Separating mixtures

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Catatan pelajaran

Big idea: separating what has been mixed

Big idea (key concept): Change. A mixture can be changed back into its separate parts, because nothing has been chemically joined.
Related concept: Function. Each separation method has a job: it works because it uses one property that is different between the substances.
Global context: Globalization and sustainability. Clean drinking water, salt, medicines and metals all depend on separating mixtures, and many places use distillation to get fresh water from sea water.
A mixture contains two or more substances that are not chemically joined. Each substance keeps its own properties, so a mixture can be separated by physical methods.
A pure substance is one substance only, with nothing else mixed in. Pure water is just water.
When a solid dissolves in a liquid it makes a solution. The solid is the solute and the liquid is the solvent. A solid that does not dissolve is insoluble.
To choose a method, ask: what is different about the substances? Size of pieces, solubility, boiling point and magnetism can all be used.

Filtration, evaporation and crystallisation

Filtration separates an insoluble solid from a liquid. The mixture is poured through filter paper in a funnel. The paper has tiny holes: liquid particles pass through, but pieces of insoluble solid are too big.
The solid left on the paper is the residue. The liquid that drips through is the filtrate. Sand in water separates this way.
Filtration cannot separate a dissolved solid. Dissolved salt particles are tiny and pass through the paper with the water.
To get a dissolved solid back, use evaporation. Heating the solution gives the water particles enough energy to escape as vapour. The solute stays behind.
Crystallisation gives better, larger crystals: the teacher heats the solution gently to evaporate some of the water, then leaves it to cool slowly. Crystals form, and they are filtered off and dried.
A teacher stops heating before the dish is dry. A hot solid left to dry out can spit, so the last of the water is left to evaporate slowly.
Safety: eye protection is worn, the teacher does the heating, and hot equipment is left to cool before anyone touches it.

Filtration: residue and filtrate

Simple distillation: getting the solvent back

Simple distillation collects the solvent from a solution, for example pure water from salty water. It uses the fact that water boils but salt does not.
The solution is heated in a flask until it boils. The solvent turns to vapour and leaves through the side arm. The salt stays in the flask.
The vapour passes into a condenser, a tube surrounded by a jacket of cold flowing water. Cooling water goes in at the lower end and out at the top, so the jacket stays full.
In the cold tube the vapour condenses to liquid and drips into a beaker. The pure liquid collected is called the distillate.
A thermometer at the side arm reads the temperature of the vapour. For pure water it should stay at about 100 °C while water is coming over.
Distillation changes state twice: liquid to gas (boiling), then gas to liquid (condensing). No new substance is made, so it is a physical change.
Distillation is always teacher-run or done with school safety rules: the glass is hot, so eye protection is worn and nobody touches the apparatus.

Simple distillation of salty water

Chromatography: separating dissolved colours

Paper chromatography separates dissolved substances, such as the dyes in an ink. It is useful when the substances are all soluble, so filtering cannot help.
Draw a start line in pencil near the bottom of the paper. Pencil lead does not dissolve, so it will not travel with the solvent. Put a small spot of each ink on the line.
Stand the paper in a beaker with a little solvent (often water) that is below the pencil line. If the line were under the solvent, the ink would wash away into the beaker.
The solvent soaks up the paper and carries the dyes with it. Each dye moves at its own speed, depending on how well it dissolves in the solvent and how strongly it sticks to the paper.
Take the paper out when the solvent is near the top and mark the solvent front straight away. The finished paper is a chromatogram.
Each separate spot is a different substance. A pure substance gives one spot. A mixture gives two or more.
Reference spots of known dyes are run on the same paper. A spot in the ink that lines up with a reference spot is probably that dye.

