In which one of the following cases do both physical and chemical changes take place?
- A.Burning of candle
- B.Freezing of water
- C.Cooking of food
- D.Rusting of iron
Correct answer
Explanation
The correct answer is A, Burning of candle. When a candle burns, the wax first melts and turns to vapour, which is a physical change, and the vapour then burns in air, which is a chemical change.
A physical change alters only the state or shape of a substance and can usually be reversed; melted wax can set again into solid wax. A chemical change makes new substances: burning wax vapour reacts with oxygen to form carbon dioxide and water vapour and gives out heat and light. That is why the burning candle is the standard textbook example of both kinds of change happening together.
B is wrong because freezing of water is only a physical change of state, from liquid to solid. C is wrong because cooking is treated as a chemical change, since it forms new substances and cannot be reversed. D is wrong because rusting of iron is a chemical change in which iron reacts with oxygen and moisture to form hydrated iron oxide.
Exam tip: candle burning means melting (physical) plus combustion (chemical); rusting, cooking and digestion are chemical changes.
Practice Questions
View allThe mass number of an element is NOT changed when it emits
- A.Alpha and Beta radiations only
- B.Alpha and Gamma radiations only
- C.Beta and Gamma radiations only
- D.Alpha, Beta and Gamma radiations
Show answer
Neither beta nor gamma emission changes the total number of protons and neutrons in a nucleus, so the mass number stays the same.
Read the full explanation →Which one among the following statements about facilitated transport in plants is correct?
- A.All passive transports are types of facilitated transport.
- B.Facilitated transport is an example of active transport.
- C.Water moves across plasma membrane by active transport.
- D.Facilitated transport requires assistance of carrier proteins.
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In facilitated transport, molecules cross the membrane with the help of membrane proteins, while still moving down their concentration gradient.
Read the full explanation →Who among the following has coined the term 'Quark', the fundamental particles that make up protons and neutrons in an atomic nucleus ?
- A.Richard Feynman
- B.Murray Gell-Mann
- C.Albert Einstein
- D.Niels Bohr
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The American physicist Murray Gell-Mann proposed quarks in 1964 and gave them their name. Gell-Mann, and independently George Zweig, suggested that protons, neutrons and other such particles are built from smaller units.
Read the full explanation →The spindle fibre is attached to which of the following specific regions of the chromosomes?
- A.Kinetochore
- B.Chromonema
- C.Telomere
- D.Centriole
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Spindle fibres attach to the kinetochore, a disc-shaped protein structure on the centromere of each chromosome. During cell division the spindle, made of microtubules, pulls the copies of each chromosome apart.
Read the full explanation →Which one of the following is the chemical name of heavy water?
- A.Hydrogen oxide
- B.Deuterium dioxide
- C.Deuterium oxide
- D.Heavy hydrogen oxide
Show answer
Heavy water is D₂O: two atoms of deuterium joined to one atom of oxygen. Deuterium is the isotope of hydrogen with one proton and one neutron in its nucleus, so it is about twice as heavy as ordinary hydrogen.
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