chemical-changes MCQs for UPSC Prelims

30 practice questions on chemical-changes from the Changes Around Us: Physical and Chemical section of the UPSC Prelims syllabus. 30 come with a written explanation. Try the sample set below - the answer stays hidden until you ask for it.

9 Easy 15 Medium 6 Hard

Sample questions

Q1
medium

Which observation best shows that rust is a different substance from the iron it forms on?

  1. A Rust forms on the outer surface of the iron object
  2. B Rust appears only after the object has been left for many days
  3. C Rust makes the iron object look old and neglected
  4. D Rust is a brown, flaky powder that is neither shiny nor strong like iron
Show answer and explanation

Correct answer: D - Rust is a brown, flaky powder that is neither shiny nor strong like iron

Different properties mean a different substance, and rust differs from iron in colour, strength and shine. Forming on the surface, taking time to appear and looking untidy are all true of rust but say nothing about whether the material itself has changed.

Q2
hard

Assertion (A): A change in which one or more new substances are formed is called a chemical change. Reason (R): Chemical changes are often accompanied by a rise or fall in temperature.

  1. A Both A and R are true, and R is the correct explanation of A
  2. B Both A and R are true, but R is not the correct explanation of A
  3. C A is true, but R is false
  4. D A is false, but R is true
Show answer and explanation

Correct answer: B - Both A and R are true, but R is not the correct explanation of A

A states the correct definition of a chemical change, so it is true. Many chemical changes do give out or take in heat, so R is true as well. But a temperature change is only a common sign, not the reason for the definition, and it happens in some physical changes too, so R does not explain A.

Q3
medium

An LPG stove burns gas to heat a pan of water. How should the two changes be classified?

  1. A Burning of the gas is chemical; heating of the water is physical
  2. B Burning of the gas is physical; heating of the water is chemical
  3. C Both changes are physical
  4. D Both changes are chemical
Show answer and explanation

Correct answer: A - Burning of the gas is chemical; heating of the water is physical

The burning gas turns into new substances such as carbon dioxide and water vapour, so that change is chemical. The water only gets hotter and may turn to steam, and it remains water throughout, so that change is physical. The remaining choices swap or wrongly group these two very different changes.

Q4
easy

When a sheet of paper is burnt, the grey ash that is left behind

  1. A is a new substance and cannot be turned back into paper
  2. B is the same paper in powder form
  3. C can be pressed together to make paper again
  4. D is paper that has only changed its state
Show answer and explanation

Correct answer: A - is a new substance and cannot be turned back into paper

Ash does not tear, fold or write like paper, so it is a different substance produced by burning. It is not paper in another form, and no amount of pressing will bring the paper back. A change of state would leave the substance unchanged, and ash clearly is not paper.

Q5
easy

Which of these is NOT a sign that a chemical change may have taken place?

  1. A A solid being broken into smaller pieces
  2. B A gas bubbling out of a mixture
  3. C A sudden change of colour
  4. D Heat being given out
Show answer and explanation

Correct answer: A - A solid being broken into smaller pieces

Breaking a solid changes only its size, and the pieces are still the same substance, so it is a physical change with no sign of anything new. A gas appearing, a sharp change of colour and heat being released are all common pointers to a chemical change.

Q6
medium

A spoon of baking soda is dropped into a glass of vinegar and the mixture fizzes strongly for a while. Which sign of a chemical change is being observed?

  1. A The vinegar is changing state
  2. B A gas is being produced
  3. C The soda is dissolving without any change
  4. D Light is being given out
Show answer and explanation

Correct answer: B - A gas is being produced

The fizzing is caused by bubbles of a gas that was not there before mixing, and producing a gas is a classic sign of a chemical change. The vinegar remains a liquid throughout, so no change of state occurs. Plain dissolving is silent and produces no bubbles, and no light is seen.

Q7
medium

Vegetable peels left in a covered bin for a few days give off a strong unpleasant smell. What does the smell tell us?

  1. A The peels have dissolved in the air inside the bin
  2. B Only the shape of the peels has changed
  3. C The peels have simply changed state
  4. D New substances with new smells have been formed in the peels
Show answer and explanation

Correct answer: D - New substances with new smells have been formed in the peels

A smell that was not there earlier comes from substances that were not there earlier, so rotting is a chemical change. The peels are still solid, so no change of state has happened. Solids do not dissolve in air, and a mere change of shape could never create a new smell.

Q8
easy

A cut apple slowly turns brown when it is left in the open air. Which sign of a chemical change is seen here?

  1. A Dissolving of the apple
  2. B A change of shape
  3. C A change of state
  4. D A change of colour
Show answer and explanation

Correct answer: D - A change of colour

The fresh pale surface darkens to brown, and that colour change points to a new substance forming on the cut surface. The apple stays a solid, so no change of state occurs. It is not cut or squeezed further, so its shape is unchanged, and it does not dissolve in air.

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