Paper chromatography in three steps

Rf values, purity and choosing a method

The Rf value (retention factor) describes how far a spot travels compared with the solvent: Rf = distance moved by the spot / distance moved by the solvent front. Both distances are measured from the pencil line.
Worked example: the solvent front is 8.0 cm above the line and a spot is 5.6 cm above it, so Rf = 5.6 / 8.0 = 0.70. An Rf is never more than 1, because a spot cannot overtake the solvent.
The same substance gives the same Rf on the same paper with the same solvent, so Rf values can be compared with a table to identify a substance.
Purity can be tested by heating. A pure substance has a sharp melting or boiling point: pure water boils at 100 °C and ice melts at 0 °C. A mixture melts or boils over a range. Dissolved impurities usually lower a melting point and raise a boiling point.
Fractional distillation separates liquids that mix together, such as water and ethanol. A fractionating column above the flask lets the vapour rise and condense again and again. Ethanol boils at 78 °C, lower than water at 100 °C, so it reaches the top and is collected first.
Choosing a method: insoluble solid in liquid, use filtration. Dissolved solid, use evaporation or crystallisation. Pure solvent from a solution, use simple distillation. Liquids with different boiling points, use fractional distillation. Dissolved coloured substances, use chromatography.

Fractional distillation of a water and ethanol mixture

Think like a scientist: do all black pens contain the same dyes?

The question: do three brands of black pen (A, B and C) give the same chromatogram? Black is often a mixture of several coloured dyes.
The independent variable is the brand of pen. The dependent variable is the pattern of spots: how many there are and their Rf values.
Control variables to keep the same: the type of paper, the solvent, the height of the pencil line, the size of each ink spot, and how long the paper stays in the solvent.
Method (safe school practical): draw a pencil line, add a small spot of each pen, stand the paper in a little water below the line, remove it when the solvent is near the top and mark the solvent front at once.
Results: measure each spot and the solvent front in millimetres from the line, then calculate each Rf value and record them in a table.
Evaluate: is one spot enough to say a pen is pure? A second solvent could split it. Repeating the test makes the results more reliable.
Your task: write a plan to test whether the same pen gives the same Rf in two different solvents. State the variable you change, the variable you measure and three things you will keep the same.

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Soal latihan

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1.Which statement about a mixture is correct?
Easy
- AIts substances are chemically joined in a fixed ratio each time
- BIts substances are not chemically joined, so physical methods can separate them
- CIt always melts or boils at one single sharp temperature
- DIt can only be separated by chemical reactions such as burning
2.Sand mixed with water is a mixture.
Easy
True or false?
3.Which method would separate sand from a mixture of sand and water?
Easy
- ASimple distillation
- BFiltration
- CChromatography
- DCrystallisation
4.Complete the sentence about filtration.
Easy
The solid left on the filter paper is called the ____ and the liquid that passes through is called the ____.
5.Why does filtration work for a mixture of sand and water?
Easy
- ASand dissolves in water and passes through the paper
- BThe paper is chemically attracted to the sand
- CSand pieces are too big to fit through the tiny holes in the paper
- DWater particles are too big for the holes in the paper
6.In salt solution, what is the salt called?
Easy
- AThe solvent
- BThe solute
- CThe filtrate
- DThe residue
7.In salt solution, which substance is the solvent?
Easy
- ASalt
- BSand
- CAir
- DWater
8.Filtration can separate salt that has fully dissolved in water.
Easy
True or false?

