Purification of Substances

Know their principles, procedures and application Objectives: The need for pure substance Criteria for purity Methods of Purification: Filtration Crystallization Know their principles, procedures and application

The NEED for pure substance Importance of purity of substances used in everyday life: Chemist need pure substances to study their properties. Pure substances are used in industry to make useful products such as food and drugs.

The NEED for pure substance In pharmaceutical industry, medicines must be tested for purity before they are sold. Impurities in drugs and food may produce undesirable side effects.

The NEED for pure substance In highly precision engineering e.g. Production of silicon chips, even small amount of impurities can greatly reduce the effectiveness of a component in an electronic device.

The NEED for pure substance In food and beverage industry, the types of chemicals which can be added into our food are controlled by certain governmental guidelines to ensure that our food and drinks contain chemicals that are safe for consumption.

Criteria for purity A pure substance is made up of only one substance and not mixed with anything else. In nature, many substances are not pure. Most are found impure as mixtures.

The need for pure substance A mixture is a substance that contains two or more substances. The substances do not react with one another chemically. They can be easily separated into pure substances by purification techniques. (Physical method)

Criteria for purity Pure Substances Mixtures Not Definite Properties Boiling Point Exact and Fixed Not Fixed (Higher) Exact and Fixed Not Fixed (Lower) Melting Point

Criteria for purity Impurities affect the melting and boiling point in the following ways: 1) Decrease the melting point; E.g. Frozen sea water melts below 0°C at -2.5°C 2) Increase the boiling point E.g. Seawater boils at about 102°C

Criteria for purity 3) Increase the range at which melting and boiling occurs; E.g. Coconut oil melts over a range of temperature. Starts melting at 14°C and complete melting at 22°C. Petrol fuel for motorcars boils over a range of 35°C to 75°C.

Melting point of pure water = 0 ºC A pure substance contains only one type of substance (that is, one type of molecule or atom) Melting point of pure water = 0 ºC Boiling point of pure water = 100 ºC So, do you think the temperature for sea water is also 100 ºC?

Criteria for purity Pure solid/liquid will melt/ boil at only ONE temperature. E.g. Water melts at 0°C and boils at 100°C. To find out if the substance is pure: Melt at its true melting point Measure its melting point Boil at its true boiling point Measure its boiling point

Criteria for purity Pure Water melts at 0°C and boils at 100°C. When impure: Melting point decreases further, Boiling point increases Solid Gas Liquid -2oC 0oC 100oC 102oC

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Some important terms Some terms that you should know: A solute is a dissolved substance (e.g. salt in salt solution). A solvent is a substance which dissolve a solute (e.g. water in salt solution. Miscible liquids are liquids that are completely soluble in each other.

Mixtures Solutions and suspensions What is a solution? A solution is a mixture that appears to be made of one substance. copper (II) sulphate solution, fizzy drinks What is a suspension? A suspension is a mixture in which two or more parts can be seen with our eyes oil + water, muddy water, chalk water

How do we obtain Pure substances from a mixture? We have to first consider what are characteristics of the substances involved before we decide on the suitable method of purification.

Possible scenarios of mixtures! To separate a soluble and insoluble solid (E.g. Salt and sand)/ To separate insoluble solids from a solution (E.g. Sand from water) To separate pure solid from a solution (E.g sugar from sugar solution)

Possible scenarios of mixtures! To separate or identify colors, dyes and pigment. To obtain a pure liquid from a solution (E.g. water from salt solution). To separate miscible liquids with different boiling points.

Types of separation techniques Aim: To separate a soluble and insoluble solid (E.g. Salt and sand)/ To separate insoluble solids from a solution (E.g. Sand from water) Suitable method of purification: Filtration

Solid-Solid mixture Solid-Liquid mixture Filtration 1. Glass rod 3. Mixture 2. Retort stand 4. Filter paper 5. Filter funnel Filtration is usual method of separating solid from a liquid. 6. Beaker 7. Filtrate

A Glimpse of our First mini Practical on FILTRATION!!

Do you remember how to do filtration?

