Separation and Purification Techniques

Question 1

What are the steps to separating sodium chloride from a solid mixture of sodium chloride and sand.

Answer 1

Step 1: Dissolving
Add excess water to the mixture in a beaker. Stir well to dissolve the _______. (In this case sodium chloride)

Step 2:
Filter the mixture and collect the filtrate.

Step. 3:
Heat the filtrate in an evaporating dish until all the water evaporates, i.e. evaporating to dryness. The solid obtained is _________ (in this case: sodium chloride

Question 2

Evaporating to dryness is suitable for ______________.The solid obtained does not easily _______ when heated. Sodium chloride is one ________.

Answer 2

Evaporating to dryness is suitable for thermally stable compounds. The solid obtained does not easily decompose when heated. Sodium Chloride is one example.

Question 3

If given a mixture of solids A and B where A is soluble in water and B is not soluble in water, what would you to obtain both solids.

Answer 3

Step 1: Dissolving
Specific step (Need to know). Add excess water to the mixture in a beaker. Stir well to dissolve the sodium chloride.
(A solution containing A and B remains undissolved)

Step 2: Filtering
Specific step ( need to know): Filter the mixture and collect the filtrate.
A is collected as filtrate and B is collected as the residue.

Step 3:
Evaporating

Specific step (need to know) Heat the filtrate in evaporating dish until all the water evaporates, i.e. evaporating to dryness. The solid obtained is sodium chloride.
(Solid A is obtained)

To obtain dry solid B (residue)
Wash residue with distilled water as it may be containing solid A. Pat dry residue between pieces of filter paper.

Question 4

Decanting is only used if the solids are in ______ and do not _____ with the liquid during pouring out. Decanting is ________ used in chemistry procedures because usually the _______ is a _____ solid and would ________ flow out with the liquid during _________. Thus, filtration is _____, as all the solid _____ is trapped in the filer paper and collected.

Answer 4

Decanting is only used if the solids are in lumps and do not flow out with the liquid during pouring out. Decanting is seldom used in chemistry procedures because usually the precipitate formed is a powdery solid and would easily flow out with the liquid during decanting. Thus, filtration is preferred, as all the solid precipitate is trapped in the filter paper and collected.

Question 5

They are solid compounds, which on heating, would ______. These compounds are thermally _______. These solid compounds may exist as ______, containing water of ___________ that are easily expelled when heated, causing the crystal structure to _________.

Given a solution of the above kind of compounds, evaporating by ________ the solutions to dryness ______ produce the desired solid crystals. Strong heating will _______ the water to crystallisation. Thus, the method of crystallisation is required.

Answer 5

There are solid compounds, which on heating, would decompose. These compounds are thermally unstable. These solid compounds may exist as crystals, containing water of crystallisation that are easily expelled when heated, causing the crystal structure to crumble.

Given a solution of the above kind of compounds, evaporating by heating the solutions to dryness will not produce the desired solid crystals. Strong heating will drive off the water to crystallisation. Thus, the method of crystallisation is required.

Question 6

What are the steps to obtaining crystals of copper (II) sulfate from a solution of copper (II) sulfate?

Answer 6

Step 1:
Heat the copper (II) sulfate solution in the evaporating dish.

Step 2:
Stop heating when about one-third of the solution is left- a saturated solution is obtained. This means that it cannot be dissolved further.

Step 3:
Let the solution cool down to allow crystals to form. Cooling the solutions will decrease the solubility of copper (II) sulfate, causing crystals to form.

Step 4:
Filter the mixture to obtain the crystals on the filter paper, then air-dry them or pat dry them between pieces of filter paper.

Question 7

Why do we use heat to dryness for sodium chloride?

Answer 7

Sodium chloride does not decompose and its solubility remain constant at all temperatures, thus it is still suitable and appropriate to use heat to dryness.

Question 8

What happens to the solubility of most solids when a solution cools?

Answer 8

The solubility decreases, and the solution can no longer hold as much of the dissolved substance.

Question 9

Why does a less soluble substance crystallize out when the solution cools?

Answer 9

Because it has difficulty staying dissolved at lower temperatures, so the particles come closer together and form solid crystals.

