Which one of the following substances do silver articles react with, that makes the silver articles black?
- A.Sulphur
- B.Oxygen
- C.Carbon dioxide
- D.Phosphorus
Correct answer
Explanation
The correct answer is A, Sulphur. Silver reacts with sulphur compounds in the air, mainly hydrogen sulphide, and forms a black coating of silver sulphide.
This blackening is the tarnishing of silver, a kind of corrosion. Traces of hydrogen sulphide come from polluted air, rubber, eggs and some foods, and silver combines with them to form Ag2S. The layer is thin and can be removed by polishing or by wrapping the article in aluminium foil in a hot baking-soda solution. Copper behaves differently: it gains a green coat of basic copper carbonate in moist air.
B is wrong because silver hardly reacts with oxygen at room temperature, which is why it is called a noble metal. C is wrong because carbon dioxide does not blacken silver; with moisture it helps form the green coat on copper. D is wrong because phosphorus is not present in air in a form that reacts with silver.
Exam tip: Silver turns black (Ag2S, sulphur); copper turns green (basic copper carbonate); iron rusts (hydrated iron oxide).
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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Liver cells turn the toxic ammonia released from the breakdown of amino acids into urea through the ornithine cycle, also called the urea cycle.
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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It binds oxygen at its iron-containing haem groups and carries some carbon dioxide on its protein chains. Each haemoglobin molecule has four polypeptide chains and four haem groups, and each haem iron can hold one oxygen molecule, forming oxyhaemoglobin in the lungs.
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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