Study Notes for CBSE Class 9 Science Chapter 2 Is Matter Around Us Pure
‘Pure’ word means that there is no mixing in a substance. But according to scientific language all things are mixture of so many substances, not of single one. That’s why they are not pure. e.g., Milk, water, fat, etc.
• Pure substances means that all elements have same chemical properties.
• A pure substance is made up of same kind of elements.
Substance : A substance is a kind of matter that cannot be separated into other kind of matter by any physical process. A pure substance is made up of same kind of elements.
What is a mixture ?
It is a substance in which two or more substances (element or compound) are simply mixed together in any proportion. Examples : The air is a mixture of oxygen, nitrogen, argon, carbon dioxide and water vapour.
Types of Mixture :
Mixtures are of two types :
(i) Homogenous mixture
(ii) Heterogenous mixture
Homogenous Mixture : It has no visible boundaries of separation between the various constituents, so they are called homogenous mixtures.
Example : Sugar in water. It has a uniform composition throughout its mass.
Heterogenous Mixture : These types of mixtures has visible boundaries of separation between the various constituents.
Example : Mixture of sugar and sand. It does not have a uniform composition throughout its mass.
Solution : A solution is a homogenous mixture of two or more substances. E.g., Nimboo pani, soda water.
A solution has a solvent and a solute as its components. The component of the solution that dissolves the other component in it is called the solvent. The component of the solution that is dissolved in the solvent is called the solute.
Types of Solution
True , Colloidal and Suspension
Methods of Separation of Mixtures
Basic principle : Out of the two components of a mixture one can evaporate [i.e., has less boiling point] and other has higher boiling point.
Example : Mixture of dye [higher boiling point] and water. Out of water and dye, water evaporates but dye is left behind in petri dish.
(2) Centrifugation :
Basic principle : When mixture is rotated very fast, then denser particles are forced at the bottom and lighter particles stay above.
Example : Separating cream from milk.
(a) Used in diagnostic labs for blood and urine tests.
(b) Used in dairies and home to separate butter from cream.
(c) Used in washing machines dryers to squeeze out water from clothes.
(3) Differential extraction :
Basic principle : Two immiscible liquids (which do not dissolve in each other) can be easily separated by putting in a differential extraction funnel/separating funnel.
Example : Water from oil can be separated by first opening the stop cock till water is removed in one beaker, then afterwards oil can be collected in a separate beaker.
(a) Separation of oil from water.
(b) Extraction of iron from its ore. Lighter slag is removed from above the molten iron.
(4) Sublimation :
Basic principle : Out of the two components, one will sublime (directly converts to gas from solid) and other will not.
Example : NH4Cl (ammonium chloride) and NaCl common salt mixture can be easily separated by heating so that NH4Cl sublimes but common salt remains behind.
(a) Camphor, naphthalene, anthracene, NH4Cl can sublime.
(5) Chromatography :
Basic principle : Coloured components of a mixture can be separated by using an adsorbent on which they are adsorbed at different rates. (Absorption is the process of surface absorption.)
When water/any suitable solvent moves up, the chromatography paper ink with two different colours separates because both colours are adsorbed at different speeds.
(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 : Out of the two components one has a lower boiling point and other has higher boiling point. This is used to separate two or more miscible liquids.
(7) Crystallisation :
Basic principle : To remove impurities from a mixture by first dissolving in a suitable solvent and then crystallising out one component.
For example : Copper sulphate crystals (impure) are first dissolved in sulphuric acid and then heated to saturated solution. Now, this solution is left overnight. So, only pure copper sulphate crystals are formed whereas impurities are left behind in the solution. This solution can be thus filtered so as to get pure copper sulphate crystals on filter paper.
Why is crystallisation better than evaporation ?
(i) Some solids decompose or get charred upon heating to dryness during evaporation. E.g., sugar.
(ii) Some impurities remain dissolved in solution after filtration. On evaporation, these impurities do not evaporate and remain with the mixture.
(a) Purification of salt from sea water.
(b) Separation of crystals [e.g., alum (phitkari, copper sulphate)] from their impure crystals.
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