Which one of the following is an oxidation-reduction reaction?
- A.NaOH + HCl → NaCl + H2O
- B.CaO + H2O → Ca(OH)2
- C.2Mg + O2 → 2MgO
- D.Na2SO4 + BaCl2 → BaSO4 + 2NaCl
Correct answer
Explanation
The correct answer is C, 2Mg + O2 → 2MgO. When magnesium burns, magnesium loses electrons and is oxidised, while oxygen gains them and is reduced, so oxidation and reduction happen together. In a redox reaction the oxidation numbers of the atoms change. Here magnesium goes from 0 to +2 and oxygen goes from 0 to −2. Burning magnesium ribbon gives a dazzling white light and leaves a white powder of magnesium oxide, a reaction shown in every school laboratory. Option A is wrong because it is neutralisation: an acid and a base form salt and water, and no oxidation number changes. Option B is wrong because quicklime simply combines with water to form slaked lime; calcium stays +2 and oxygen −2 throughout. Option D is wrong because it is a double displacement that forms a white precipitate of barium sulphate, with every ion keeping its charge. Exam tip: redox = change in oxidation number; neutralisation and precipitation are not redox.
Practice Questions
View allA recent movie titled The Man Who Knew Infinity is based on the biography of
- A.S. Ramanujan
- B.S. Chandrasekhar
- C.S. N. Bose
- D.C. V. Raman
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The 2015 film 'The Man Who Knew Infinity' tells the life of the Indian mathematician Srinivasa Ramanujan and is based on Robert Kanigel's 1991 biography of the same name.
Read the full explanation →The word 'Denisovan' is sometimes mentioned in media in reference to
- A.fossils of a kind of dinosaurs
- B.an early human species
- C.a cave system found in North-East India
- D.a geological period in the history of Indian subcontinent
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Denisovans were an archaic group of humans, close relatives of the Neanderthals, first identified in 2010 from a finger bone found in Denisova Cave in the Altai Mountains of Siberia.
Read the full explanation →Which one of the following statements best describes the role of B cells and T cells in the human body?
- A.They protect the body from environmental allergens.
- B.They alleviate the body's pain and inflammation.
- C.They act as immunosuppressants in the body.
- D.They protect the body from the diseases caused by pathogens.
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B cells and T cells are lymphocytes, the white blood cells of the adaptive immune system that recognise and fight specific bacteria, viruses and other pathogens.
Read the full explanation →Which one of the following is not used as fertilizer?
- A.Ammonium nitrate
- B.Ammonium sulphide
- C.Ammonium phosphate
- D.Ammonium sulphate
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Ammonium sulphide is an unstable, foul-smelling compound that breaks down to release ammonia and toxic hydrogen sulphide, so it is not used as a fertiliser.
Read the full explanation →The focal length of the objective lens of a telescope is 50 cm. If the magnification of the telescope is 25, then the focal length of the eye-piece is
- A.12.5 cm
- B.5 cm
- C.2 cm
- D.10 cm
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For an astronomical telescope in normal adjustment the magnifying power equals the focal length of the objective divided by that of the eyepiece, so the eyepiece focal length is 50 ÷ 25 = 2 cm.
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