PHYSICAL AND CHEMICAL CHANGES CLASS 7. Types of changes: The changes are of two kinds, physical and chemical..

Transcription

1 PHYSICAL AND CHEMICAL CHANGES CLASS 7 Types of changes: The changes are of two kinds, physical and chemical.. Physical Properties of Substances Properties such as shape, size, colour and state of a substance are called its physical properties. Physical Change A change in which a substance undergoes a change in its physical properties is called a physical change. A physical change is generally reversible. In such a change no new substance is formed. Examples of Physical changes 1. Boiling of water 2. Melting of Ice 3. Melting of Wax 4. Cutting paper into small pieces 5. Crushing of calk onto dust Chemical Changes Burning of Magnesium: Symbol of Magnesium Mg Magnesium is kept in lab as Magnesium Ribbon Surface of Magnesium looks dull.( Surface of Magnesium id cleaned with sand paper) During burning magnesium is reacting with Oxygen Magnesium burns with Dazzling silver white colour. After burning Magnesium oxide is left as white powder. Magnesium+ Oxygen Magnesium oxide 2Mg + O 2 2MgO Reactants : Substances that take part in a chemical reaction are called as Reactants. Products : Newly formed substances in a chemical reaction are called products 2Mg + O 2 2MgO Reactants: Magnesium and Oxygen Products: Magnesium oxide Litmus Test for Magnesium oxide Dissolve Magnesium oxide in water in a petri dish. Magnesium oxide dissolve in water slowly. Magnesium oxide dissolves in water to form,magnesium hydroxide.

2 Dip a red litmus paper in it Red litmus changes to blue Magnesium oxide + Water Magnesium hydroxide MgO + H 2 O Mg(OH) 2 Since new substance is formed, this is a chemical change Displacement Reaction Reaction between Iron nail and Copper Sulphate Iron- is a metal, silver grey colour, Symbol- Fe Magnesium hydroxide isa bas and changes red litmus to blue Copper Sulphate Blue crystal, Formula: CuSO4, Solution Blue colour Take a 10 ml Copper sulphate solution in a test tube, and put a clean iron nail. Keep it for 3-4 hours Blue colour of the solution changes to green Brown deposit over the iron nail Green colour is due to the formation of Iron sulphate Brown deposit id due to the formation of Copper Copper sulphate + Iron Iron sulphate + Copper CuSO 4 + Fe FeSO 4 + Cu Since new substances are formed this is a chemical change Reaction between Vinegar and Baking Soda Vinegar Acetic Acid, Formula CH 3 -COOH Baking Soda Sodium hydrogen carbonate, Formula NaHCO 3 Take small pinch of baking soda in a test tube and add small quantity of Vinegar Bubble come out suddenly with a hissing sound The gas is Carbon dioxide Vinegar + Baking soda Carbon dioxide Since new substances are formed, this is a chemical change

3 Reaction between Lime Water and CO 2 Lime Water: Calcium hydroxide ( Ca (OH)2 Dissolve calcium hydroxide and filter with filter paper Take 2 ml of lime water and pass CO2 by blowing exhaled air. Shake the tube. Lime water changes milky Calcium hydroxide + Carbon dioxide calcium carbonate + water Ca ( OH) 2 + CO 2 CaCO 3 + H 2 O Since new substances are formed, this is a chemical change USES OF CHEMICAL REACTION The milky colour is due to the formation of a new substance called calcium carbonate Changes that help us to know that a chemical change has taken place Burning In addition to new products, the following may accompany a chemical change: Heat, light or any other radiation (ultraviolet, for example) may be given off or absorbed. Sound may be produced. A change in smell may take place or a new smell may be given off. A colour change may take place. A gas may be formed Burning is a chemical change. Burning of any substance produces new substance. Burning is always accompanied by production of heat or light or both We should not play with fire crackers.: Explosion of a firework is a chemical change. An explosion produces heat, light, sound and unpleasant gases that pollute the atmosphere. That is why we are advised not to play with fireworks. A cut piece of Apple changes to red colour: a slice of an apple acquires a brown colour if it is not consumed immediately.it is due to chemical reaction A slice of potato or brinjal changes colour if kept open in air: A slice of potato or brinjal. change colour due to the formation of new substances. Or chemical changes

4 Neutralization is a chemical reaction: ACID + BASE SALT + WATER More acid in stomach causes acidity. Acidity is controlled by using Antacids. Rusting: Rusting ia a chemical change. Iron changes to a brown powder. Conditions needed for Rusting Presence of oxygen Presence of moisture Methods to prevent Rusting 1. Applying grease, oil or paint- Prevent contact between iron and oxygen/moisture 2. Galvanisation:- Coating objects with a thin layer of zinc metal 3. Alloying:- an alloy is a mixture of one metal with other metal or non-metal Stainless Steel is an alloy of Iron Does not rest Made by mixing carbon, chromium and nickel with molten iron Alloy of Gold Pure gold is 24 carat Not good for making ornaments 22 carat Gold 22 pars of Gold + 2 parts of silver/ copper Uses of Chemical Reaction: Chemical changes are very important in our lives. All new substances are formed as a result of chemical changes.,if a metal is extracted from an ore, by a series of chemical changes. A medicine is the end productof a chain of chemical reactions. Materials, such as plastics and detergents, are produced by chemical reactions. CRYSTLLISATION ;- Some substances can be obtained in pure state from their solutions by crystallisation. Crystallisation. is a.physical change. Method Observation Result Prepare saturatd solution of copper sulphate in After some time we see the crystals of copper On evaporation, pure crystals are formed from a water. Add few drops of sulphate saturated solution by sulphuric crystallisation Filter the solution. Allow it to cool. Do not disturb the solution when it is cooling. Look at the solution after some

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