Introduction
How do we define pure?
The use of word ‘Pure’ in science means that all the constituent particles of a substance are the same in their chemical nature. That is it is made up of same type of particles and is hence pure.
A pure substance therefore has same chemical properties for all its constituents.
A pure substance consists of only one type of atom, molecule, or compound.
Example: Diamond, Sulphur, Tin, Crystals of Salt or any other substance that is consistent in its structure.
But this is subjective.
All elements are pure substances.
Crystals of compounds like Salt, Sugar, Copper Sulphate etc. even though compounds are pure substances.
A mixture which is consistent in its constituents i.e. if it is homogenous mixture even then it may be referred to as a pure substance. (Pure Oil, Pure Honey or Air are pure substances as the composition of these substances is uniform)
What is a mixture?
Most of the things around us exist as mixtures. Mixtures are made up of more than one kind of pure substances present in any fixed proportion.
Types of Mixtures
1. Homogenous Mixture:
It is a mixture that has no visible boundaries i.e. you cannot distinguish between the components just by looking.
For example: Air is a mixture of several gases but it cannot be separated into its components as it is uniform in its composition.
Similarly, a mixture of salt and water is uniform throughout and hence is a homogenous mixture.
2. Heterogeneous Mixture:
It is a mixture that has visible boundaries. For example, a mixture of sand and sugar can be visually distinguished into its components.
Similarly, a mixture of oil and water is visually distinguishable as oil floats over water.
What is a Solution?
A solution is a homogeneous mixture of two or more substances.
It has two components:
Solvent: It is the part of the mixture in which the other component is dissolved. It is usually present in larger quantity.
Solute : It is the part of the mixture that is dissolved. It is usually present in lesser quantity.
For example : In a Sugar Solution; Sugar is the Solute and Water is the Solvent.
Does a solution necessarily have to be liquid?
No, solutions can be solids, liquids as well as in gaseous state.
Alloys
A Solid Solution: An alloy is a homogenous mixture of two or more metals or a metal and a non-metal and they cannot be separated by physical methods.
A mixture of 30% Zinc and 70% copper is called as Brass. The proportions of metals can be varied to achieve varying mechanical and electrical properties.
Air is also a solution in gaseous form as it’s a homogenous mixture of different gases.
“Tincture of Iodine” is a solution of solid iodine in liquid alcohol. -
Properties of Solution:
1. They are homogenous mixtures.
2. The size of the particles is generally very small; usually smaller than 1nm. (10-9 m)- Therefore not visible by naked eye.
3. Since the particles are very small, they do scatter a beam of light (Tyndall Effect). That is the path of light is not visible in the solution.
4. The solute and solvent cannot be separated by filtration or by sedimentation as they do not settle.
Tyndall Effect: The Tyndall effect is light scattering by particles in a colloid or in a very fine suspension.
True Solution | Colloidal Solution | Suspension |
Size of solute is generally smaller than 10-9 m | Size of the solute particles lies between 10-6 to 10-9m. | Size of the solute particles is generally greater than 10-6m |
The particles cannot be seen by naked eye | Particles cannot be seen by naked eye | Particles can be seen be naked eye. |
It is a homogenous mixture | Seems homogenous but is actually heterogeneous | It is a heterogeneous mixture |
It is transparent | It is translucent | It is opaque |
It does not scatter light: i.e does not show Tyndall Effect | It shows Tyndall effect | It may or may not show Tyndall effect |
They are stable | They are stable | They are unstable |
The solute and solvent cannot be separated by process of filtration or sedimentation. | The solute particles pass through filter paper but not through parchment paper | The solute particles do not pass through filter or parchment paper |
Common Examples of Colloids
Dispersing Phase | Dispersing Medium | Type | Example |
Liquid | Gas | Aerosol | Fog, Clouds, Mist |
Solid | Gas | Aerosol | Smoke, Automobile Exhaust |
Gas | Liquid | Foam | Shaving Cream |
Liquid | Liquid | Emulsion | Milk, Face Cream |
Solid | Liquid | Sol | Milk of Magnesia, Mud |
Gas | Solid | Foam | Foam, Rubber, Sponge, Pumice |
Liquid | Solid | Gel | Jelly, Cheese, Butter |
Solid | Solid | Solid Sol | Colored Gemstone, Milky Glass |
Gas in Gas is not a colloid but a mixture!!
