On exposure to moist air, copper gains a green coat on its surface due to formation of which one of the following compounds ?
- A.Copper carbonate
- B.Copper oxide
- C.Copper sulphate
- D.Copper nitrate
Correct answer
Explanation
The correct answer is A, Copper carbonate. When copper is left in moist air for a long time, it slowly reacts with oxygen, water vapour and carbon dioxide. The product is basic copper carbonate, a mixture of copper carbonate and copper hydroxide, CuCO₃·Cu(OH)₂, which forms a dull green layer called patina or verdigris on the surface. This is the corrosion of copper, just as rusting is the corrosion of iron, and it is why old copper roofs, vessels and statues, such as the Statue of Liberty, look green. The layer also protects the metal beneath from further attack. Option B is wrong because copper oxide is black (copper(II) oxide), formed when copper is heated in air, not green. Option C is wrong because copper sulphate is formed by reaction with sulphuric acid and is blue in its hydrated form, not a coating from moist air. Option D is wrong because copper nitrate forms only with nitric acid, which is not present in ordinary air. Exam tip: copper turns green (basic copper carbonate), silver turns black (silver sulphide), iron turns reddish-brown (rust).
Practice Questions
View allChlorophyll in photosynthetic prokaryotic bacteria is associated with
- A.plastids
- B.membranous vesicles
- C.nucleoids
- D.chromosomes
Show answer
Photosynthetic bacteria have no chloroplasts; their pigments sit on infoldings of the cell membrane that form vesicles or sacs, often called chromatophores.
Read the full explanation →Fat in human body is stored in
- A.Adipose tissue
- B.Red blood cells
- C.Muscle tissue
- D.Epithelial tissue
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Adipose tissue is a loose connective tissue whose cells, called adipocytes, are built to store fat. Each cell holds a large droplet of fat that pushes the nucleus and cytoplasm to one side.
Read the full explanation →Which one of the following is a metalloid ?
- A.Germanium
- B.Coke
- C.Iodine
- D.Sugar
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Germanium is a metalloid, an element whose properties lie between those of metals and non-metals. It is a hard, shiny, grey-white solid like a metal, yet it is brittle and conducts electricity far less well than a metal, which makes it a semiconductor.
Read the full explanation →A photon of X-ray has energy of 1 keV. A photon of visible radiation has energy of 3 eV. In this context, which one of the following statements is not correct?
- A.The wavelength of X-ray photon is less than the wavelength of visible radiation photon.
- B.Both the photons have different energies.
- C.The speeds of both the photons in vacuum are different.
- D.The frequency of X-ray photon is higher than the frequency of visible radiation photon.
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This statement is wrong because every photon, whatever its energy, travels in vacuum at the same speed of light, about 3 × 10^8 metres per second.
Read the full explanation →Which of the following substances cause temporary hardness in water?
1. Mg(HCO3)2
2. Ca(HCO3)2
3. CaCl2
4. MgSO4
Select the correct answer using the code given below.
- A.3 and 4
- B.2 and 3
- C.1 and 4
- D.1 and 2
Show answer
Temporary hardness is caused by the bicarbonates of magnesium and calcium, Mg(HCO3)2 and Ca(HCO3)2. It is called temporary because simple boiling removes it: on heating, the soluble bicarbonates break down into insoluble carbonate or hydroxide, carbon dioxide and water, and the solid settles out.
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