Is Matter Around Us Pure Notes Class 9
Pure substance
A substance that consists of only a single type of constituent particles is called pure substance, e.g. gold, water, etc.
1. Elements
Elements can be broadly classified as metals, non-metals and metalloids.
(i) Metals
A metal is an element that is malleable, ductile and conduct heat and electricity.
(ii) Non-metals
A non-metal is an element that is neither malleable nor ductile and does not conduct heat and electricity.
(iii) Metalloids
Elements having intermediate properties between those of metals and non-metals are called Metalloids, e.g. boron, silicon, etc.
2. Compounds
A compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion.
| Compound | Mixture |
| Definite elements are present in a definite ratio. | Substances mix in any ratio. |
| Elements combine chemically to form a compound. | The chemical reaction does not take place during its formation. |
| Constituents lose their properties. | Constituents retain their properties. |
| Their constituents cannot be separated by simple physical methods. | Their constituents can be seperated by simple physical methods. |
| Considerable energy changes take place during the formation or decomposition of a compound. | Energy changes do not take place (or negligible) during the formation or decomposition of a mixture. |
| A compound is always homogeneous in nature. | A mixture may be homogeneous or heterogeneous in nature. |
Types of mixture
(i) Homogeneous mixture
A mixture in which the constituents are uniformly distributed throughout without any clear boundary of separation is called a homogeneous mixture.
(ii) Heterogeneous mixture
A mixture that does not have uniform composition, i.e. has visible boundaries of separation between in constituents is called heterogeneous mixture.
Solution
A homogeneous mixture of two or more substances is called a solution.
There are two main components of a solution:
(i) Solvent (Dissolving Phase): The component (usually present in a larger amount) of the solution that dissolves the other component in it is called the solvent.
(ii) Solute (Dissolved Phase): The component (usually present in lesser quantity) of the solution that is dissolved in the solvent is called the solute.
Properties of a solution
(i) A solution is a homogeneous mixture.
(ii) The particles of a solution are smaller than 1nm (10-9m) in diameter. Therefore, they cannot be seen by naked eyes.
(iii) Due to very small particles, they do not scatter a beam of light passing through the solution. So, the path of light is not visible in a solution.
(iv) A solution is stable, i.e. the solute particles do not settle down when left undisturbed. The solute particles cannot be separated from the mixture by the process of filtration.
Concentration of a solution
(i) Saturated solution: A solution in which no more amount of solvent can be dissolved at a given temperature, is called saturated solution.
(ii) Unsaturated solution: If the amount of solute contained in a solution is less than the saturation level, it is called an unsaturated solution.
Suspension
A suspension is a heterogeneous mixture in which the solute particles do not dissolve, but remain suspended throughout the bulk of the medium.
Separating the components of a mixture
1. Evaporation (Separation of volatile component from non-volatile component)
The process of conversion of a substance from a liquid state to a gaseous state is called evaporation and the substance is said to be volatile.
This method can be used to separate a volatile component (solvent) from a non-volatile component (solute) of a mixture. On heating the mixture, the volatile component evaporates leaving behind the non-volatile component, e.g. a mixture of dye in water (ink) can be separated by this method. The heat of sun gradually evaporates water in the shallow lakes and common salt is left behind as a solid.
2. Centrifugation (Separation of colloidal particles from solution)
Two components having difference in densities can be separated by centrifugation. This method is based on the principal that, the denser particles are forced to the bottom and the lighter particles stay at the top when spun rapidly. A device used to separate liquids from solids by spinning is called centrifuge. Centrifugal machines are commonly used for this method. The machine can be rotated either by hand, i.e. manually or electrically. Applications centrifugation technique is used in
•diagnostic laboratories for blood and urine tests.
•diaries and homes to separate butter from cream.
3. Separation by using separating funnel (Separation of mixture of two immiscible liquids)
This method is used to separate a mixture of two immiscible liquids. This method is based upon the principle that immiscible liquids separate out in layers depending on their densities. Applications separating funnel is used in
•separation technique used in separation of mixture of pull and water.
•during extraction of iron from its ore, the lighter slag is removed from molten iron.
