Answer: Option A. -> True
:
A
A change in which new substance is formed, whose composition and properties are different from those of the original substance is known as a chemical change. The change is permanent and cannot be reversed. So, chemical changes are mostly irreversible and physical changes are reversible. Hence, the given statement is true.

Answer: Option A. -> No new substance is formed
:
A
A chemical change is a permanent change in which new substances are formed that are entirely different from the original substance.
It is usually accompanied by:
$\bullet$ change in colour
$\bullet$ formation of new substances
$\bullet$ evolution of gas
$\bullet$release or absorption of energy in the form of heat or light or sound, etc.

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Naming: 1 Mark
Equation: 1 Mark
The two conditions essential for rusting are presence of
(i) Oxygen
(ii) Water or water vapour
The chemical equation for rusting of Iron is:
Iron(Fe) + Oxygen$(O_2$) + Water$(H_{2}O$) $\to$ rust(Iron Oxide$F{e}_2{O}_3$)

:
(1) Each part: 1 Mark
(2) Answer: 1 Mark
(1) a) In case of water, when it is cooled up tozero 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.

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Changes: 1 Mark
Explanation: 2 Marks
When a candle is burnt, two types of changes occur
i) Physical Change
ii) Chemical Change
When the candle is lit, the wax at the top melts. This is a physical change.
The wick absorbs liquid wax. The liquid wax vaporizes due to the heat produced by the flame. This wax vapour near to flameburns and gives new substances like Carbon Dioxide, Carbon soot, water vapour, heat, and light. This is an irreversible change.

:
Physical change definition: 1 Mark
Chemical change definition: 1 Mark
Examples: 1 Mark
Physical changes are the changes where the physical property of the substance varies without any formation of new products. These changes are reversible and the products are identical to the reactants.
Examples: Melting of ice,melting of wax.
Chemical changes are the changes where the chemical property of the substance varies with the formation of new products.These changes are irreversible and the products are not identical to the reactants.
Examples: Formation of curd from milk, rusting of iron.

:
Each part: 1.5 Marks
(a) Yes, the rusting of iron nails is faster in coastal areas than in desserts.
For rusting, the presence of both oxygen and water is essential.
In coastal areas humidity is much higherthan in desserts.
The content of moisture in the air is more in coastal areas than in dessert. So the process of rusting becomes faster in coastal areas.
(b)The correct sequence of processes in water purification is:
Sedimentation - For removal of heavy particles.
Loading - For removal of the suspended particles.
Filtration - For removal of small particles which cannot be removed by sedimentation and loading.

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(a) Products: 1 Mark
Equation: 1 Mark
(b) Explanation: 1 Mark
(a) When a magnesium ribbon is burnt, magnesium oxide is formed. The equation for the reaction is given below.
$Mg+O_2\to MgO$
When magnesium oxide is dissolved in water, magnesium hydroxide is formed. The equation for the reaction is as follow:
$MgO+H_{2}O\to Mg(OH{)}_2$
(b)When wood is burnt, a new substanceis formed. Therefore, it is a chemical change. However, when we cut wood, only the shape and size of the wood changes but no new substance is formed. Therefore, it is a physical change.

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Change: 1 Mark
Justification: 2 Marks
Blending a smoothieinvolves two physical changes: the change in shape of each fruit and the mixing together of many different pieces of fruit. There is no chemical change in the process.
This is a physical change because blending does not cause the fruits to become a fundamentally different substance. A new substance is not formed. It is a mixture of substances. So, it is a physical change.

:
a) Type of change: 1 Mark
Irreversible change: 1 Mark
b) Answer: 1 Mark
When vinegar is mixed with baking soda, the release of carbon dioxide gas is seen in the form of bubbles.
We cannot get back our original substances since it is an irreversible change.
It is a chemical change because a new substance (carbon dioxide) is formed.
b) When carbon dioxide is passed through lime water, calcium carbonate is formed, which makes the lime water milky. The turning of lime water milky is astandard test for carbon dioxide.