Question

E. ANSWER THE FOLLOWING QUESTIONS :
1. What is the formula of quick lime ?
2. Which arrow is used to indicate a reversible change?
3. What is the colour of flame when magnesium burns in air?
4. Give two factors on which a change depends.
5. Name the reactions in which heat is absorbed.
6. Define dissolution.
7. How does rate of evaporation change when temperature is increased ?
8. Why is rusting referred as slow eating of metals ?
9. Which changes are referred to as undesirable changes ?
10. 'Pulling of a rubber string is a reversible change'. Explain.
11. Differentiate between :
(a) Rusting and burning.
(b) Burning and respiration
12. What is evaporation ? How does water change to its vapour state?
13. Differentiate between physical and chemical changes.

Answer 1

1. The chemical formula for quick lime is CaO .
2. The double arrow is the indication that the reaction is reversible.
3. In its pure form, it is silvery white, and relatively soft. It burns in air with a brilliant white light, and for this reason is often used in flares and fireworks.
4. Factors on which change depends: So, pressure and temperature will also influence change.
5. A reaction in which heat is absorbed is called an endothermic reaction.
6. don't know
7. Evaporation increases with the increase in temperature as more molecules get kinetic energy to convert into vapor. When the water is heated, the water molecules tend to move rapidly. This makes the molecules escape faster.
8. Rusting is referred to as the slow eating of metal because it is an oxidation process and the oxide which is formed is attached with very weak bonds with the metal, due to this, it wears off from the metal. This rust gets shred off from the metal surface leads to the thinning of the iron surface.
9. Those changes which we do not want to take place are called undesirable changes. Examples: rusting of iron, souring of milk, rottening of fruits etc.
10. When we strech a rubber with force of our hands, it undergoes a change and its length increases. But, on releasing the force, rubber comes back to it's original length. So the streching of rubber is a reversible change.
11. Difference between a)rusting and burning:

Rusting:

Rusting is the process of formation of iron oxide or red oxide formed on surface of iron and its alloys when exposed to “air and moisture”.

1. It requires both oxygen and moisture.
2. It is a gradual process.
3. Rusting occurs at low temperatures.
4. Rusting takes place only in iron and its alloys.
5. Rusting results in the reddish coating on the surface due to red or iron oxide formation.

Burning:

2. Burning is a process where a substance reacts with the oxygen with the release of “heat and light”.
3. It requires only oxygen, moisture is not required.
4. It is rapid process
5. It needs high temperature.
6. Burning takes place in fuel, rubber, paper,and many more substances.
7. Burning results in black soot and smoke particles.

b) answer in attachment

12.Evaporation happens when a liquid turns into a gas. When liquid water reaches a low enough temperature, it freezes and becomes a solid—ice. When solid water is exposed to enough heat, it will melt and return to a liquid. As that liquid water is further heated, it evaporates and becomes a gas—water vapor.

13. Physical change

(a) This change is temporary and reversible.

(b) There is no formation of new products.

(c) There is no energy change in these changes. (

Example - Melting of ice.

chemical change

(a) These changes are permanent and irreversible.

(b) There is formation of new products.

(c) Energy is either absorbed or evolved in these changes.

Example - Change of milk into curd.