Question 10

How do particles arrange themselves during crystallisation?

Answer 10

The particles arrange themselves in an orderly pattern to form solid crystals.

Question 11

What happens to impurities during crystallisation?

Answer 11

Impurities remain dissolved in the solution because they have different solubilities and do not crystallize as easily.

Question 12

Why does cooling favor the crystallization of the primary solute?

Answer 12

Cooling reduces solubility, which causes the main solute to solidify, while impurities stay in the liquid.

Question 13

Why is crystallisation used instead of heating to dryness for substances that are less soluble?

Answer 13

Crystallisation allows for the formation of pure crystals, while heating to dryness may decompose the substance or retain impurities.

Question 14

What happens to solute particles when a solution cools?

Answer 14

The solute particles lose energy, come closer together, and form a solid.

Question 15

Why do impurities not crystallize with the main solute?

Answer 15

Impurities usually have different solubilities and remain in the liquid as the solution cools.

Question 16

Some ______ substances sublime to become gaseous when heated, without __________, to form a _________. Examples of solids that are observed to sublime are ________ ______, ______, ___ _____ (_______) and __________.

Answer 16

Some substances sublime to become gaseous when heated, without melting to form a liquid. Examples of solids that are observed to sublime are ammonium chloride, iodine, dry ice (solid carbon dioxide) and naphthalene (mothball).

Question 17

What are the steps to separating a solid mixture of iodine (black solid) and sodium chloride (white solid)?

Answer 17

Use sublimation
Step 1: Place the solid mixture in an evaporating dish.

Step 2: Place an inverted filter funnel over the mixture.

Step 3: heat the dish using a Bunsen burner.

Solid iodine which is black, will sublime on heating to form a purple vapour. Upon contact with the cooler surface of the funnel, decomposition (gas to solid) takes place, i.e. the vapour changes to a solid.

Question 18

Draw the diagram for sublimation.

Answer 18

Check your answer in your worksheet : “ Notes- Separation and purification techniques”

Question 19

A solution comprises a ______ dissolved in a _________.
Simple distillation is used to ______ and collect the ______ from a solution.

Answer 19

A solution comprises a solute dissolved in a solvent.

Simple distillation is used to separate and collect the solvent from a solution.

Question 20

What are the steps to separating and collecting pure water from sea water by simple distillation?

Answer 20

Step 1:
Place the sea water with some boiling chips in a round bottom flask. Connect the flask to a condenser and attach a thermometer as show below.

Step 2: pump water into the condenser, to cool the condenser.

Step 3: heat the flask using a Bunsen burner to boil the mixture.

Question 21

Explain how pure water can be separated from sea water.

Answer 21

Need to check with Ms Low but I think sublimation should be incorrect

Use sublimation.

Steam is produced from the boiling mixture and rises. Upon entering the condenser, the steam undergoes condensation to form liquid water, which is collected as the distillate in the conical flask. The substance left in the flask is mainly salt.

Question 22

What is the purpose of boiling chips when separating and collecting the solvent from a solution by using simple distillation?

Answer 22

The chips allow for smoother boiling and prevents ‘bumping’ of the liquid.

Question 23

Why is the thermometer bulb placed at the position shown above when separating and collecting the solvent from a solution using simple distillation?

Answer 23

To measure the temperature of the vapour/gas when it leaves the flask (this is also the condensation temperature) and hence the boiling point of the distillate (i.e. the solvent collected). In the above example, the temperature should read 100 degrees celcius (as it is water in this case) as water is collected as the distillate.

Question 24

Why does the cooling water enter the bottom of the condenser rather than the top?

Answer 24

This makes the bottom of the condenser the coolest part, so that any vapour that may not have condensed initially would condense as it moves further down the condenser. Hence this prevents/reduces the loss of the vapour/gas.

Question 25

Fractional distillation is used to separate a __________ of ___________ ________ which have ________ _________ ____________.

Answer 25

Fractional distillation is used to separate a mixture of miscible liquids, which have different boiling points. (b.p.)

Question 26

What are the steps to separating a mixture of water (b.p. 100 degrees celcius) and ethanol (b.p. 78 degrees celcius)?

