid substance changes to liquid state at atmospheric pressure.  Boiling point is the temperature at which the vapour pressure of the substance equals the atmospheric pressure.

(i) To convey alcohol vapour from the drum to the collection point.

(ii) It is resistant to decomposition by heat.

(iii) To increase the surface area for condensation of the alcohol vapour.

6. Distillation and centrifuge.

7. Impurities decrease or increase melting and boiling points.

TASK 2

EXTENSION WORK AND STUDENT’S PROJECTS

These individual projects are intended to help you apply basic chemistry concepts and skills acquired in class to separate mixtures and pure substances.  You are required to formulate your own procedures for carrying out the project.  Then, you will write a report about your project that may include name of project, apparatus and procedures used, results obtained, and conclusions. You will hand in your report to the teacher. Your work may be displayed in class for your peers to see.
Outlined below are some of the possible individual projects you will carry out.

Decanting

• Extraction oil from seeds:  Roast groundnuts or Simsim, pound, grind them, boil with water until oil floats then decant off the oil.

Filtration

• Prepare sand filter beds.
  
• Make filter funnels from plastic bottles and large leaves.

Distillation

• Model distillation kit using clay.
• Distil local alcohol.

Crystallisation

• Growing of copper(II) sulphate crystals.
  
  
• Chemical forest made growing crystals of different chemicals in glass water.

Chromatography

• Analysis of dyes in coloured sweets, sodas and a mixture of phenolphthalein and methyl orange indicators.

TASK 3

STUDY TOURS

This activity will help you learn how the basic principles/methods of separating mixtures and pure substances are carried out in real world situations such as factories, laboratories, distilleries, and sewage treatment plants. During the study tour, carry with you pencils and notebooks for taking notes. Be prepared to ask relevant questions to the staff at the facility visited. You may ask your teacher for help in formulating the relevant questionnaire.

Some examples of facilities that operate on the principles of separating mixtures and pure substances include:

• Filtration: National water and sewage treatment plants (e.g. Bugolobi sewage treatment plant and Gaba Water Treatment plant in Uganda).
  
  
• Fractional distillation: Manufacture of oxygen at Uganda Oxygen Limited (UOL)
  
  
• Distilleries: Uganda distilleries Luzira- Kampala and Kenya petroleum refinery Mombasa.
  
  
• Chromatography: Chemotherapeutic research laboratories( e.g. Chemotherapeutics research laboratory Wandegeya- Kampala Uganda
  
  
• Centrifugal: Clinical laboratories (e.g. in hospitals).

Application of methods of separating mixtures

• Filtration: Sewage and water treatment (stone and sand filter beds).see pictures attached.

• Simple distillation: Production of local alcohol e.g Kasese-Kasese, Lira-Lira.
  
  
• Fractional distillation: Petroleum refinery, manufacture of spirits and separation of liquid air into oxygen and nitrogen.

• Fractional crystallisation: Used in the extraction of sodium chloride (common salt) from Lake Magadi in Kenya and Lake Katwe in Uganda.
  
  
• Chromatography: Used in analysing chemicals e.g. plant extracts in chemotherapeutics research laboratories.

Teachers' Notes

Scheme of work

Student activity

Learners' activities