Which one among the following is an example of chemical change ?
- A.Sublimation of camphor
- B.Melting of ice
- C.Heating of an iron rod
- D.Mixing of NaOH and HCl
Correct answer
Explanation
The correct answer is D, Mixing of NaOH and HCl. Sodium hydroxide and hydrochloric acid react to form new substances, sodium chloride and water, so this is a chemical change.
This reaction, NaOH plus HCl giving NaCl and H2O, is called neutralisation: a base and an acid cancel each other and form a salt and water. Heat is given out, so the mixture warms up. A chemical change produces new substances with new properties and usually cannot be reversed by simple means. A physical change only alters the state, shape or size of a substance, which stays the same substance throughout.
A is wrong because camphor turning directly into vapour is only a change of state; the vapour is still camphor. B is wrong because melting ice gives water, the same substance, and freezing reverses it. C is wrong because heating an iron rod only makes it hot and slightly longer; it is still iron and returns to normal on cooling.
Exam tip: acid plus base gives salt plus water, a chemical change; melting, sublimation and heating are physical changes.
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.
Read the full explanation →Which one of the following metals is used in the filaments of photo-electric cells that convert light energy into electric energy ?
- A.Tungsten
- B.Copper
- C.Rubidium
- D.Aluminium
Show answer
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
Show answer
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
Show answer
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
Show answer
After fertilisation the ovary of the flower grows into the fruit, and each ovule inside it develops into a seed.
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