CHAPTER - 6 PHYSICAL AND CHEMICAL CHANGES

1. CHAPTER - 6PHYSICAL AND CHEMICAL CHANGES

2. 1) Changes around us :- In our daily life we come across several changes. Eg :- Dissolving sugar in water. Formation of curd from milk. Cooking of food. Cutting of a piece of paper. Stretching a rubber band. Melting of ice. Melting of wax Boiling of water. Rusting of iron. Burning of candle Burning of wood. Burning of coal.

3. 2) Types of changes :- Changes are of two main types. They are :- i) Physical change ii) Chemical change Physical change :- is a change in which no new substance is formed. Chemical :- change is a change in which new substances are formed.

4. 3) Physical changes :- Physical change is a change in which no new substance is formed. In a physical change the physical properties like shape, size, state etc. of the substance changes. Eg :- Cutting of paper into pieces, Melting of ice, Boiling of water, Stretching a rubber band etc. Some physical changes are reversible. Eg :- Melting of ice, Boiling of water, Stretching a rubber band etc.

5. 4) Chemical changes :- Chemical change is a change in which new substances are formed. A chemical change is also called a chemical reaction. Eg :- Rusting of iron, Burning of wood, Formation of curd from milk, Cooking of food etc. During a chemical change the following changes take place :- i) Formation of new substances. ii) Heat or light may be absorbed or released. iii) Sound may be produced. iv) A colour change may take place. v) A change in smell may take place. vi) A gas may be formed.

6. 5) Burning of magnesium ribbon :- When a strip of magnesium ribbon is heated over a flame, it combines with oxygen and burns with a bright white light to form a powdery ash called magnesium oxide. The reaction can be written as :- Magnesium (Mg) + Oxygen (O2) Magnesium oxide (MgO) If the magnesium oxide is dissolved in water, it forms a new substance called Magnesium hydroxide. It turns red litmus to blue. So it is basic. The reaction can be written as :- Magnesium oxide (MgO) + Water (H2O) Magnesium hydroxide [ Mg(OH)2]

7. 6) Reaction between copper sulphate and iron :- Dissolve some copper sulphate in a beaker and add a few drops of dilute sulphuric acid to it. The solution is blue coloured. Drop a nail or a shaving blade into it. Observe the colour of the nail or blade after half an hour. A brown deposit of copper is formed over the nail or blade and the solution colour changes to green due to the formation of iron sulphate. The reaction can be written as :- Copper sulphate (CuSO4) + Iron (Fe) Iron sulphate (FeSO4) + Copper (Cu) After half an hour

8. 7) Reaction between vinegar and baking soda :- Take some vinegar in a test tube and add some baking soda to it. Gas bubbles are formed with a hissing sound. The gas formed is carbon dioxide. The reaction is :- Vinegar (Acetic acid) + Baking soda (Sodium hydrogen carbonate) Carbon dioxide + Other substances If the carbon dioxide gas is passed through lime water, the lime water turns milky due to the formation of calcium carbonate. This is a test for carbon dioxide. The reaction is:- Carbon dioxide (CO2) + Lime water [ Ca(OH)2 ] Calcium carbonate (CaCO3) + Water (H2O)

9. 8) Rusting of iron :- Rusting of iron causes damage to articles made of iron. Rusting of iron takes place due to the reaction between iron and oxygen and water (or water vapour). The reaction of rusting of iron is :- Iron (Fe) + Oxygen (O2) + Water (H2O) Rust (Iron oxide) Rusting of iron can be prevented by :- i) Applying oil or grease. ii) Applying paint. iii) Coating a layer of metals like zinc or chromium. The process of coating a layer of zinc on iron is called galvanisation. It prevents rusting of iron.

10. 9) Crystallisation :- The process of obtaining large crystals of a pure substance from its solution is called crystallisation. To prepare crystals of copper sulphate :- Take some water in a beaker. Add a few drops of dilute sulphuric acid to it. Heat the water. When it starts boiling add powdered copper sulphate slowly and stir the solution till no more powder can be dissolved. Then filter the solution and allow it to cool. Crystals of copper sulphate are formed.