Answer 26

Step 1: Place the ethanol-water mixture with some boiling chips in a round bottom flask. Connect the flask to a fractionating column with a thermometer, and attach a condenser as shown above. Step 2: Pump water into the condenser to cool the condenser. Step 3: heat the mixture with an electric heater (not a Bunsen burner)

Question 27

Does ethanol has a higher or lower boiling point than water? What is ethanol’s boiling point?

Answer 27

Ethanol has a lower boiling point than water. Ethanol’s boiling point is 78 degrees celcius.

Question 28

Gaseous ethanol and steam are ______________ from the boiling mixture.

Answer 28

Gaseous ethanol and steam are both produced from the boiling mixture.

Question 29

Explain how a mixture of water and ethanol Can be separated.

Answer 29

Gaseous ethanol and steam are both produced from the boiling mixture. The vapours rise through the fractionating column that is packed with glass beads. At the cooler parts of the column, steam condenses into water which drips back into the flask, since water has a higher boiling point than ethanol. The ethanol vapour reaches the top of the column, passes into the condenser and undergoes condensation. Liquid ethanol is collected as the distillate.

Question 30

Why is an electric heater, instead of a Bunsen burner, used to heat the mixture of ethanol and water for the fractional distillation?

Answer 30

Ethanol is flammable and may catch fire when exposed to an open flame.

Question 31

Why is the fractionating column packed with glass beads?

Answer 31

The beads increases the surface areas for the condensation of vapour, making separation of the two vapours more efficient. (Refer to note: explaining fractional distillation)

Question 32

What would be the readings of the thermometer as the fractional distillation takes place for a mixture of water and ethanol?

Answer 32

The thermometer first records the rising temperature, and eventually stays at 78 degrees celcius, the temperature of the ethanol vapour, or the boiling point of the liquid (fraction) with the lower b.p. After all the ethanol vapour has entered the condenser, the thermometer will then record the temperature of stream, 100 degrees celcius,

Question 33

What happens during fractional distillation of ethanol and water, and what are the purity considerations?

Answer 33

Fractional distillation separates ethanol (boiling point 78.4°C) from water (boiling point 100°C) by heating the mixture. Ethanol vaporizes first, but because ethanol and water are miscible, the distillate (ethanol) often contains traces of water. This makes ethanol less than 100% pure. The residue in the flask becomes richer in water, which can be considered purer. To further purify ethanol, additional steps like chemical drying or azeotropic distillation are needed. (these statement is not tested)

Question 34

When two liquids, such as water and oil, do not mix together, but form separate layers, the liquids are said to be ___________. A __________________ is used to separate such liquids.

Answer 34

When two liquids, such as water and oil, do not mix together, but form separate layers, the liquids are said to be immiscible. A separating funnel is used to separate such liquids.

Question 35

How can immiscible liquids be separated using a separating funnel?

Answer 35

Turning on the tap allows the bottom liquid to be dispensed first. The tap is quickly closed before the top liquid can escape, thus separating the 2 liquids.

Question 36

In a separating funnel containing water and oil, which one is below and why?

Answer 36

In a separating funnel, water is below oil in the separating funnel because water has a high density than oil. Hence water sinks below oil.

Question 37

A pure substance has a ______, ____ melting point and boiling point.

Answer 37

A pure substance has a fixed, constant melting and boiling point.

Question 38

The presence of impurity _________ the melting point of the substance and causes it to ___________________________________________.

Answer 38

The presence of impurity decreases the melting point of the substance and causes it to melt over the range of temperatures.

Question 39

The presence of impurity _________ the boiling point of the substance and causes it to _________________________

Answer 39

The presence of impurity increases the boiling point of the substance and causes it to boil over a range of temperatures.

Question 40

Being able to measure the purity of substances such as drugs (e.g. Panadol) and food additives (e.g. food dyes) is very important. Why?

