Chemical Changes
Those changes in which new substances are formed, are called chemical changes.In a chemical change, the substances involved change their identity. They get converted into entirely new substances.The new substances usually cannot be returned to their original form. This means that chemical changes are usually irreversible.
Some common examples of chemical changes are : Burning of a magnesium wire ; Burning of paper; Rusting of iron ; Ripening of fruits, Formation of curd from milk; and Cooking of food.Some of these chemical changes are discussed below.
(i) When a magnesium wire is heated, it burns in air to form a white powder called 'magnesium oxide'.This magnesium oxide is an entirely new substance. Thus, a new chemical substance is formed during the burning of a magnesium metal wire.So,
the burning of a magnesium wire is a chemical change.
(ii) If we bum a piece of paper with a lighted match stick then entirely new substances like carbon dioxide, water vapour, smoke and ash are produced. So,
the burning of paper is a chemical change.
The chemical changes are permanent changes which are usually irreversible. For example, the burning of paper is a permanent change which cannot be reversed. This is because we cannot combine the products of burning of paper to form the original paper once again.
Some more examples of chemical changes are : Burning of candle wax ;Burning of charcoal; Burning of fuels (like Coal, Wood and LPG); Burning of hydrogen in oxygen to form water ; Decomposition of water into hydrogen and oxygen by passing electric current (Electrolysis of water) ; Burning of incense stick (agarbatti); Formation of iron sulphide from iron and sulphur ; Growth of a plant; Cutting of trees; and Digestion of food. During all these changes, new chemical substances are formed. Please note that chemical changes are also known as chemical reactions. The following diagrams show a physical change and a chemical change which take place in the kitchen of our homes everyday :
The main differences between physical and chemical changes are given below.
| Physical change | Chemical Change |
1.No new substancc is formed in a physical change.
2.A physical change is a temporary change.
3.A physical change is easily reversible.
4.Very little heat (or light) energy is usually absorbed or given out in a physical change.
5.The mass of a substance does not alter in a physical change. | 1.A new substance is formed in a chemical change.
2.A chemical change is a permanent change.
3.A chemical change is usually irreversible.
4.A lot of heat (or light) energy is absorbed or given out in a chemical change.
5.The mass of a substance does alter in a chemical change. |
Notes
Matter in Our Surroundings - Notes
2. 7. Metals are Solids at the Room Temperature
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4. 3. Non-Metals are Bad Conductors of Heat and Electricity.
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8. 11. Non-Metals Have Many Different Colours.
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9. 7. Non-Metals may be Solid, Liquid or Gases at the Room Temperature.
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15. 8. Metals Generally Have High Melting Points and Boiling Points.
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19. 8. Non-Metals Have Comparatively Low Melting Points and Boiling Points
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23. Chemical Formula for daily use material
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24. 1. Non-Metals are Not Malleable. Non-Metals are Brittle.
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25. To Study the Properties of a Suspension
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26. Effect of Temperature and Pressure on Solubility
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31. Pure Substances : Elements and Compounds
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32. 3. Metals are Good Conductors of Heat and Electricity.
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36. 1. Separation by a Suitable Solvent
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37. 4. Non-Metals are Not Lustrous (Not Shiny). They are Dull in Appearance.
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38. Solutions, Suspensions And Colloids
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46. Differences Between Mixtures and Compounds
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48. To Separate the Salt-Water Mixture (or Salt-Solution)
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