Methods of Purification:

a.Filtration

b.Crystallization

The need for pure substance

Criteria for purity

Objectives:

Know their principles, procedures

and application

The NEED for pure substanceImportance 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.

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.

The NEED for pure substance

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.

A mixture is a substance that contains two or more substances.

The need for pure substance

They can be easily separated into pure substances by purification techniques.

The substances do not react with one another chemically.

(Physical method)

Criteria for purity

MixturesPure Substances

Properties

Boiling Point

Melting Point

Exact and Fixed

Not Fixed (Higher)

Not Fixed (Lower)

DefiniteNot

Definite

Exact and Fixed

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.

Criteria for purity

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?

Pure solid/liquid will melt/ boil at only ONE temperature. E.g. Water melts at 0°C and boils at 100°C.

Criteria for purity

To find out if the substance is pure:Melt at its true melting point

Boil at its true boiling point

Measure its melting point

Measure its boiling point

Pure Water melts at 0°C and boils at 100°C.

Criteria for purity

When impure: Melting point decreases further, Boiling point increases

0oC 100oC-2oC 102oC

LiquidSolid Gas

WEBSITE TO VISIT:http://environmentalchemistry.com/

You may start by zooming in on:• PCBs (Polychlorinated Biphenyls) are in

the Foods You Love • Norwegian farmed salmon production

raises global concern • Mercury in Fish vs. Omega-3 Fatty Acids

Health Benefits • PCBs (Polychlorinated Biphenyls) in New

York's Hudson River

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.

MixturesSolutions 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!

1) To separate a soluble and insoluble solid (E.g. Salt and sand)/ To separate insoluble solids from a solution (E.g. Sand from water)

2) To separate pure solid from a solution (E.g sugar from sugar solution)

3) To separate or identify colors, dyes and pigment.

4) To obtain a pure liquid from a solution (E.g. water from salt solution).

5) To separate miscible liquids with different boiling points.

Possible scenarios of mixtures!

Types of separation techniques

Suitable method of purification:

Filtration

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)

Filtration

1. Glass rod

2. Retort stand

3. Mixture

4. Filter paper5. Filter funnel

6. Beaker

7. Filtrate

Filtration is usual method of separating solid from a liquid.

Solid-Solid mixture

Solid-Liquid mixture

A Glimpse of our First mini Practical on FILTRATION!!

Do you remember how to do filtration?

Filtration – How it works

• Mixture poured through filter paper with tiny

holes (“pores”).

• Large particles of the solid gets

trapped.

- Residue

• Small particles of liquid passes through the filter.

- Filtrate

Filter paper with small holes

Small particles of liquid

Large particles of solid

Solid-Solid mixture

Solid-Liquid mixture

Applications:

Filtration – How it works

Solid-Solid mixture

Solid-Liquid mixture

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

What happens to the filtrate?

Can filtration be done to separate solids dissolved in

solution?

Filtration

Solid-Solid mixture

Solid-Liquid mixture

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.

Possible Scenario

? ?

Evaporate

Crystallisation

Suitable method of purification:Which should we use?

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

• 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.

EvaporationSolid-Liquid mixture

salt solutionevaporating dish

Salt solution

heat

• 2 types of evaporation technique:

Solid-Liquid mixture

Evaporation

Boiling to drynessEvaporation using water bath (slower)

Conditions!!!!!

Evaporation

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.

There is Always

conditions involved!!!

Solid-Liquid mixture

Crystallisation

Aim: To obtain PURE solids from a solution (solid will decompose upon strong heating).

E.g. To obtain sugar from sugar solution, Copper (II) Sulphate from Copper (II) Sulphate solution.

Do you know

others?

Solid-Liquid mixture

solution

CRYSTALLISATION

Glass Rod

SolventImpure Solid

1. 2.

SolutionEvaporating dish

Crystals

3.Pure crystals appear when the hot solution cools.

4.

Filter paper

Crystals

Crystals are then collected and dried between filter papers.

The solution must be

Often after filtration.

Crystallisation

saturated.

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.

Solid-Liquid mixture

Crystals

Salt crystalsSugar crystals

Copper(II) sulphate crystals

Solid-Liquid mixture

Have you noticed the difference of evaporation from Crystallisation?

Evaporation Crystallisation

Does solid decompose upon strong heating?

No Yes

How much solvent have to be vaporised?

All Most

In a Solid-Liquid mixture, both Evaporation and Crystallisation is used to obtained solid from its solution.

Summary:Evaporation & Crystallization

To obtain aTo obtain a solublesoluble solidsolid from afrom a liquid liquid

in solid-liquid in solid-liquid mixturemixture

EvaporatioEvaporationn

CrystallizatioCrystallizationnIf solidIf solid does does

not not decomposedecompose on on

heatingheating

If solidIf solid decomposesdecomposes on heatingon heating

1. Which of these is a pure substance?Salt, Cooking oil, Tea leaves, Milk

2. 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

Questions 3. 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

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