Unit 4: Separation Techniques

Question 1

What is a pure substance? (2 points)

Answer 1

• A pure substance is made up of one type of element or compound.
• It is not mixed with any other substance.

Question 2

How do we deduce the purity of a substance?

Answer 2

• A pure substance has a fixed melting and boiling points at 1 atmospheric pressure.
• For example, at 1 atmospheric pressure,
• Ice melts at 0 °C.
• Magnesium melts at 650°C.
• Pure water boils at 100 °C.
• Ethanol boils at 78 °C.

Question 3

How do we deduce the identity of a substance? (2 points, 2 sub-points)

Answer 3

• The melting and boiling points and density of pure substances are unique.
• What it means is there is no two pure substances with the same melting point. There is also no two pure substances with the same boiling point.
• E.g. if the melting point of a substance is 0 °C and the boiling point of the substance is 100 °C, then it must be pure water.
• However, there is a possibility that an impure substance may have the same melting point as the pure substance.

Question 4

What is the effect of impurities on melting point?

3 points, 1 sub-point

Answer 4

What is the effect of impurities on melting point?
• Impurities lower the melting point of a substance.
• E.g. impure ice will start to melt at temperature below 0 °C.
• The greater the amount of impurities, the lower the melting point of the substance.
• Impurities cause melting to take place over a range of temperatures.

Question 5

Why and when do we need pure substances? (2 examples, optional)

Answer 5

Example 1
When? Production of medicines
Why? Undesirable side effects due to impurities Example 2
When? Production of food and beverages
Why? Have to ensure chemicals used are pure and safe for human consumption

Question 6

Types of Mixtures and Separation Techniques

Answer 6

• The constituents of mixtures can be separated by physical means such as filtration, evaporation, distillation and fractional distillation.
• Each method makes use of the different physical properties of the constituents in order to separate them.
• However, the method of separation depends on the type of mixtures and what is to be obtained from the mixture.
• The different types of mixtures are solid-solid mixture, solid-liquid mixture and liquid-liquid mixture.
• The 9 techniques that are covered are:
• Magnetic Attraction
• Using a suitable solvent
• Filtration using a funnel
• Evaporation till dryness
• Crystallisation
• Distillation
• Separating Funnel
• Paper Chromatography
• Reverse Osmosis

Question 7

Magnetic attraction

Physical properties of the components in a mixture

Answer 7

Magnetic attraction
• Separating solids from the mixture of solids (solid-solid mixture)
Physical properties of the components in a mixture
• Magnetic substances
- E.g. cobalt, Iron, nickel, steel (alloy of iron) (COINS)
• Non-magnetic substances
- E.g. aluminium, copper, gold, tin

• Magnetic materials like iron, nickel and cobalt can be separated from non-magnetic materials using a magnet.

• E.g. we can separate sulfur and iron filings using a magnet.
• Magnet attracts iron filings, hence sulfur is left behind.

Question 8

Applications of magnetic separation

Answer 8

Electromagnets
• are used to remove scrap steel and iron at the junkyard.
• Magnetic attraction is useful in industries such as waste recycling centres where magnets are used to separate iron and steel from waste products. In these centres, a mixture of objects (scrap metals) is passed along a conveyor belt near strong magnets. Objects made from magnetic materials (iron, steel) are then attracted to the magnets and removed.
• Magnets are used to clean agricultural products such as sugar. The sugar is placed on conveyor belt and passed near magnets so that any pieces of machinery parts in the sugar can be removed.
• In hospitals, a magnet is used to remove iron splinters from the patient’s eye

Question 9

Using a Suitable Solvent (4 points, 2 subpoints)

Answer 9

• Separating solids from the mixture of solids (solid-solid mixture)
• Solubility: Different solids dissolve in different solvents.
- Some common solvents are water and ethanol.
• E.g mixture of salt and sand
• Physical property of the components in the mixture: salt is soluble in water but not sand
- You dissolve the mixture in water to obtain salt water as the filtrate and sand as the residue

Question 10

Filtration using a funnel (7 points, 2 subpoints)

Answer 10

Filtration using a funnel
• Separating insoluble solids from a liquid (solid-solid mixture)
(mixture of insoluble solid in a liquid – suspension)

• Filtration is used to separate insoluble solids from the solid-liquid mixture.
• In filtration, the insoluble solid particles are trapped on the filter paper as residue while the liquid that passes through the filter paper is called the filtrate.
• The filter paper has microscopic holes to allow liquid particles to pass through but does not allow solid particles to pass through.
• For example, a sand and water solution. The particles are being separated by their size:
- The smaller particles (the water molecules) can easily pass through the small pores in the filter paper.
- The larger particles of sand cannot and so they get trapped in the filter.