Answer 40

It does not need to be this precise but you should have the general idea: Measuring the purity of substances like drugs (e.g., Panadol) and food additives (e.g., food dyes) is crucial in chemistry because impurities can significantly alter the intended chemical properties, efficacy, and safety of these substances. In pharmaceuticals, even trace amounts of impurities can reduce the effectiveness of a drug or cause harmful side effects, compromising patient health. For food additives, impurities may lead to toxicity, allergic reactions, or undesired changes in taste, color, or texture. Chemically, purity ensures that the substance behaves predictably in reactions and formulations, maintaining consistency and quality. Analytical techniques like chromatography and spectroscopy are employed to quantify purity, ensuring compliance with regulatory standards and safeguarding public health. Thus, purity measurement is fundamental to both chemical integrity and practical applications in medicine and food science.

Question 41

Which test could be used to show that a sample of water is pure? A It freezes at exactly 0 degrees celcius B It has no effect on red or blue litmus paper. C It turns anhydrous copper (II) sulfate blue. D When it evaporates it leaves no residue.

Answer 41

A B is incorrect because if it has no effect on red or blue litmus paper, it means that it is neutral but it does not tell you anything about purity. C is incorrect because the test involving anhydrous copper(II) sulfate turning blue only indicates the presence of water (hydration of the salt), not its purity. Anhydrous copper(II) sulfate is a white powder that turns blue in the presence of water, but this reaction occurs with any water, whether pure or impure. Therefore, it cannot be used to determine the purity of water, as it does not distinguish between pure water and water containing dissolved substances. D is incorrect because ethanol also evaporate without leaving residue.

Question 42

The diagram shows apparatus used to separate hexane (boiling point 70 degrees celsius) and heptane (boiling point 98 degrees celsius). Which graph (in this case, I can only give a general description) would be obtained if the temperature at point T was plotted against the total volume of distillate collected? A from 70 degree celsius (maintain at 70) and then suddenly increases to 98 and maintains at 98 B starts from 98 degrees celsius and then suddenly decreases or drop to 70 degrees celsius and maintains at 70 degrees celsius C starts from 70 degree celsius and then increase gradually to 98degrees celcius D starts from 98 degree celsius and then gradually decreases to 70 degrees celsius

Answer 42

A Explanation: Hexane (boiling point = 70°C) distills first: When the mixture is heated, hexane, which has a lower boiling point, will vaporize first. The temperature at point T will remain constant at 70°C while hexane is being distilled. This is because the heat energy is being used to vaporize hexane, not to increase the temperature. Heptane (boiling point = 98°C) distills next: Once most of the hexane has been distilled, the temperature will rise because heptane, which has a higher boiling point, begins to vaporize. The temperature will then remain constant at 98°C (not 100°C, as stated in the question, but close to it) while heptane is being distilled. Why the graph matches A: The temperature starts at 70°C and remains constant while hexane is being collected. After hexane is mostly distilled, the temperature suddenly increases to the boiling point of heptane (98°C) and remains constant again while heptane is being collected. Why the other options are incorrect: B: The temperature cannot start at 100°C and then drop to 70°C. This is the opposite of what happens during distillation. C: The temperature does not increase gradually. It remains constant at each boiling point during the distillation of each component. D: The temperature cannot start at 100°C and gradually decrease to 70°C. This is not how distillation works.

Question 43

What separation technique should be used to obtain pure sugar from a sugar solution?

Answer 43

Crystallisation

Question 44

What separation technique should be used to separate petrol from crude oil?

Answer 44

Fractional distillation

Question 45

What separation technique should be used to remove leaves from a swimming pool?

Answer 45

Filtration

Question 46

What separation technique should be used to obtain drinking water from muddy water?

Answer 46

Distillation

Question 47

What separation technique should be used to separate olive oil from water?

Answer 47

Use a separating funnel.

Question 48

Why do we need to increase the temperature gradually if I want to determine the melting point of a solid?

Answer 48

This allows the melting point to be obtained accurately.

Question 49

How does the melting point of a solid show that the solid is impure?

Answer 49

If the solid melts over a range of temperatures or if it melts at a temperature lower than its melting point (if the melting point is known), then it is impure.