4. Sublimation (Separation of a solid volatile component from mixture)
Some solids have a tendency to sublime on heating, i.e. they convert directly from solid to gaseous/ vapour phase on heating without passing through the liquid phase. A mixture containing such solid with any other (normal) solid can be separated by sublimation. Examples of solids that sublime are camphor, ammonium, chloride, iodine, etc. Hence, sublimation is used to separate such mixtures that contain a sublimable volatile component from a non-sublimable impurity, e.g. a mixture of common salt and ammonium chloride can be separated by sublimation.
5. Chromatography (Separation of components with the help of same solvent)
The term chromatography is based on Greek word, kroma means colour. This technique was first used for the separation of colour, so this name was given.
Chromatography is the technique used for the separation of those solutes that dissolve in the same solvent. The separation of different components of a mixture is depend upon their different solubilities in the same solvent, e.g. separation of dyes present in ink by paper chromatography.
The ink that we use has water as the solvent and the dye is soluble in it. 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 colours. The coloured component that is more soluble in water, rises faster and in this way the colour get separated.
Applications Chromatography is used to separate
(i) drugs from blood
(ii) pigments from natural colours.
(iii) colours in a dye.
6. Distillation (Separation of two Miscible liquids)
If liquids in a mixture are miscible, boil without decomposition and possess different boiling points, then they can be separated by distillation. Distillation involves the conversion of a liquid into vapour followed by the condensation of vapour back into the liquid. Distillation is used only if the liquids have a difference in boiling points of more than 25K.
Fractional distillation
To separate a mixture of two or more miscible liquids for which the differences in boiling points is less than 25K, fractional distillation process is used, e.g. separation of different gases from air, different fraction from petroleum products, etc.
•The apparatus is similar to that for simple distillation, except that a fractionating column is fitted in between the distillation flask and the condenser.
•A simple fractionating column is a tube packed with glass beads. The beads provide surface for the vapours to cool and condense repeatedly.
7. Crystallisation(Separation of pure substance from its impure form)
Crystallisation is a process that separates a pure solid in the form of its crystals from a solution. This method is used to purify solids, e.g. the salt we get from sea water can have impurities in it. To remove these impurities, the process of crystallisation is used.
Crystallisation technique is better than simple evaporation technique as:
•Some solids decompose or some, like sugar, may get charred during heating to dryness.
•Some impurities may remain dissolved in the solution even after filtration. On evaporation these contaminate the solid.
Applications crystallisation technique is used in:
- Purification of salt obtained from sea water.
- Separation of crystals of alum (phitkari) from impure samples.
Purification of drinking water
(i) Sedimentation to remove suspended solids (water is allowed to stand for some time so that suspended impurities settle down).
(ii) Loading with alum to remove small particles like clay present in the colloidal state. Particles like clay being negatively charged are neutralised by alum and are coagulated at the bottom of tank.
(iii) Filtration to remove dissolved solids by passing the water through filtration tank which possess three layers. Coarse gravel at the bottom, fine gravel in the centre and fine sand art the top acts as filters. Impure water is introduced from the bottom such that impurities are retained in three layers of gravel and pure water goes to the top and sent to chlorination tank.
(iv) Chlorination to kill bacteria. Filtered water is treated with bleaching powder to kill bacteria. The pure water is now supplied to homes for drinking purpose.
Comparative study of physical and chemical changes
| Physical change | Chemical change |
| Only physical properties of the substance change. | Chemical properties also change. |
| No new substance is formed. | One or more new substances are formed. |
| These changes are generally reversible. | These changes are not easily reversible. Only some of them are reversible. |
| Energy changes normally do not accompanied the change. | Energy changes normally occur during these changes. |
| e.g. melting of ice, cutting off paper, melting of wax, etc. | e.g. burning of coal, formation of curd from milk, etc. |
Q1. What is a Compound?
Q2. Difference between Physical and Chemical?
Q3. How Crystallisation technique is better than the simple evaporation technique?
More
1 thought on “Is Matter Around Us Pure Notes Science Chapter 2 Class 9”