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Question (31)

QN001111

Which of the following changes is irreversible because a new substance forms?

(A) Melting ice
(B) Folding paper
(C) Burning wood
(D) Inflating a balloon

Correct Answer: C

Explanation:

Burning wood forms ash and gases.

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Question (32)

QN001112

A student mixed baking soda with water and observed no bubbles. This indicates that

(A) no chemical reaction occurred
(B) oxygen was produced
(C) combustion took place
(D) magnesium oxide formed

Correct Answer: A

Explanation:

No new substance or gas formed.

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Question (33)

QN001113

The milky appearance of lime water is due to the formation of a substance that is

(A) soluble in water
(B) colourless
(C) insoluble in water
(D) gaseous in nature

Correct Answer: C

Explanation:

Insoluble calcium carbonate makes the solution milky.

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Question (34)

QN001114

Which observation would best help identify carbon dioxide experimentally?

(A) It smells sweet
(B) It turns lime water milky
(C) It burns with blue flame
(D) It changes colour of paper

Correct Answer: B

Explanation:

Carbon dioxide is tested using lime water.

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Question (35)

QN001115

During combustion, oxygen mainly acts as

(A) fuel
(B) coolant
(C) supporter of burning
(D) solidifying agent

Correct Answer: C

Explanation:

Oxygen supports the burning process.

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Question (36)

QN001116

Which of the following is an example of Science and Society from the chapter?

(A) Folding paper boats
(B) Making chalk powder
(C) Extinguishing fire by cutting off air supply
(D) Inflating balloons

Correct Answer: C

Explanation:

Fire safety is related to society and daily life.

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Question (37)

QN001117

A student says that combustion cannot occur without heat. Which activity supports this idea?

(A) Rusting iron nails
(B) Blowing air in water
(C) Burning paper with focused sunlight
(D) Folding paper

Correct Answer: C

Explanation:

Heat from focused sunlight raises paper to ignition temperature.

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Question (38)

QN001118

Which statement about chemical changes is correct?

(A) They never release energy
(B) They always involve only state change
(C) They produce one or more new substances
(D) They can always be reversed easily

Correct Answer: C

Explanation:

Formation of new substances defines a chemical change.

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Question (39)

QN001119

Which of the following combinations will produce carbon dioxide gas?

(A) Baking soda and vinegar
(B) Sand and water
(C) Salt and sugar
(D) Chalk and paper

Correct Answer: A

Explanation:

The reaction releases carbon dioxide gas.

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Question (40)

QN001120

Why does lime water remain unchanged when ordinary air is not blown continuously into it?

(A) Carbon dioxide reaction becomes insufficient
(B) Lime water becomes combustible
(C) Oxygen changes to nitrogen
(D) Water freezes immediately

Correct Answer: A

Explanation:

Continuous carbon dioxide supply is needed to turn lime water milky.

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Question (31)

Which of the following changes is irreversible because a new substance forms?

Explanation:

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Question (32)

A student mixed baking soda with water and observed no bubbles. This indicates that

Explanation:

Link:

Question (33)

The milky appearance of lime water is due to the formation of a substance that is

Explanation:

Link:

Question (34)

Which observation would best help identify carbon dioxide experimentally?

Explanation:

Link:

Question (35)

During combustion, oxygen mainly acts as

Explanation:

Link:

Question (36)

Which of the following is an example of Science and Society from the chapter?

Explanation:

Link:

Question (37)

A student says that combustion cannot occur without heat. Which activity supports this idea?

Explanation:

Link:

Question (38)

Which statement about chemical changes is correct?

Explanation:

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Question (39)

Which of the following combinations will produce carbon dioxide gas?

Explanation:

Link:

Question (40)

Why does lime water remain unchanged when ordinary air is not blown continuously into it?

Explanation:

Link:

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