Question 50

When a mixture of two miscible liquids have different _______ points is heated in a flask till it boils, the vapour produced will ______up the fractionating column and contain a higher ___________ of the substance having the _____ boiling point (which is the more volatile liquid). When the vapour encounter a cooler surface in the column it __________ into liquid. Rising hot vapours will re-boil this liquid and the vapour formed will contain an even ________ proportion of the more volatile component. Any liquid along the column will fall back to the flask to be re-boiled. As the vapours rise up the fractionating column, the ________________________ ultimately cause the more volatile component to be obtained as the ________. The less volatile component is obtained as the residue in the flask where the original mixture was.

Answer 50

When a mixture of two miscible liquids have different boiling points is heated in a flask till it boils, the vapour produced will rise up the fractionating column and contain a higher proportion of the substance having the lower boiling point (which is the more volatile liquid). When the vapour encounter a cooler surface in the column it condenses into liquid. Rising hot vapours will re-boil this liquid and the vapour formed will contain an even higher proportion of the more volatile component. Any liquid along the column will fall back to the flask to be re-boiled. As the vapours rise up the fractionating column, the repeated re-boiling and re-condensation ultimately cause the more volatile component to be obtained as the distillate. The less volatile component is obtained as the residue in the flask where the original mixture was.

Question 51

To facilitate separation of the components, a sufficiently _________ column is used. Instead of a hollow column, one that is ____________________ is used. The structures may be ____________ or ____________ to ______________. This facilitates_______________________________>

Answer 51

To facilitate separation of the components, a sufficiently long column is used. Instead of a hollow column, one that is structure is used. The structures may be glass beads or protrusions to increase the surface area of contact by the vapour. This facilitates condensation and subsequent re-boiling of the liquid that formed.

Question 52

What is the precision of the pipette (how many d.p.)?

Answer 52

1 decimal place

Question 53

What is the precision of beretta (how many d.p.)?

Answer 53

2 decimal place (second d.p. Is 0 or 5

Question 54

For hydrogen, what is the solubility in water, density relative to air and collection method?

Answer 54

The solubility in water of hydrogen is not soluble. Hydrogen is less dense relative to air. The collection method for hydrogen is displacement of water.

Question 55

For carbon dioxide, what is the solubility in water, density relative to air and collection method?

Answer 55

Carbon dioxide is slightly soluble in water. Carbon dioxide is denser than air. The collection method for carbon dioxide is the displacement of water.

Question 56

For oxygen, what is the solubility in water, density relative to air and collection method?

Answer 56

Oxygen is slightly soluble in water. Oxygen is slightly denser than air. The collection method for oxygen is displacement of water.

Question 57

For chlorine, what is the solubility in water, density relative to air and collection method?

Answer 57

Chlorine is soluble in water. Chlorine is denser than air, The collection method for chlorine is downward delivery.

Question 58

For ammonia, what is the solubility in water, density relative to air and collection method?

Answer 58

Ammonia is very soluble in water. Ammonia is less dense than air. The collection method for ammonia is upward delivery.

Question 59

For sulfur dioxide, what is the solubility in water, density relative to air and collection method?

Answer 59

Sulfur dioxide is very soluble in water. Sulfur dioxide is denser than air The collection method for sulfur dioxide is downward delivery.

Question 60

Which of the following mixtures cannot be separated by performing the following separation techniques in sequence: dissolving in water followed by filtration? You may choose more than one answer. (A) Iron filings and sulfur powder. (B) Sand and salt. (C) Sugar and salt.

Answer 60

A is correct because both iron filings and sulfur powder are insoluble in water and hence cannot be separated with this proposed sequence as filtration will not be able to separate these two substances from each other. C is correct because both sugar and salt are soluble in water to form a solution. Subsequent filtration cannot separate them from each other.

Question 61

A substance is condensed using a water condenser. What are the melting point and boiling point of the substance? A melting point/ degrees celcius (-115) boiling point/ degrees celcius (-85) B melting point/ degrees celcius (-16.6) boiling point/degrees celcius (9.5) C melting point/degree celcius (-73) boiling point/degrees celcius (-10) D melting point/degrees celcius (-130) boiling point/degrees celcius (36)

Answer 61

D The temperature of water is typically 20-25 degrees and substances with a boiling point of above 25 degrees will be a gas but at a temperature lower than 25 degrees, the substance will condense. Since D is the only option with boiling point above 20-25 degrees, it means that D is the melting point and the boiling point of the substance.