Measuring Concentration of Solution:
Depending upon the amount of solute present in the solution, the solution can be called : Dilute, Concentrated or Saturated.
These terms dilute and concentrated are comparative. – A sugar solution containing 100gms of Sugar in 100ml of solution will be called concentrated in relation to a sugar solution containing 80gms of Sugar in 100 ml of solution .
What is a saturated solution?
A solution in which no more than solute can be dissolved at the given temperature is called a saturated solution. If you wish to dissolve more solute, then increasing the temperature will allow more solute to be to be dissolved.
If the amount of solute contained in a solution is less than the saturation level, it is called an unsaturated solution.
What is Solubility?
The amount of the solute present in the saturated solution at this temperature is called its solubility.
What would happen if you were to take a saturated solution at a certain temperature and cool it slowly?
The solubility of such a solution will reduce and crystals would form. Upon heating again, the crystals would dissolve and the solution will become saturated again.
There are 3 methods of measuring concentration of solution
3. Volume by Percentage = Volume of Solute / Volume of Solution x 100
Example: 10 ml of iodine dissolved in 90ml of water
Total Volume of Solute = 10ml
Total volume of solution = 10ml + 90ml = 100ml
Therefore, mass by volume percentage of solution = 10/100 x 100 = 10%
Methods of Separation of Mixtures
(1) Evaporation
Basic principle: Out of the two components of a mixture one having lower boiling point can evaporate and other having higher boiling point will remain in liquid state.
Therefore, a mixture of volatile and non-volatile components can be separated by evaporation.
What is a volatile substance?
A volatile substance is one which evaporates readily on its own. Acetone, Alcohol Based Solutions are volatile substances.
(2) Centrifugation
Basic principle: Based on varying density of solute and solvent; the process allows separation of materials based on their density when rotated very fast.
The heavier material gets settled at the bottom and the lighter one floats. The process is used in separating the components of blood into its constituents such as Plasma and Erythrocytes.
Applications:
Used in diagnostic labs for blood and urine test.
Used to separate cream and butter from milk
Used in drying out clothes in a dryer as it squeezes out the water due to acceleration of the circular motion.
Toned and Double-Toned varieties of milk contain different amounts of fat which is obtained by churning fat from the full cream milk.
A centrifuge is a machine that is used to spin the test tubes/material rapidly.
(3) Separating Funnel
Basic Principle: Immiscible liquids (which do not dissolve) can be separated out in layers depending on their densities.
The stop cock then be used to extract the fluid having greater density at the bottom.
Applications
Used for separating oil from water
Used in extraction of lighter slag during the process of extraction of molten iron from iron ore.
(4) Sublimation
Basic Principle: Separation is based on the property of sublimation. Of the components, the one which is volatile and sublime gets converted directly into gas and can then be solidified later.
Ammonium chloride, naphthalene, camphor and anthracene are some materials which are sublime.
(5) Chromatography
Basic principle: Coloured components of a mixture can be separated by using an absorbent on which they are absorbed at different rates.
The process of separation of components of a mixture is known as chromatography.
Origin of Word: Kroma in Greek means colour. This technique was first used for separation of colors, so this name was given. Chromatography is the technique used for separation of those solutes that dissolve in the same solvent.
As the water rises on the filter paper it takes along with it the dye particles. Usually, a dye is a mixture of two or more colors. The coloured component that is more soluble in water, rises faster and in this way the colors get separated.
Applications:
a) To separate colours of a dye.
b) To separate pigments from natural colours like chlorophyll.
c) To separate drugs from blood.
(6) Distillation
Basic Principle: Two miscible liquids having sufficient difference (atleast 25K) in their boiling points. The liquids should not decompose on heating.
In there are more than 2 liquids or liquids having a difference of boiling point less than 25K, the process of fractional distillation should be used.
