Class 8th (Chemistry) chp-3 Elements, Compounds & Mixtures (part-2) Matter is classified into two groups: 1. Pure substances (i.e. elements and compounds) 2. Impure substances (mixture) Element: An element is matter which is made up of only one type of atom. There are total 118 elements out of which 92 occur naturally and rest of them are made by humans. IUPAC (International Union of pure and applied chemistry): IUPAC system is used to name the element and compounds. Some names elements derived from their Latin name; Eg. Sodium: Natrium (Na) Potassium: Kalium (K) Iron: Ferrum (Fe) Copper: Cuprum (Cu) Silver: Argentum (Ag) Gold: Aurum (Au) Atomicity: The number of atoms that are present in a molecule of an element is it’s called atomicity. A molecule can be: Monoatomic. Eg. Na, K Diatomic. Eg. H2,O2 Triatomic. Eg. O3 Polyatomic. Eg. S8, P4 Q. Explain what is atomicity. Difference between element and compound: Elements Compounds i. An element is a pure chemical substance made of same type of i. A compound contains atoms of different elements chemically atom. combined together in a fixed ratio. ii. An element is represented using symbols. ii. A compound is represented using a formula. Q. Write difference between elements and compound. Different type of mixture: solid - solid: sand & chalk powder, rice & chaff solid - liquid: sand & water, chalk powder & water liquid - liquid oil & water, alcohol & water liquid - gas: oxygen & water, aerated drink solid - gas: smoke(shoot particles + air) gas - gas: air(O2 + CO2 + N2 etc) Sublimation: The change of a solid directly into its vapour state without passing through the liquid state is called sublimation. Example: Camphor, ammonium chloride, dry ice, iodine, Benzoic acid, anthracene, naphthalene. Crystallization: It is the process of separating a pure solid in the form of its crystals from its solution. Separation of solid liquid mixture: There are two methods to separate solid – liquid mixture. Sedimentation Decantation Example of solid – liquid mixture: sand in water, muddy water. Sedimentation: The settling down of insoluble heavy solid particles in a solid liquid mixture at the bottom of container is called sedimentation. During sedimentation, the heavier coarse particles settle down quickly. But the finer particles settle down very slowly. The process of making the finer particles settle down quickly is known as loading. It can be done using an alum. Alum dissolves in water and makes the clay particles heavier and increases the rate of sedimentation. Q. Write about sedimentation in brief. Decantation: The process of pouring the clear supernatant liquid without disturbing the sentiment is called decantation. Activity 1: Aim: To separate calcium carbonate powder mixed with common salt (sodium chloride) by filtration. Materials required: Beakers, a piece of filter paper, a funnel, a glass rod, a funnel stand and mixture of common salt and calcium carbonate. Procedure: Take the mixture in a beaker and add water to it to make a solution. Take a circular piece of filter paper and fold it in half and then in quarter. Then, open it out in the form of a cone to fit it properly. You can put a few drops of water on the filter paper cone to fit it properly. Fix the funnel in a funnel stand and place a beaker below the funnel. Now stir the mixture and pour it slowly along a glass rod into the filter paper cone. Observation: The common salt dissolves in water passes through the filter paper and collects in the beaker as filtrate. The calcium carbonate is left on the filter paper as the residue. Conclusion: As calcium carbonate is insoluble in water, we can separate if by filtration. Q. How to separate a mixture of solid – solid (calcium carbonate and common salt)? Explain with the help of an activity. Activity2: Aim: To obtain the common salt (sodium chloride) dissolved in water by evaporation Materials required: An evaporating dish, the filtrate containing salt obtained from Activity 1, a Bunsen burner, a tripod stand, a wire gauze. Procedure: Take the filtrate containing common salt in an evaporating dish. Now heat the filtrate Observation: As the solution is heated, water gets evaporated from the dish. Let the water evaporate completely. You will find that it is nothing but salt. Conclusion: We can separate common salt dissolved in water by evaporation. Q. Explain how to separate a solid which is dissolved in a liquid with the help of an activity. Activity 3: Aim: To separate a mixture of kerosene and water. Materials required: A mixture of kerosene and water, a separating funnel, a tripod stand, a beaker Procedure: Pour a mixture of kerosene and water in a separating funnel. Shake up the mixture and let it stand for some time. Carefully open the stopper of the funnel and allow the water to flow out into a beaker. Stop as soon as the layer of the kerosene reaches the stopper. Observation: The kerosene remains in the separating funnel and the water in the beaker. Conclusion: Kerosene and water can be separated using a separating funnel. Q. How to separate two immiscible liquids? Explain with the help of an activity. .