• Upon filtration, the solid that remains on the filter paper is called the residue.
• The liquid or solution that passes through the filter paper is called the filtrate.

Question 11

Applications of filtration in daily life

Answer 11

• Filtration is very useful in industries and everyday life.
• Filtration is one of the several stages used to purify water in a water treatment plant. Impurities are removed by passing water through a bed of sand, gravel and pebbles.
• A vacuum cleaner uses a filter bag to collect dust and dirt.
• The vacuum bag traps the dust and dirt and allows clean air to escape.
• The hairs in our nose also act as a filter to sieve out the dust from the air when we breathe in.
• Our kidneys use filtration to separate waste (and extra water) from our blood. The waste and water then pass out of the body as urine.

Question 12

Evaporation till dryness (6 points, 1 subpoint)

Answer 12

• Separating a soluble solid from a liquid (solution mixture)

*Evaporation till dryness is only for substances that do not decompose on heating!
• A solute which dissolves completely in a solvent cannot be separated using filtration.
- E.g. common salt solution. Instead, evaporation to dryness is used to separate the solute (salt) from the solvent (water).
• The solution is heated so that the solvent evaporates, leaving the solid behind. In this process, only the solute can be obtained and the solvent will evaporate away.
• Not all soluble substances can be obtained by evaporation till dryness.
- Example: Sugar and hydrated copper(II) sulfate decomposes on heating .

• The solid obtained by evaporation till dryness is not always pure. Any soluble impurities will be left together with the solid after the process is completed.

Question 13

Applications of evaporation in daily life

Answer 13

• To produce evaporated milk – this is milk with more than half the water removed by evaporation.
• Sea water is left to evaporate under the Sun in salt pans. When the water has evaporated, salt will be left behind in the pans. The salt is then piled into heaps to be dried further.
• Evaporation is also used to dispose of brine in desalination plants.
• Miners use ponds to separate ore from water.
• Evaporation ponds at contaminated sites remove water from hazardous waste, which greatly reduces its mass and volume and allows the waste to be transported more easily to be treated and disposed of.

Question 14

Crystallisation

Answer 14

• Separating a soluble solid from a liquid (solution mixture)
• It is a process of forming crystals. This method is used for separating dissolved solids from a solution where the solutes would decompose upon direct heating.
- Sugar cannot be separated by evaporation of sugar solution since sugar decomposes (breaks down into simpler substances) when heated. Hence sugar can be obtained using a process known as crystallisation. In this process, sugar solution is first heated until saturated. It is then cooled, forming crystals upon cooling.

• Two common techniques of crystallisation are:
- By cooling down a hot concentrated solution (rapid cooling)
→ The solution has to be heated to get rid of some water in order to obtain crystals from an unsaturated aqueous (water solvent) solution. The solution becomes more concentrated as the water boils and evaporates away. Upon cooling of the hot concentrated solution, pure crystals of the solute will be formed.
→ The crystals formed from this technique are smaller than crystals formed from slow cooling.

• Slow evaporation of solution at room temperature (slow cooling)
  → Crystals can be obtained by evaporating a solution at room temperature. After the solvent in the solution has been evaporated, the remaining solution will become saturated. Further evaporation will cause the formation of crystals.
  → The crystals formed from this technique are larger than the crystals formed from rapid cooling.

• Crystals formed by slow cooling or evaporation are large. Small crystals are formed when a saturated solution is cooled down quickly.
- This is because solute particles need time to arrange themselves in regular shapes in order to form crystals.

• After crystallisation, crystals can then be separated from the solution by filtration. Use cold distilled water to wash the crystals two or three times after filtration. Collect the crystals with a spatula and dry them by pressing them gently between filter papers.

• Crystallisation can also be used to purify solids.
- Assume a sample of cane sugar contains a small amount of glucose as impurities. They are both soluble in water. Pure cane sugar can be crystallised and removed from the solution. As the glucose does not crystallise, it will remain dissolved in the solution.

