Silver articles turn black when kept in the open for longer time due to the formation of
- A.H₂S
- B.AgS
- C.AgSO₄
- D.Ag₂S
Correct answer
Explanation
The correct answer is D, Ag₂S. Silver reacts with traces of hydrogen sulphide in the air and forms a black coating of silver sulphide.
This slow attack on a metal by its surroundings is called corrosion, and on silver it is usually called tarnishing. The reaction needs only very small amounts of sulphur compounds, which come from polluted air, rubber, eggs and some foods: 4Ag + 2H₂S + O₂ → 2Ag₂S + 2H₂O. Silver shows a valency of one, so two silver atoms join one sulphur atom. In the same way copper turns green in moist air because it gains a coating of basic copper carbonate, and iron rusts to hydrated iron oxide.
Option A is wrong because hydrogen sulphide is the gas that causes the tarnish, not the black layer itself. Option B is wrong because AgS does not match the valency of silver; the correct formula is Ag₂S. Option C is wrong because a sulphate is not formed in tarnishing, and silver sulphate would in any case be Ag₂SO₄, a white solid.
Exam tip: corrosion colours: silver black (Ag₂S), copper green (basic copper carbonate), iron reddish-brown rust.
Practice Questions
View allThe fact that the planets move around the Sun, not in circles but in ellipses, was first demonstrated by
- A.Galileo
- B.Martin Luther
- C.Johannes Kepler
- D.Copernicus
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In 1609 Kepler showed that the planets move around the Sun in elliptical orbits, with the Sun at one focus of the ellipse.
Read the full explanation →Which one of the following statements is correct?
- A.Urea is produced in liver.
- B.Urea is produced in blood.
- C.Urea is produced from digestion of starch.
- D.Urea is produced in lung and kidney.
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Read the full explanation →Which one of the following is the correct sequence of passage of light in a compound microscope?
- A.Condenser-Objective lens-Eyepiece-Body tube
- B.Objective lens-Condenser-Body tube-Eyepiece
- C.Condenser-Objective lens-Body tube-Eyepiece
- D.Eyepiece-Objective lens-Body tube-Mirror
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Light passes through the condenser to the specimen, is picked up by the objective lens, travels up the body tube and reaches the eye through the eyepiece.
Read the full explanation →Which one of the following statements regarding haemoglobin is correct?
- A.Haemoglobin present in RBC can carry only oxygen but not carbon dioxide.
- B.Haemoglobin of RBC can carry both oxygen and carbon dioxide.
- C.Haemoglobin of RBC can carry only carbon dioxide.
- D.Haemoglobin is only used for blood clotting and not for carrying gases.
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Read the full explanation →Which one of the following statements regarding histone proteins is correct?
- A.Histones are proteins that are present in mitochondrial membrane.
- B.Histones are proteins that are present in nucleus in association with DNA.
- C.Histones are proteins associated with lipids in the cytosol.
- D.Histones are proteins associated with carbohydrates in the cytosol.
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They are the proteins around which DNA winds to form chromatin in the nucleus of eukaryotic cells. Histones are small basic proteins, rich in the positively charged amino acids lysine and arginine, which attract the negatively charged phosphate groups of DNA.
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