Unit 4: Separation Techniques Flashcards
What is a pure substance? (2 points)
- A pure substance is made up of one type of element or compound.
- It is not mixed with any other substance.
How do we deduce the purity of a substance?
- 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.
How do we deduce the identity of a substance? (2 points, 2 sub-points)
- 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.
What is the effect of impurities on melting point?
3 points, 1 sub-point
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.
Why and when do we need pure substances? (2 examples, optional)
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
Types of Mixtures and Separation Techniques
- 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
Magnetic attraction
Physical properties of the components in a mixture
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.
Applications of magnetic separation
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
Using a Suitable Solvent (4 points, 2 subpoints)
• 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
Filtration using a funnel (7 points, 2 subpoints)
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.
Applications of filtration in daily life
- 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.
Evaporation till dryness (6 points, 1 subpoint)
• 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.
Applications of evaporation in daily life
- 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.
Crystallisation
• 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.
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/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.
Crystallisation steps
3 steps
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.