Soal lainnya

9.Which method is used to get a dissolved solid, such as salt, back from a solution?
Easy
- AFiltration
- BEvaporation
- CUsing a magnet
- DDecanting
10.Which method would you use to collect pure water from salty water?
Easy
- AFiltration
- BChromatography
- CDecanting
- DSimple distillation
11.What does paper chromatography separate?
Easy
- ASand that is mixed into water
- BSalt that has dissolved in water
- CDifferent coloured dyes dissolved in an ink
- DIron filings that are mixed with sand
12.Why is the start line in chromatography drawn in pencil and not in pen?
Easy
- APen ink is too light to see on paper
- BPen ink would not dissolve so it would stay still
- CPencil makes the paper stick to the beaker
- DPencil lead does not dissolve, so it will not run up the paper
13.Why must the solvent start below the pencil line?
Easy
- ASo the paper stays completely dry
- BOtherwise the ink would wash off into the beaker
- CSo the solvent cannot climb the paper
- DSo the paper is easier to lift out
14.In chromatography, the solvent moves up the paper and carries the dissolved substances with it.
Easy
True or false?
15.Complete the sentence.
Easy
Chromatography separates substances that are ____ in a solvent.
16.A pure substance melts or boils at one sharp temperature.
Easy
True or false?
17.A teacher is heating a liquid in a practical. What should you wear?
Easy
- ASunglasses
- BEye protection
- COven gloves only
- DNothing, because water is safe
18.An evaporating dish has just been heated. What should you do?
Easy
- APick it up to check how hot it is
- BRinse it under cold water straight away
- CPut it in your bag to take home
- DLeave it to cool and do not touch it
19.Which method of separation relies on a difference in boiling points?
Easy
- AFiltration
- BChromatography
- CDistillation
- DUsing a magnet
20.Salt is dissolved in water. Why does the salt pass through filter paper?
Medium
- ASalt is a liquid at room temperature
- BThe paper dissolves the salt as it passes
- CThe dissolved salt particles are tiny and go through with the water
- DSalt is much heavier than water so it falls through
21.What happens to the solvent during evaporation?
Medium
- AIt turns into salt as it is heated
- BIt turns into vapour and escapes, leaving the solute behind
- CIt sinks to the bottom of the dish and stays there
- DIt reacts with the solute to make a new substance
22.Why does a teacher stop heating before all the water has gone when making crystals?
Medium
- AThe salt would evaporate away with all of the water
- BThe dish would start to dissolve into the solution
- CSlow cooling gives better crystals, and a hot dry solid can spit
- DThe filter paper would stop working properly afterwards
23.Put the steps for making salt crystals from salt solution in order.
Medium
- Leave the concentrated solution to cool so crystals form
- Heat the solution gently to evaporate some of the water
- Dry the crystals between sheets of filter paper
- Filter the crystals out of the leftover solution
24.In simple distillation, what is the job of the condenser?
Medium
- ATo heat the liquid to its boiling point
- BTo filter out insoluble solids
- CTo cool the vapour so it turns back into a liquid
- DTo measure the temperature
25.At the end of simple distillation of salty water, what is left in the flask?
Medium
- APure water that has been cooled
- BConcentrated salt solution or salt
- CSand left over from the mixture
- DSteam trapped inside the flask
26.Put these stages of simple distillation in order.
Medium
- The vapour travels into the condenser
- The salty water is heated until it boils
- The vapour cools and condenses to a liquid
- Pure water drips into the collecting beaker
27.Which changes of state happen during simple distillation? (select all that apply)
Medium
- ABoiling
- BCondensation
- CMelting
- DFreezing
- ESublimation
28.In a condenser, the cooling water enters at the top and leaves at the bottom.
Medium
True or false?
29.Why does the cooling water enter a condenser at the lower end?
Medium
- ASo the water can leave the jacket faster
- BSo the jacket stays full of water and cools the whole tube
- CSo the vapour is pushed back towards the flask
- DSo the cooling water heats up as quickly as possible
30.Why do different dyes travel different distances up the paper?
Medium
- AThey start at slightly different temperatures
- BHeavier dyes always travel further than light dyes
- CThe paper is thicker in some places than in others
- DThey differ in how well they dissolve and how strongly they stick
31.A chromatogram of an ink shows three separate coloured spots. What does this tell you?
Medium
- AThe ink is a pure substance
- BThe ink is a mixture of at least three dyes
- CThe ink contains three solvents
- DThe ink is one dye in three forms
32.A pure dye is tested by chromatography. How many spots do you expect?
Medium
- AOne
- BTwo
- CThree
- DNone
33.Match each part of a chromatography set-up to its job.
