Which one of the following is a physical change?
- A.Burning of coal
- B.Burning of wood
- C.Heating of a platinum crucible
- D.Heating of potassium chlorate
Correct answer
Explanation
The correct answer is C, Heating of a platinum crucible. Platinum only gets hot and may glow; it does not react, and on cooling the crucible is exactly as it was, so the change is physical.
A physical change alters state, shape or temperature without forming a new substance and is usually reversible, like melting ice or heating a metal. A chemical change forms new substances with new properties and cannot be undone easily. Platinum is a noble metal that resists oxidation even at high temperature, which is why laboratories use platinum crucibles and wires.
A is wrong because burning coal is combustion: carbon combines with oxygen to form carbon dioxide, a new substance.
B is wrong because burning wood produces carbon dioxide, water vapour and ash, which cannot be turned back into wood.
D is wrong because heating potassium chlorate decomposes it into potassium chloride and oxygen, a chemical change.
Exam tip: new substance formed = chemical change; only state or temperature changes = physical change.
Practice Questions
View allBronze is an alloy of copper and
- A.nickel
- B.iron
- C.tin
- D.aluminium
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Bronze is an alloy of copper and tin, usually about 88 parts copper to 12 parts tin. Adding tin makes copper harder, stronger and easier to cast, and bronze also resists corrosion well.
Read the full explanation →Which one of the following instruments is used for measuring moisture content of air?
- A.Hydrometer
- B.Hygrometer
- C.Hypsometer
- D.Pycnometer
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A hygrometer measures humidity, that is, the amount of water vapour or moisture in the air. The name comes from the Greek "hygros", meaning wet.
Read the full explanation →Which one of the following elements is present in the green pigment of leaves?
- A.Magnesium
- B.Iron
- C.Calcium
- D.Copper
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The green pigment of leaves is chlorophyll, and a magnesium atom sits at the centre of every chlorophyll molecule.
Read the full explanation →Consider the following actions :
1. Detection of car crash/collision which results in the deployment of airbags almost instantaneously
2. Detection of accidental free fall of a laptop towards the ground which results in the immediate turning off of the hard drive
3. Detection of the tilt of the smartphone which results in the rotation of display between portrait and landscape mode
In how many of the above actions is the function of accelerometer required?
- A.Only one
- B.Only two
- C.All three
- D.None
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An accelerometer measures acceleration, including the pull of gravity, and each of the three actions depends on reading it. An accelerometer is a small sensor, today usually a MEMS (micro-electro-mechanical system) chip, that senses changes in speed and the direction of gravity.
Read the full explanation →Consider the following statements :
1. Carbon fibres are used in the manufacture of components used in automobiles and aircrafts.
2. Carbon fibres once used cannot be recycled.
Which of the statements given above is/are correct?
- A.1 only
- B.2 only
- C.Both 1 and 2
- D.Neither 1 nor 2
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Carbon fibre is widely used in cars and aircraft for its strength and low weight, and it can be recovered and recycled.
Read the full explanation →