A simple fractionating column is a tube packed with glass beads. The beads provide surface for the vapors to cool and condense repeatedly
When mixture of acetone and water is heated, acetone having lesser boiling point, boils and moves to delivery tube, within which it condenses back to liquid with the help of a condenser clamped to it.
Thus, acetone is separated out in a beaker and water is left in the distillation flask.
(7) Fractional Distillation of Gases
Air is a homogeneous mixture and can be separated into its components by fractional distillation.
The air is compressed by increasing the pressure and is then cooled by decreasing the temperature to get liquid air. This liquid air is allowed to warm-up slowly in a fractional distillation column, where gases get separated at different heights depending upon their boiling points.
Simple Fractional Distillation
(8) Crystallization
Crystallization is a process that separates a pure solid in the form of its crystals from a solution.
Basic principle: To remove impurities from a mixture by first dissolving in a suitable solvent and then crystallizing out one component.
Why is Crystallization better than Evaporation?
- Some solids decompose or some, like sugar, may get charred on heating to dryness.
- Some impurities may remain dissolved in the solution even after filtration. On evaporation these contaminate the solid.
Process
1. Impure Copper sulphate crystals are first dissolved in sulphuric acid and then heated to saturated solution.
2. Now, this solution is left overnight. So, only pure copper sulphate crystals are formed whereas impurities are left behind in the solution.
3. This solution can be thus filtered so as to get pure copper sulphate crystals on filter paper.
Applications
- Purification of salt from sea water.
- Separation of crystals [e.g., alum (phitkari, copper sulphate)] from their impure crystals.
Water purification in water treatment plants
Difference between Physical and Chemical Changes
Chemical Changes | Physical Changes |
They are generally non-reversible | They are easily reversible |
New Products are formed | No New Products are Formed |
Reactants are used up | There is often just a change in the state |
Heat/Light/Sound is produces or Gas is released | Such changes may or may not occur |
Types of Pure Substances
1. Elements 2. Compounds
1. Elements: The term was first used by Robert Boyle.
Definition of element as per Antoine Laurent Lavoisier
“Element is a basic form of matter that cannot be broken down into simpler substances by chemical reactions."
- Majority of the elements are solid.
- The number of elements known at present are more than 100.Ninety-two elements are naturally occurring and the rest are manmade.
Elements can be classified into Metals, Non-Metals and Metalloids
Some elements have intermediate properties between those of metals and non-metals, they are called metalloids; examples are boron, silicon, germanium etc.
Metals | Non-Metals |
Metals in their pure state have shiny surface. This property is called metallic lustre. | Non-Metals are generally non-lustrous. Except Iodine - Which is lustrous. |
They are generally hard. The hardness varies from metal to metal. They are generally solids. (Except Mercury which is liquid at room temperature). Also, Akali metals are soft solids and can be cut down with a knife. | They are generally soft but carbon - a non metal can exist in various allotropes such as 'Diamond' which is the hardest known substance. |
They generally have high melting points. Except 'Gallium' and 'Caesium' which have low melting points and can meant on your palm. | They generally have low melting points. Except Diamond |
Some metal can be beaten in sheets, this property is called malleability. e.g. Gold and Silver. | They are non-malleable |
They can be drawn into this wires. This property is called ductility. e.g Gold. | They are non-ductile. |
Metals are good conductor of heat and electricity. Silver and copper are best conductors. Mercury and lead are poor conductors | Non-metals are generally bad conductors of heat and electricity. Except allotrope of carbon; 'Graphite' which is a good conductor of electricity. |
They are sonorous, as they produce sound after striking them. | They are non sonorous. |
2. Compounds: Compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion.
Mixtures | Compounds |
Elements or compounds just mix and no new compound is formed. | Elements react to form new compounds. Together to form a mixture |
A mixture has a variable composition | The composition of each new substance is always fixed. |
A mixture shows the properties of the constituent substances. | The new substance has totally different properties |
The constituents can be separated easily by physical methods | The constituents cannot be separated by physical methods. |