Filtration – How it works Solid-Solid mixture Solid-Liquid mixture Filtration – How it works Mixture poured through filter paper with tiny holes (“pores”). Filter paper with small holes Small particles of liquid Large particles of solid Large particles of the solid gets trapped. - Residue Small particles of liquid passes through the filter. - Filtrate

Filtration – How it works Solid-Solid mixture Solid-Liquid mixture Filtration – How it works Applications:

A little extra: Buchner funnel Vacuum filtration is used primarily to collect a desired solid, Vacuum filtration is faster than gravity filtration, because the solvent or solution and air is forced through the filter paper by the application of reduced pressure. Note: Do not use vacuum filtration to filter a solid from a liquid if it is the liquid that you want and if the liquid is low boiling. Any solvent which boils at about 125 degrees or lower will boil off under the reduced pressure in the vacuum flask.

Buchner funnel Vacuum filtration

Solid-Solid mixture Solid-Liquid mixture Filtration What happens to the filtrate? Can filtration be done to separate solids dissolved in solution?

Notes Can Dyes be separated by filtration? Can salt be obtained from a mixture of salt solution by filtration? Dyes in ink go through the filter As dye molecules are smaller than the pores in the filter paper and go straight through them. Salt in sea water cannot be separated from water by filtration, as the sodium chloride molecules are much smaller than the pores in the paper.

Suitable method of purification: Possible Scenario ? Evaporate Crystallisation Suitable method of purification: Which should we use?

Solid-Liquid mixture Evaporation Aim: To obtain a solute from its solution by vaporising ALL the solvent. (E.g. To obtain salt from salt solution.) How it works? When a solution is heated, only the solvent boils away while the solute remains. Evaporation is a process where ALL the liquid has been vaporised.

Solid-Liquid mixture Evaporation When a solution is heated, the liquid (solvent) evaporates, leaving behind the solid (solute) as residue. You will have only the solid component, as the liquid component is lost as vapour to the surrounding. salt solution evaporating dish Salt solution heat

Evaporation 2 types of evaporation technique: Boiling to dryness Solid-Liquid mixture Evaporation 2 types of evaporation technique: Boiling to dryness Evaporation using water bath (slower)

There is Always conditions involved!!! Solid-Liquid mixture Evaporation There is Always conditions involved!!! Conditions!!!!! This method cannot be used for substance which will decompose upon strong heating. E.g. Sugar, potassium nitrate etc cannot be obtained from their solution by evaporation them to dryness. They will decompose.

Solid-Liquid mixture Crystallisation Aim: To obtain PURE solids from a solution (solid will decompose upon strong heating). Do you know others? E.g. To obtain sugar from sugar solution, Copper (II) Sulphate from Copper (II) Sulphate solution. solution

CRYSTALLISATION

Crystallisation Often after filtration. The solution must be Solid-Liquid mixture Crystallisation Often after filtration. The solution must be saturated. Pure crystals appear when the hot solution cools. Crystals are then collected and dried between filter papers. 1. 2. 3. 4. Glass Rod Evaporating dish Solution Crystals Solvent Impure Solid Crystals Filter paper Heat the solution until MOST of the solvent evaporate off. Allow it to cool. (Steps to make solution saturated) Dissolved solid appears as pure crystals as the solution cools. Impurities remain dissolved in the solution.

Crystals Solid-Liquid mixture Sugar crystals Salt crystals Copper(II) sulphate crystals

Have you noticed the difference of evaporation from Crystallisation? In a Solid-Liquid mixture, both Evaporation and Crystallisation is used to obtained solid from its solution. Evaporation Crystallisation Does solid decompose upon strong heating? No Yes How much solvent have to be vaporised? All Most

Summary: Evaporation & Crystallization To obtain a soluble solid from a liquid in solid-liquid mixture Evaporation Crystallization If solid decomposes on heating If solid does not decompose on heating

Questions 1. Which of these is a pure substance? Salt, Cooking oil, Tea leaves, Milk A liquid is likely to be pure if A. it is colourless. B. it boils at an exact temperature. C. it dissolves in water. D. it is neutral.

Questions Which of these techniques would produce the largest crystals from an aqueous solution of copper (II) sulphate? A) Allowing the solution to evaporate naturally. B) Boiling off the water from the aqueous solution. C) Boiling off the water until crystals just start to appear. D) Heating the solution strongly then cooling it using ice.

Questions A) dissolve – filter – evaporate -crystallise 4. Which of the following is the correct order for purifying potassium nitrate from impure saltpetre (naturally occuring potassium nitrate) using water as a solvent? A) dissolve – filter – evaporate -crystallise B) dissolve – evaporate – crystallise - filter C) filter – dissolve – evaporate - crystallise D) evaporate – crystallise – dissolve - filter