Question 15

Two common techniques of crystallisation are:

Answer 15

• By cooling down a hot concentrated solution (rapid cooling)
  → The solution has to be heated to get rid of some water in order to obtain crystals from an unsaturated aqueous (water solvent) solution. The solution becomes more concentrated as the water boils and evaporates away. Upon cooling of the hot concentrated solution, pure crystals of the solute will be formed.
  → The crystals formed from this technique are smaller than crystals formed from slow cooling.
• Slow evaporation of solution at room temperature (slow cooling/evaporation)
  → Crystals can be obtained by evaporating a solution at room temperature. After the solvent in the solution has been evaporated, the remaining solution will become saturated. Further evaporation will cause the formation of crystals.
  → The crystals formed from this technique are larger than the crystals formed from rapid cooling.

Question 16

Crystallisation steps

3 steps

Answer 16

Step 1:
• The solution is heated to remove most of the solvent (water).
• Heating is stopped when a saturated solution is formed.

Step 2:
• The hot, saturated solution is allowed to cool.
- Saturated solution = a solution that contains as much dissolved solute as it can at a given temperature.
• The dissolved copper(II) sulfate appears as pure crystals.
- Rapid cooling produces small crystals while slow cooling produces large crystals.
Step 3:
• The cold solution with crystals is filtered.
• The crystals (residue) are washed with small volume of cold distilled water and filtered again. The filtrate contains soluble impurities.
• The crystals (the residue) are then dried by pressing them between pieces of filter paper.

• Example: To obtain copper (II) sulfate crystals from copper (II) sulfate solution using either crystallisation or evaporation till dryness, which method should be used to obtain copper(II) sulfate crystals from copper(II) sulfate solution?

• Crystallisation. This is because blue hydrated copper(II) sulfate decomposes on heating to form white anhydrous (no water) copper(II) sulfate.

Question 17

Applications of crystallisation in daily life

Answer 17

• Production of silicon wafers for microchips used in electronic equipment and solar cells (the silicon has to be very pure!)
• Production of sucrose from sugar beets and sugar cane

Question 18

Distillation

Answer 18

• Separating a liquid from a solution
- The mixture (solution) can be a solid-liquid mixture or a liquid-liquid (miscible) mixture.
• Distillation is the process by which a solid-liquid mixture can be separated. In this process, pure liquid can be obtained as the distillate, leaving behind the solid residue in the flask.

Question 19

What are the definitions of miscibility, miscible and immiscible?

Answer 19

Miscibility is the property of two substances to be mixed together, forming a solution.
Miscible = can be mixed
Immiscible = cannot be mixed

Question 20

Parts of Simple Distillation (4)

Answer 20

Water enters the condenser from the bottom and leaves from the top. Condenser must be sloping downwards so that it will be filled fully with water because of gravity.

Thermometer bulb should be just beside the side arm leading to the condenser.

Boiling chips are placed in the flask to ensure smooth boiling.

Volatile (easily evaporated at normal temperatures) liquids can be kept in the liquid state by placing conical flask in ice.

Question 21

Processes involved in Simple Distillation (2)

Answer 21

• There are two processes involved in this method. They are boiling and condensation.
• First, the solution is heated so that the component to be separated escapes as vapour. The vapour is then cooled and condensed into a liquid called distillate which is a pure liquid.

Question 22

• Simple Distillation of Salt Solution (5 steps)

Answer 22

1. Water boils and becomes water vapour. Water vapour rises and enters the condenser.
2. Thermometer measures boiling point of the vapour.
3. Water vapour is cooled and condenses into pure liquid (distillate).
4. The salt solution becomes more concentrated as distillation continues. Salt will be collected as residue.
5. Pure water is collected as the distillate in a flask.

Question 23

Uses and purpose of thermometer and condenser in simple distillation?