Medium
- Pencil line
- Solvent
- Chromatography paper
- Solvent front
- Marks where the ink samples are placed
- Carries the dissolved substances upwards
- Holds the spots as they separate
- Shows how far the solvent travelled
34.In a chromatography experiment a spot moved 3.0 cm and the solvent front moved 10.0 cm from the pencil line. Rf = distance moved by the spot / distance moved by the solvent front. What is the Rf value?
Medium
35.A spot moved 4.5 cm and the solvent front moved 9.0 cm. Use Rf = distance moved by the spot / distance moved by the solvent front. What is the Rf value?
Medium
36.Why is the Rf value useful when you compare chromatograms made on different days?
Medium
- AIt is always exactly 1 for a pure dye
- BIt removes the need to use a solvent at all
- CIt tells you the boiling point of the dye used
- DIt does not depend on how far the solvent went, so runs can be compared
37.Pure water boils at 100 °C. A liquid boils over a range from 101 °C to 106 °C. What does this suggest?
Medium
- AThe liquid is pure water that was heated too fast
- BThe liquid is pure ethanol that was heated too slowly
- CThe liquid is a mixture, probably with something dissolved in it
- DThe thermometer must be broken whatever the liquid is
38.How do impurities usually change the melting point of a solid?
Medium
- AThey make it lower and spread over a range
- BThey make it higher and sharper
- CThey make it exactly 0 °C
- DThey do not change it
39.Which of these are mixtures? (select all that apply)
Medium
- ASea water
- BPure distilled water
- CAir
- DInk
- EOxygen gas
40.A mixture contains sand, salt and water. Which plan separates all three?
Medium
- AEvaporate all the water first, then filter out the sand
- BUse chromatography first, then filter the paper
- CFilter out the sand, then evaporate the filtrate for the salt
- DDistil the water first, then use a magnet on the rest
41.Iron filings are mixed with sand. Which property gives the quickest way to separate them?
Medium
- AIron dissolves in water and sand does not
- BSand has a lower boiling point
- CSand is a liquid
- DIron is magnetic and sand is not
42.A student wants to know whether different black pens give the same chromatogram. Which is the independent variable?
Medium
- AThe distance the solvent travels
- BThe size of the paper
- CThe brand of pen
- DThe solvent used
43.For a fair test of three pens, which things should be kept the same?
Medium
- AA different paper for each pen
- BThe same paper, solvent, start line height and time
- CA different solvent for each pen
- DA different start line height for each pen
44.Water boils at 100 °C and ethanol boils at 78 °C. How many degrees Celsius lower is the boiling point of ethanol?
Medium
45.A mixture of water (boils at 100 °C) and ethanol (boils at 78 °C) is heated in fractional distillation. Which liquid is collected first, and why?
Hard
- AWater, because there is more of it
- BBoth together, because they are mixed
- CEthanol, because it has the lower boiling point
- DNeither, because they react
46.In distillation the thermometer bulb is placed level with the side arm of the flask. Why?
Hard
- AIt heats the vapour so that it can leave the flask
- BIt stops the liquid from boiling over the top
- CIt reads the temperature of the vapour entering the condenser
- DIt cools the flask down once the liquid has boiled
47.A spot moved 6.4 cm and the solvent front moved 8.0 cm from the pencil line. Use Rf = distance moved by the spot / distance moved by the solvent front. What is the Rf value?
Hard
48.A chromatogram has the solvent front 8.0 cm above the pencil line. Spot A is 2.0 cm above it, spot B is 5.6 cm and spot C is 6.4 cm. What is the Rf value of spot B?
Hard
- A0.25
- B0.70
- C0.80
- D1.40
49.Why can an Rf value never be greater than 1?
Hard
- AThe paper is only ever 1 cm long in practice
- BRf is always measured in centimetres, up to 1 cm
- CThe solvent always moves exactly 1 cm up the paper
- DA spot can never travel further than the solvent front
50.A brown ink gives spots with Rf values 0.25, 0.70 and 0.80. Four pure dyes made on the same paper with the same solvent have Rf values: dye P = 0.25, dye Q = 0.70, dye R = 0.50, dye S = 0.80. Which dye is NOT in the ink?
Hard
- ADye P
- BDye Q
- CDye S
- DDye R
51.A student says: "Pen B made only one spot, so it must be a pure dye." Which comment evaluates this claim best?
Hard
- ACorrect: one spot always means a pure substance
- BWrong: pure substances never give a spot
- CCorrect: one spot means the pen is empty
- DNot certain: a different solvent might split it into more spots
52.Explain how you would obtain pure water from salty water. Name the method and say what happens to the water.
Hard
53.Describe how you would separate a mixture of sand and salt.
Hard
54.Complete the description of simple distillation.
Hard
The liquid is heated until it ____, the vapour is cooled in the ____, and the pure liquid that is collected is called the ____.

