Copper sulphate crystals available in the market are blue coloured crystals. By careful heating, they turn to white colour. Which one of the following is responsible for the blue colour?
- A.Oxygen
- B.Nitrogen
- C.Water
- D.Hydrogen
Correct answer
Explanation
The correct answer is C, Water. The blue colour comes from water of crystallisation: blue vitriol is CuSO4·5H2O, and heating drives off these five water molecules to leave white anhydrous copper sulphate.
Water of crystallisation is a fixed number of water molecules held within one formula unit of a salt in its crystal. When the blue crystals are heated gently in a test tube, droplets of water collect on the cooler sides and the solid turns white. The change can be reversed: adding a few drops of water to the white powder turns it blue again, which is why anhydrous copper sulphate is used as a simple test for water.
A is wrong because oxygen is already part of the sulphate ion in both forms and is not lost on gentle heating. B is wrong because copper sulphate contains no nitrogen at all. D is wrong because hydrogen is present only as part of the water molecules; it is the water as a whole that gives the colour.
Exam tip: CuSO4·5H2O is blue vitriol; Na2CO3·10H2O is washing soda; CaSO4·2H2O is gypsum.
Practice Questions
View allWhich one of the following features is an indication for modification of stem of a plant?
- A.Presence of 'eye' on potato
- B.'Scale' found in onion
- C.'Tendril' found in pea
- D.'Hair' present in carrot
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The eyes of a potato are nodes with buds, and only a stem bears nodes and buds, so they prove that the potato is a modified stem.
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- A.Tungsten
- B.Copper
- C.Rubidium
- D.Aluminium
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Rubidium, an alkali metal like caesium, gives off electrons easily when light falls on it, so it is used in photoelectric cells.
Read the full explanation →Tooth enamel is made up of which one of the following calcium compounds ?
- A.Calcium carbonate
- B.Calcium sulphate
- C.Calcium hydroxide
- D.Calcium phosphate
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Tooth enamel is made mainly of calcium phosphate in the form of a hard crystal called hydroxyapatite. Enamel is the hardest substance in the human body, harder even than bone.
Read the full explanation →Which one of the following viruses used to be responsible for highly infectious disease Smallpox ?
- A.Adenovirus
- B.Variola virus
- C.Aichi virus
- D.Coxsackie virus
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Smallpox was caused by the variola virus, a large DNA virus of the pox family that spread from person to person through the air.
Read the full explanation →After fertilization, the fruit and the seed are produced by
- A.ovule and ovary, respectively
- B.ovary and ovule, respectively
- C.ovary, no ovule required
- D.ovule, no ovary required
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After fertilisation the ovary of the flower grows into the fruit, and each ovule inside it develops into a seed.
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