Answer 23

• The bulb of the thermometer is to be placed beside the side arm of the distillation flask. It should not be dipped into the solution. This ensures that the thermometer measures the boiling point of the vapour that enters the condenser. The purpose of the thermometer is to measure the boiling point of vapour about to be condensed.
• The set-up for distillation uses a condenser. This condenses the vapour into liquid. The condenser consists of two tubes - a water inlet tube and a water outlet tube. Cold running water enters through the water inlet which is found at the bottom of the condenser and leaves through the outlet found at the top of the condenser. This arrangement enhances the cooling effect of the vapour to be distilled so as to fill the whole condenser with water since gravity would push the water down if the water inlet was at the top which would not allow the water to fill up the whole condenser. (oh and there is a tube in the middle of the condenser where the vapour condenses at)
• The purpose of a condenser is to condense a gas to a liquid/ to condense water vapour to water.
• The water inlet is at the lower end of the condenser while the water outlet is at the upper end of the condenser.
• The condenser is placed in a slanted position so that the pure solvent formed can run into the receiver easily.
• If the distillate is volatile (i.e. evaporates easily), the receiver can be placed in a large container filled with ice so that it can stay in the liquid state for a longer period of time.
• Using this process, we can obtain pure water from salt solution (sea water).
• Simple distillation is used to recover a solvent from a non-volatile solute. The solute should not decompose on heating.

• The difference in boiling points of the solids/liquids allows them to be separated.

Question 24

What property allows the solids/liquids to be separated in simple distillation?

Answer 24

the difference in boiling points

Question 25

purpose of condenser in simple distillation?

Answer 25

• The purpose of a condenser is to condense a gas to a liquid/ to condense water vapour to water.

Question 26

where are the water inlet and outlet located in simple distillation?

Answer 26

• The water inlet is at the lower end of the condenser while the water outlet is at the upper end of the condenser.

Question 27

how is the condenser placed and why is it placed like that in simple distillation?

Answer 27

• The condenser is placed in a slanted position so that the pure solvent formed can run into the receiver easily.

Question 28

Applications of distillation in daily life

Answer 28

* Distillation is used widely in the perfume industry. Many plants contain fragrant oils which can be used to make perfume. The oil is extracted by heating the plant material and distilling the vapour. * Distillation is also used to obtain pure water from sea water. This method is called desalination. In the Middle Eastern countries like Saudi Arabia, this method is used to obtain pure water. Although this method looks simple and easy to use, it is actually a very expensive process and requires a lot of energy. * Production of distilled water, purification of essential oil, production of alcoholic drinks, making perfume.

Question 29

Separating funnel (2 points, 2 subpoints)

Answer 29

• Separating liquids from a mixture of immiscible (cannot be mixed) liquids • Separating funnel is used to separate 2 or more immiscible liquids of different densities. - Immiscible liquids such as oil and water can be separated by using a separating funnel as shown on the right. The mixture is put into a separating funnel, shaken and allowed to settle. The oil and water would form two separate layers. The liquid with a higher density (water) forms the lower layer. - Remove the stopper (covers the gap at the top) and open the tap at the bottom of the separating funnel. Water would first be collected as it forms the bottom layer. The oil can then be collected in another beaker.

Question 30

Chromatography (3 points)

Answer 30

* Chromatography is a technique used for separating small samples of mixtures of gases, liquids or dissolved substances. * This method can be used to identify and analyse the substances in the mixtures although it does not separate the mixtures like filtration, evaporation and distillation. However, unlike other separation processes, only a small amount of the sample is needed to carry out the process of chromatography. • There are several types of chromatography: Paper chromatography, Thin-layer Chromatography (TLC), High Performance Liquid Chromatography (HPLC), Gas Chromatography (GC), Two-dimensional chromatography.