Catatan ulasan

## The big idea - A **mixture** is not chemically joined, so **physical methods** can separate it (**Change**, **Function**). - Choose a method by a **property that differs**. ## The methods - **Filtration**: insoluble solid (**residue**) from liquid (**filtrate**). - **Evaporation / crystallisation**: dissolved solid from a solution. - **Simple distillation**: pure solvent from a solution; boil, then **condense**. - **Fractional distillation**: liquids with different boiling points. - **Chromatography**: dissolved coloured substances. ## Numbers and purity - **Rf = distance moved by spot / distance moved by solvent front** (never more than 1). - Pure substances have **sharp** melting and boiling points; mixtures have a **range**. - Water boils at 100 °C, ethanol at 78 °C. ## Practical sense - Eye protection, teacher-run heating, let hot equipment cool. - Fair test: change one variable, keep the rest the same.

Mixture: Two or more substances that are not chemically joined and can be separated by physical methods.
Pure substance: One substance only, with nothing else mixed in.
Solute: The substance that dissolves in a solution.
Solvent: The liquid that a solute dissolves in.
Insoluble: Does not dissolve in a given solvent.
Filtration: Separating an insoluble solid from a liquid using filter paper.
Residue: The solid left on the filter paper.
Filtrate: The liquid that passes through the filter paper.
Crystallisation: Forming solid crystals from a solution by evaporating some solvent and cooling.
Simple distillation: Boiling a solution and condensing the vapour to collect the solvent.
Condenser: A cooled tube where vapour turns back into a liquid.
Fractional distillation: Separating liquids with different boiling points using a fractionating column.
Chromatography: Separating dissolved substances as a solvent moves up paper.
Rf value: Distance moved by the spot divided by distance moved by the solvent front.

Kartu flash

What is a mixture?

Two or more substances that are not chemically joined. They can be separated by physical methods.

What is a solute, and what is a solvent?

The solute is the substance that dissolves; the solvent is the liquid it dissolves in.

Which method separates an insoluble solid from a liquid?

Filtration.

What are the residue and the filtrate?

The residue is the solid left on the paper; the filtrate is the liquid that passes through.

Why can't filtration separate dissolved salt from water?

Dissolved salt particles are tiny and pass through the filter paper with the water.

How do you get salt crystals from salt solution?

Gently evaporate some water, leave the solution to cool so crystals form, then filter and dry them.

Which method gives pure water from salty water?

Simple distillation: boil the water, condense the vapour and collect it.

Which way does cooling water flow through a condenser?

In at the lower end and out at the top, so the jacket stays full.

What is the distillate?

The pure liquid that is collected after condensing.

When is fractional distillation used?

To separate liquids that mix, using their different boiling points, for example ethanol and water.

Why is the chromatography start line drawn in pencil?

Pencil lead does not dissolve, so it does not run up the paper.

What does one spot on a chromatogram suggest?

A pure substance. Two or more spots mean a mixture.

What is the formula for the Rf value?

Rf = distance moved by the spot / distance moved by the solvent front.

A spot moved 3.0 cm and the solvent front moved 10.0 cm. What is the Rf?

3.0 / 10.0 = 0.30.

How do you tell if a substance is pure by heating it?

A pure substance melts or boils at one sharp temperature. A mixture does so over a range.

What temperature does pure water boil at, and ethanol?

Water boils at 100 °C and ethanol at 78 °C.

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