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Each part: 1.5 Marks
(a) When candle burns, both physical and chemical changes take place. On burning, the candle wax melts but can be solidified again on cooling. This shows that melting of wax is a physical change.
The burning of candle also produces light and carbon dioxide. Hence, it is also a chemical change.
(b) A reversible change is a type of change in which a substance that is undergoing a change can be recovered in its original form. An irreversible change is a type of change in which a substance cannot be reobtained in its original form. In the cooking of rice, raw rice cannot be obtained from cooked rice. Therefore, this change is an irreversible change.

Answer: Option A. ->
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Explanation: 1 Mark
Chemical Equation: 1 Mark
The standard test of carbon dioxide is by passing it through lime water.
When carbon dioxide is passed through lime water, calcium carbonate is formed, which turns lime water milky.
Carbon dioxide ($C{O}_2$) + Lime water [$Ca(OH{)}_2$] → Calcium Carbonate($CaC{O}_3$) + Water ($H_{2}O$) 

Answer: Option A. ->
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Each point: 1 Mark
1) Evaporation is the process of loss of water as water vapour from the surface of water.
2) Evaporation is a physical change. When the water vapour is allowed to be cooled, its physical state changes back to liquid form. 

Answer: Option A. ->
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Each part: 1 Mark
(a) A change in which one or more new substances are formed is called a chemical change. When steam is condensed, it becomes water. Only the state of $H_{2}O$ changes. A new substance is not formed. So, it is not a chemical change.
(b) When two liquids are immiscible and are taken together in a funnel, the heavier liquid forms the bottom layer and the lighter liquid forms the top layer.
 

Answer: Option A. ->
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(1) Each part: 1 Mark
(2) Answer: 1 Mark
(1)  a) In case of water, when it is cooled up to zero degree Celsius it becomes ice. However, when the ice is left in a warm room, it becomes liquid again. So, the change occurs only in its phase which is a physical change and reversible.
b) Rusting of iron is an example of a chemical change in which colour change is observed. If we leave a piece of iron in open for some time, it acquires a film of brownish substance.This substance is called rust and the process is called rusting.
$Iron\left(Fe\right)+Oxygen(O_2,fromtheair)+water\left(H_{2}O\right)\to Rust\left(ironoxide\right)(F{e}_2{O}_3.H_{2}O)$
(2) After the formation of curd, milk cannot be re-obtained from it. Also, both milk and curd have different properties. Since these are the properties of a chemical change, setting of curd is a chemical change.
 

Answer: Option A. ->
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Explanation: 2 Marks
Figure: 1 Mark
 
(i) Take a separating funnel. After setting up the separating funnel, pour the kerosene oil and water mixture in it.
(ii) Let it stand undisturbed for some time. Soon two distinct and separate layers of oil and water will be seen.
(iii) Next, open the stopcock of this separating funnel. The aim is to drain out the water slowly which has collected at the base of the funnel.

(iv) Close the stopcock of this separating funnel as soon as the level of oil reaches the stopcock.
 

Answer: Option A. ->
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Definition: 1 Mark
Factors: 0.5 Mark each
Corrosion is a natural process, which converts a refined metal to a more chemically-stable form, such as its oxide, hydroxide, or sulfide. It is the gradual destruction of materials (usually metals) by chemical and/or electrochemical reaction with their environment.
 

The factors which affect corrosion are:

1. More the reactivity of the metal, more will be the possibility of the metal getting corroded.

2. Presence of carbon dioxide in natural water increases rusting of iron.

3. When the iron surface is coated with layers of metals more active than iron, then the rate of corrosion is retarded.

4. A rise in temperature (within a reasonable limit) increases the rate of corrosion.

 

Answer: Option A. ->
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(a) Characteristics: 2 Marks
(b) Explanation: 1 Mark
(a) The characteristics of chemical changes are:

1) It is a permanent change and is irreversible.
2) New substances are formed in this change.
3) Energy in the form of heat or light is either absorbed or released.
(b) When wood is lit up, it burns gently and after a while, it turns into ashes. As it burns, it produces heat, light and smoke which escapes through the chimney. The heat, light, fire and smoke are all good characteristics of a chemical reaction, which result in a completely new matter—ash.

Answer: Option A. ->
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(a) Explanation: 2 Marks
(b) Explanation: 1 Mark
(a) When baking soda is added to lemon juice, a chemical change occurs. In this reaction, a new substance is formed which is carbon dioxide along with the evolution of heat. It is a chemical change and the reaction is irreversible.
The equation for the reaction is as follows:

Sodium Bicarbonate + Citric acid $\to$ Sodium Citrate + Carbon dioxide + Water + Heat
(b) Food is chemically digested by enzymes. Once food has been broken down, the nutrients are sent into the bloodstream where they find their way into the liver. Food undergoes many chemical changes before being absorbed by the body. Hence digestion of food is a chemical change as the complex substances that we eat is changed into simpler substances (new products).

Answer: Option A. ->
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Reason: 3 Marks
Rotten food smells bad because of the chemical reactions occurring in it due to the action of microorganisms. Rotting alters the composition of the substance, the old bonds of the reactants are broken down and fresh bonds are formed, resulting in the formation of new products with different properties.