Basic 8 · Science · Separation of Mixtures

Separating Mixtures

In this topic you will design and perform processes for separating kinds of mixtures, using methods based on differences in particle size, density, solubility, boiling point and other properties.

By the end of these notes you should be able to:

Separate solid and liquid mixtures by filtration, sieving and decanting
Separate dissolved substances by evaporation and distillation
Choose a suitable method using properties of substances

What you already know

Types of mixtures

Copy everything that has a blue line on its left into your notebook. The other parts are for reading.

Lesson 1

Strand: Diversity Of Matter
Sub-strand: Separation of Mixtures
Indicator: B8.1.1.1.2 – Design and perform processes for separating kinds of mixtures

Introduction

Each morning somebody in Ghana sieves flour, strains tea or lets muddy water settle. These are real scientific methods of separation.

1. Choosing a Method

Separation works because substances in a mixture differ in properties, such as size, density, solubility, boiling point or magnetism. To separate a mixture, find the property that differs and choose a method using it. A method such as sieving depends on size, while magnetic separation depends on magnetism.

2. Physical Methods for Solids

Separating solids and insoluble solids

Method	Property used	Example
hand-picking	size and colour	stones from rice
sieving	particle size	flour from husks
magnetic separation	magnetism	iron from sand
winnowing	density, wind	chaff from rice
decantation	settling	pouring water off sediment
filtration	particle size	sand from water

3. Filtration

Fold a filter paper and place it in a funnel.
Place the funnel over a beaker or flask.
Pour the mixture in slowly.
The clear liquid, called the filtrate, passes through.
The solid, called the residue, stays on the paper.

Filter paper has tiny holes. Water and dissolved substances pass through, but larger particles do not.

A woman winnowing grain with a tray

Figure 1: Winnowing Name the method used for five kitchen separations.

4. Water Treatment

Water treatment works use similar processes. At Weija or Kpong, chemicals are added to clump fine particles together, which settle. The water is filtered through beds of sand and then disinfected with chlorine. At home, boiling and filtering can make water safer.

New words

filtrate: The liquid that passes through a filter.
residue: The solid left on the filter.
decantation: Pouring off liquid after settling.
winnowing: Separating chaff by wind.

Summary

Separation uses differences in properties.
Sieving, magnetism and winnowing separate solid mixtures.
Filtration and decantation separate insoluble solids from liquids.

Class activity: Separate the Mixture

Mix sand, iron filings and salt.
Plan how to separate them.
Show your method to the class.

You need: Sand-and-water mixture, Filter funnel with filter paper, Salt-water evaporation dish

Exercises

Answer in your exercise book first, then check your answers.

1. Iron filings can be removed from sand with a…
- A. magnet
- B. filter
- C. sieve only
- D. beaker
Show answer
Answer: magnetMagnetism.
2. The liquid passing through a filter is the…
- A. filtrate
- B. residue
- C. sediment
- D. solute
Show answer
Answer: filtrateFiltration.
3. Winnowing uses wind.
- A. True
- B. False
Show answer
Answer: TrueChaff is light.
4. The solid left on a filter paper is the ______.
Show answer
Answer: residueFiltration.
5. Describe how to separate sand from water.
Show answer
Answer: Filter the mixture; the sand stays on the paper and water passes through.Filtration.
6. Why can't filter paper separate salt from salt water?
Show answer
Answer: Dissolved salt particles are small enough to pass through the holes in the paper.Particle size.

Lesson 2

Strand: Diversity Of Matter
Sub-strand: Separation of Mixtures
Indicator: B8.1.1.1.2 – Design and perform processes for separating kinds of mixtures

Introduction

Salt from Ada and Keta comes from sea water left to dry in the sun. Evaporation is one of the oldest methods of separation.

1. Evaporation

Evaporation separates a dissolved solid from a liquid. The solution is heated, or left in the sun, until the water turns to vapour and the solid remains. Salt winning in places like Ada and Keta works in this way. Sea water enters shallow pans, the sun evaporates the water and salt crystals remain. This is a major industry along Ghana's coast.

2. Crystallisation

Crystallisation produces neat crystals from a solution. Heat the solution until it is nearly saturated and leave it to cool slowly. Crystals form, and they can be filtered and dried. Slow cooling produces larger crystals. This method is used for sugar and for purifying many other substances.

Salt pans along the Ghanaian coast

Figure 2: Salt pans Draw the process of getting salt from sea water.

3. Distillation

Heat the solution in a flask.
The solvent evaporates and rises.
The vapour passes into a cooled tube called a condenser.
It turns back into a liquid.
The pure liquid, called the distillate, is collected.

Unlike evaporation, distillation saves both parts of the mixture. It can make pure water from sea water. Local gin, or akpeteshie, is made by distillation of fermented palm wine or sugar cane juice, because alcohol boils at a lower temperature than water.

4. Fractional Distillation

To separate several liquids with different boiling points, scientists use fractional distillation. At the Tema Oil Refinery, crude oil is heated and separated into products such as petrol, kerosene and diesel, each of which condenses at a different temperature. Air is also separated in this way to obtain oxygen and nitrogen after it has been cooled until it turns liquid.

