Class 7 Physical Chemical Change Notes

Chapter 6: Physical and Chemical change

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Long Answer Type Question:-

  1. Give the mechanism of rusting and its prevention.

Ans. When iron is exposed to the humid atmosphere the iron gets slowly corroded by the formation of a brown flaky substance on iron objects is known as rusting of iron or corrosion.

4Fe      +       3O2       +   2H2O        →       2Fe2O3H2O

Iron            Oxygen        Water vapour            Rust

moisture

Rusting generally weakens structures such as iron railings, bridges, window grills, agricultural equipment, vehicles, iron roofs etc.

 Prevention of rusting

Rusting can be avoided by using the following methods :

(i) Painting the surface of an iron article.

(ii) Applying grease or oil on the exposed surface of iron goods.

(iii) The iron pipes that are used to supply water are coated with zinc called GI pipes (Galvanized Iron pipes) to protect iron from rusting. The process of depositing a layer of metal over another metal is called galvanization.

(iv) Iron, when mixed with certain corrosion resistant metals, forms an alloy which is resistant to rusting.

(v) Electroplating the surface of iron articles with corrosion-resistant metal, for example, the rims and handles of motorcycles are shining due to chromium coating called chrome plating that protects them from rusting.

  1. What are the characteristics of a physical change?

Ans. Physical changes can be characterized by the following points.

(a) It is a reversible change.

(b) It is a temporary change.

(c) It changes only for time being.

(d) In this change, heat is involved but very little.

  1. What is a displacement reaction? Which rule governs displacement of an element in a compound by another element? Explain with an example.

 Ans. In this reaction, one element replaces another element from a compound. It is found that a more reactive element replaces a less reactive element from its compound.

A + BC→AC + B

(i) Zinc is a more reactive element than copper. If pieces of zinc are added to a solution of copper sulphate, zinc sulphate is formed. Copper metal gets deposited on the zinc piece.

Zn      +       CuSO4       ————-     ZnSO4      +     Cu

Zinc       Copper Sulphate               Zinc Sulphate    Copper

  1. With the help of an example, distinguish between endothermic reaction and exothermic reaction.

Ans. Exothermic reaction: The reactions which give off energy are termed as exothermic reactions.

For example, burning of methane in sufficient amount of oxygen, carbon dioxide and water.

Energy is produced in the form of heat and light. So, the burning of methane is an exothermic reaction.

CH4        +     2O2—————– CO2        +     2H2O + Heat + Light

Methane       Oxygen                Carbon           Water

dioxide

  Endothermic Reaction: The reactions in which heat is absorbed by the reactants to form a product are termed as endothermic reactions.

For example, The process of photosynthesis is an endothermic reaction.In this reaction, sunlight is required as energy.

6CO2        +      6H2O    +    Light energy  →   C6H12O       +        6O2

Carbon             Water                                         Glucose                   Oxygen

dioxide

 Think and answer

  1. When a candle burns, both physical and chemical changes occur. What are the changes?

Ans- The physical changes or actions that take place during the burning of a candle include the following:

  1. Melting of the wax by the heat of the flame of a candle. The melted wax forms a pool at the top of the wax column of the candle.
  2. Liquid candle seeping up the wick of the candle by capillary action. As the liquid wax gets seeps up the wick, the level of the liquid pool of wax gets lowered and then more wax is liquefied to continuously replenish the wax being sucked up the wick.
  3. Liquid wax at the tip of the candle being vaporized by the heat of the flame. It is this vaporised wax that burns in the candle to produce heat and light.
  4. Heating of the particles produced by burning of the candle, causing them to glow and radiate light.

It is interesting to note that the vaporization of the wax in the wick keeps the temperature of the wick and area immediately surrounding it comparatively cooler. As a result, the wick itself is not burnt down by the heat of candle flame.

The chemical action produced in the burning of a candle is the oxidation of the wax, which is composed of a mixture of several different hydrocarbon compounds. In this process of oxidation, the hydrocarbons combine with the oxygen in the atmosphere to produce carbon-dioxide (CO2) and water (H2O). This chemical reaction also produces the heat required for the physical changes described above.

  1. LPG (Liquid Petroleum Gas) in a gas cylinder is in the form of a liquid. Which physical or chemical change occurs when it comes out of the cylinder and burns?

Ans-Chemical changes take place when LPG comes out of the cylinder and burns.


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