Question 62

Camphor is a substance used in skin oinments. A natural sample of it is found to contain impurities including iron (III) oxide, iron and sodium chloride. The effects of these substances with three different types of liquids are shown below. camphor (substance): no effect (water), dissolves (ethanol) , no effect (dilute nitric acid) iron (substance): no effect (water), no effect (ethanol), reacts to give a solution (dilute nitric acid) iron (III) oxide (substance): no effect (water), no effect (ethanol), reacts to give a solution (dilute nitric acid) sodium chloride (substance): dissolves (water), no effect (ethanol), dissolves (dilute nitric acid) Which is the best method to obtain pure camphor? A Add dilute nitric acid, filter and crystallise. B Add dilute nitric acid, filter, rinse and dry. C Add ethanol, filter, rinse and dry. D Add water, filter, add dilute nitric acid to filtrate and crystallise.

Answer 62

B Analysis of Each Option Option C: Add ethanol, filter, rinse, and dry Process: Ethanol dissolves camphor, leaving Fe, Fe₂O₃, and NaCl as residue. Filtering separates the ethanol solution (filtrate containing camphor) from the residue (impurities). The method instructs to rinse and dry the residue (impurities), discarding the filtrate (camphor in ethanol). Why It Fails: Camphor is lost in the filtrate. The steps do not include recovering camphor by evaporating ethanol. Final product: Impurities (Fe, Fe₂O₃, NaCl), not camphor. Option D: Add water, filter, add nitric acid to filtrate, and crystallize Process: Water dissolves NaCl, leaving camphor, Fe, and Fe₂O₃ as residue. Filtering separates the NaCl solution (filtrate) from the residue (camphor + Fe + Fe₂O₃). Adding nitric acid to the filtrate (NaCl solution) does nothing to address the Fe/Fe₂O₃ in the residue. Why It Fails: Fe and Fe₂O₃ remain mixed with camphor in the residue. Final product: Impure camphor contaminated with Fe and Fe₂O₃. Option B: Add nitric acid, filter, rinse, and dry Process: Nitric acid dissolves Fe, Fe₂O₃, and NaCl, leaving camphor undissolved. Filtering removes dissolved impurities (filtrate), leaving camphor as residue. Rinsing removes residual nitric acid; drying yields pure camphor. Why It Works: All impurities are dissolved and removed. Camphor remains intact and pure. Option A: Add nitric acid, filter, and crystallize Process: Similar to B, but "crystallize" is redundant. Camphor is not dissolved, so crystallizing the filtrate (impurities) is unnecessary. Why It Fails: Crystallization is irrelevant and adds no value. Why Evaporation/Crystallization Isn’t Used in Option C If ethanol were evaporated from the filtrate, camphor could be recovered. However, the question specifies the steps in the options. Option C does not include evaporation/crystallization, so camphor remains in the discarded filtrate. Final Answer Option B is correct because: Nitric acid dissolves all impurities (Fe, Fe₂O₃, NaCl). Camphor remains undissolved and is isolated via filtration. Rinsing and drying ensure pure camphor. Other options fail due to: C: Discarding camphor in the filtrate. D: Leaving Fe/Fe₂O₃ unaddressed. A: Redundant crystallization step. This systematic approach ensures complete impurity removal while preserving camphor.

Question 63

The table below shows some information about the solubilities of three solids; Solid P insoluble (solubility of water), soluble (solubility in ethanol) solid Q soluble (solubility in water), insoluble (solubility in ethanol) Solid R insoluble (solubility in water), insoluble (solubility in ethanol) It is given that the solubility of Q in water is not affected significantly by change in temperature. List down the steps to be carried out so to obtain pure solid Q from a mixture of P, Q and R.

Answer 63

1. Add water to the mixture of P, Q and R, STIR. 2. Filter the mixture and Q is obtained as the filtrate. 3. Evaporate filtrate to dryness to obtain pure solid Q. Notice that you need to specify what happens to the substance that you want to collect in each step. Remember to be precise and include keyword such as stir.