Safety: Do not heat a closed flask, and do not carry out alcohol distillation in class. Alcohol vapour is flammable.

New words

evaporation: A liquid changing to vapour.
crystallisation: Forming crystals from a solution.
distillation: Evaporating and condensing to purify a liquid.
condenser: Apparatus that cools vapour into liquid.

Summary

Evaporation leaves a dissolved solid behind.
Crystallisation gives pure crystals.
Distillation collects a pure liquid, and fractional distillation separates liquids.

Class activity: Salt from Salt Water

Pour salty water into a shallow dish.
Leave it in the sun for two days.
Observe and draw the crystals.

You need: Salt-water mixture, Simple distillation apparatus, Collecting beaker

Exercises

Answer in your exercise book first, then check your answers.

1. Salt is obtained from sea water by…
- A. evaporation
- B. filtration
- C. sieving
- D. magnets
Show answer
Answer: evaporationSun.
2. Which method collects pure water from salt water?
- A. distillation
- B. sieving
- C. winnowing
- D. decantation
Show answer
Answer: distillationCondense.
3. Crude oil is separated by fractional distillation.
- A. True
- B. False
Show answer
Answer: TrueRefinery.
4. Apparatus that cools vapour to liquid is a ______.
Show answer
Answer: condenserDistillation.
5. How does distillation differ from evaporation?
Show answer
Answer: Distillation collects the liquid, while evaporation loses it to the air.Comparison.
6. Why does alcohol distil before water?
Show answer
Answer: It has a lower boiling point than water.Boiling point.

Lesson 3

Strand: Diversity Of Matter
Sub-strand: Separation of Mixtures
Indicator: B8.1.1.1.2 – Design and perform processes for separating kinds of mixtures

Introduction

Some mixtures need clever methods. A camphor ball shrinks without melting, and a drop of ink hides many colours.

1. Sublimation

Sublimation: A solid changing directly into a gas without becoming a liquid.

Camphor, iodine and ammonium chloride sublime. To separate camphor from sand, heat the mixture gently in a dish with an inverted funnel above it. The camphor turns to vapour, and it forms a solid on the cool funnel while sand stays behind. This is why mothballs in a wardrobe gradually disappear.

2. Separating Liquids

Liquids that do not mix, such as palm oil and water, are called immiscible. They can be separated with a separating funnel. The denser water sinks, and it is drained from the tap, leaving the oil. Liquids that do mix, such as water and alcohol, are called miscible, and they must be separated by distillation.

3. Chromatography

Draw a pencil line near the bottom of filter paper.
Put a small ink dot on the line.
Stand the paper in a little water, with the dot above the water.
Water moves up, carrying the dyes.
Different dyes travel different distances.

Chromatography separates dissolved substances according to how readily they move. It is used to test foods, medicines and evidence in criminal investigations.

Paper chromatography strips showing separated colours

Figure 3: Paper chromatography Try chromatography with a marker pen.

4. Designing a Separation

Plan a separation

Plan how to obtain sand, salt and iron filings from a mixture of all three.

Use a magnet to remove the iron filings.
Add water and stir to dissolve the salt.
Filter to remove the sand as residue.
Evaporate the filtrate to obtain salt.

Answer: Magnet, dissolving, filtration and evaporation in that order

Choosing a method

Mixture	Best method
palm oil and water	separating funnel
camphor and sand	sublimation
dyes in ink	chromatography
salt and water	evaporation
water and alcohol	distillation

New words

sublimation: Solid changing directly to gas.
immiscible: Not able to mix.
chromatography: Separating dissolved substances on paper.
separating funnel: Apparatus for separating immiscible liquids.

Summary

Sublimation separates substances that change straight to gas.
A separating funnel separates liquids that do not mix.
Chromatography separates dyes, and each mixture needs a suitable method.

Class activity: Ink Chromatography

Place black marker dots on strips of filter paper.
Stand them in water.
Compare the colours that appear.

You need: Camphor-and-sand mixture, Covered evaporating dish, Printed sublimation-sequence chart

Exercises

Answer in your exercise book first, then check your answers.

1. Which substance sublimes?
- A. camphor
- B. salt
- C. sand
- D. sugar
Show answer
Answer: camphorSolid to gas.
2. Palm oil and water are separated with a…
- A. separating funnel
- B. magnet
- C. sieve
- D. filter paper
Show answer
Answer: separating funnelImmiscible.
3. Chromatography can separate dyes in ink.
- A. True
- B. False
Show answer
Answer: TrueColours.
4. Liquids that do not mix are ______.
Show answer
Answer: immiscibleOil and water.
5. How would you separate camphor from sand?
Show answer
Answer: Heat gently; camphor sublimes and collects on a cool funnel.Sublimation.
6. Plan the separation of salt, sand and iron filings.
Show answer
Answer: Use a magnet, dissolve in water, filter and evaporate the filtrate.Steps.

Homework

Name the separation method used in five local activities.