Question 31

``` Paper chromatography (3 points, 2 subpoints) factors affecting the movement of each component? (2) ```

Answer 31

* Paper chromatography is used to separate and identify the coloured components in dyes or inks. * In paper chromatography, there are two factors which the movement of each component in the mixture depends on: 1. The solubility of the component in the solvent - The component moves with the solvent easily if the component is very soluble in the solvent. 2. The adsorption of the component on the filter paper - Some solids are able to attract other substances strongly and hold them on their surface. This is called adsorption. The component will not move with the solvent easily if it is adsorbed strongly by the filter paper. • The components separated by paper chromatography would show coloured spots on the paper. If the components separated are colourless, these can easily be detected by spraying a locating agent on the filter paper.

Question 32

what are the two factors which the movement of each component in the mixture depends on in paper chromatography?

Answer 32

1. The solubility of the component in the solvent - The component moves with the solvent easily if the component is very soluble in the solvent. 2. The adsorption of the component on the filter paper - Some solids are able to attract other substances strongly and hold them on their surface. This is called adsorption. The component will not move with the solvent easily if it is adsorbed strongly by the filter paper. - Identical dyes produce spots at the same height. - Sample X does not contain the dyes A, B and D. However, X contains dye C. - If a sample does not contain all the spots of a particular dye (like if it only has 1 out of 2 spots), then it does not contain the dye.

Question 33

what is the definition of adsorption?

Answer 33

Adsorption is often described as a surface phenomenon where particles are attached to the top layer of material. - Some solids are able to attract other substances strongly and hold them on their surface. This is called adsorption.

Question 34

Steps for paper chromatography (10 steps, 1 subpoint at step 7)

Answer 34

1. A solution of the dye is obtained. 2. A drop of the solution is placed on the pencil line near the bottom of a strip of filter paper. 3. The paper is dipped into a solvent (e.g. water), with the solvent level below the spot. 4. The solvent moves up the paper. 5. The dyes on the pencil line dissolve in the solvent and travel up the paper at different speeds. 6. When the solvent reaches the spot of the dye, it dissolves the substances in the dye. 7. The different substances move along with the solvent but at different rates. - The more soluble substances move faster and further up the paper than those that are less soluble. Thus, the substances in the mixture are separated. 8. The chromatography paper with the separated components is called a chromatogram. 9. Locating agents is used in the chromatography of colourless compounds. The locating agent reacts with the colourless substances to produce coloured products. 10. Identical dyes travel up the same distance and produce the same colour on the paper.

Question 35

Rf value (paper chromatography)

Answer 35

* Rf value is the positions of the solvent front (position reached by solvent) and spot on a chromatogram depend on how long the experiment was allowed to run. * The ratio between the distance travelled by the substance and the distance travelled by the solvent is a constant. * This ratio is called the Rf value of the substance. * Rf value of the same substance is the same regardless of distance travelled on chromatogram.

Question 36

uses of paper chromatography in real life?

Answer 36

* To separate and identify different dyes in inks * To separate and identify coloured substances used in food products such as sweets and canned vegetables * To separate and detect pollutants in water

Question 37

Reverse osmosis (6 points)

Answer 37

* Separating a liquid from a solution * Reverse osmosis is a filtration method used to remove particles, molecules and ions from a solution by applying pressure to the solution from one side of a membrane or layer of membranes. The solution passes through a partially permeable (semi permeable) membrane, leaving the particles and molecules on one side. * This method is widely used in water desalination plants as it uses less energy compared to the desalination process of purifying water. * Reverse osmosis is most commonly known for its use in making pure drinking water from sea water (or even sewage water) by removing the salt and other substances from the sea water. * Unlike in osmosis where the water will cross the membrane to dilute the concentrated solution, in reverse osmosis, pressure is exerted on one side of the solution to force the water molecules across the membrane to the pure water side. * Reverse osmosis is often used in commercial and residential water filtration.

Question 38

Reverse osmosis vs filtration (1 similarity, 1 difference)

Answer 38

Similarity Both use the principle of particle size for purification. Difference Energy is required for reverse osmosis while no energy is required for filtration (because of the force needed to push through the membrane).

Question 39

what are the 4 national taps of SG

Answer 39

• The four national taps: - Rainwater from local catchments - Imported water - Desalinated water - NEWater * Water is essential to life. About 65% of our body mass is water. * Recreational activities such as swimming need water. * Water is needed for drinking, cooking and washing. * Industries use a lot of water to make products such as cosmetics, cleaning agents and drinks. * Water in the form of steam is also used to run machinery.

Question 40

conservation of water

Answer 40

* Need for adequate supply of water in the future. * Keeping our waterways and reservoirs litter-free. * Ways to conserve water: * Reduce – Repair leaks to prevent water wastage. * Reuse – Water plants with water used to wash vegetables.

Question 41

desalinated water (4 points, 1 subpoint)

Answer 41

• Pure water is also obtained from seawater using desalination. • Desalination can be carried out by using distillation or reverse osmosis. - Distillation is an expensive process as a large amount of fuel is required to heat the water. * In Singapore, desalination is carried out using reverse osmosis. * In reverse osmosis, a membrane is used to separate water from the salt in seawater. The pores in the membrane allow water particles to pass through but not the larger salt particles.

Question 42

NEWater (4 points)

Answer 42

* NEWater is highly purified reclaimed water. * It is produced from treated used water (using filtration) that is further purified using reverse osmosis. * As an added safety measure, the water is exposed to ultraviolet radiation for disinfection. * The water obtained in the end is ultra-clean and safe to drink.