Chemistry Questions and Answers

Answer: B

S1 is true and S2 is false.

Rayon is classed with synthetic (man-made) fibres in the school syllabus because it is made in factories by chemically treating a raw material; it resembles silk and is called artificial silk.

But the raw material is natural: rayon is made from cellulose obtained from wood pulp. So S2, that no natural raw material is used, is false. That is why rayon is often described as semi-synthetic.

Answer: B

Coke is not a product of petroleum.

Coke is obtained from coal. When coal is heated strongly in the absence of air (destructive distillation), it gives coke, coal tar and coal gas. Coke is almost pure carbon and is used in making steel and extracting metals.

Paraffin wax, bitumen and lubricating oil are all obtained by refining petroleum in a fractional distillation column, along with petrol, kerosene, diesel and LPG. Paraffin wax goes into candles and ointments, bitumen into roads and lubricating oil into machines.

Answer: B

The non-metal is iodine.

NCERT's chapter on metals and non-metals uses this example: a solution of iodine (tincture of iodine) is applied on wounds as an antiseptic. Iodine kills bacteria and other microbes on the skin.

Chlorine is used to disinfect drinking water, not as a solution applied on wounds. Bromine is a corrosive, toxic liquid. Sulphur is used in making sulphuric acid, gunpowder and in some skin ointments, but it is not the non-metal in this antiseptic solution.

Answer: B

An ant's sting contains formic acid.

When an ant stings, it injects formic acid (methanoic acid), which causes pain and irritation. Its name comes from the Latin formica, ant. Rubbing a mild base such as baking soda on the sting neutralises the acid and gives relief.

Ascorbic acid is vitamin C, found in citrus fruits. Oxalic acid occurs in tomatoes and spinach. Acetic acid is the acid of vinegar.

Answer: D

The group of alloys only is steel, brass, bronze.

Steel is iron with a small amount of carbon, brass is copper and zinc, and bronze is copper and tin.

Each of the other groups includes a pure metal: lithium, magnesium, aluminium or tin.

Answer: C

The correct classification is O - C; I - A, B, D.

An observation is something seen directly with the senses: when carbon dioxide is passed through lime water, you can see it turn milky (C).

The other three are inferences, conclusions drawn by reasoning from observations. That metal oxides are basic is inferred from tests such as their turning red litmus blue; that air expands on heating is inferred from effects such as a balloon on a heated flask inflating; and that sodium hydroxide and hydrochloric acid neutralise each other is inferred from indicator or temperature changes. Options putting B, D or A among the observations are therefore wrong.

Answer: A

Both statements are true and the Reason is the correct explanation of the Assertion.

Mixing a concentrated acid, especially sulphuric acid, with water releases a large amount of heat; the dilution is highly exothermic. If water is added to the acid, the small amount of water can boil instantly and spatter concentrated acid out of the container.

Adding the acid slowly to a large volume of water, with stirring, lets the water absorb the heat safely. The exothermic nature of dilution is exactly why the recommended order is acid into water.

Answer: A

The answer is reddish brown, green, colourless, orange.

The first three entries settle it. Turmeric is yellow and turns reddish brown in a base (A). China rose turns dark pink (magenta) in an acid and green in a base (B). Phenolphthalein is colourless in an acid and pink in a base (C). Only the first option has all three.

The table's methyl orange row is not accurate as printed: methyl orange is red in acid and yellow in base. But no other option fits A, B and C, so the first option is the answer. The others give turmeric as yellow in base or phenolphthalein as orange or pink in acid, which are wrong.

Answer: C

The colour of a flame depends on (a) and (b).

Different substances burn with different flames: sodium compounds give yellow, copper compounds blue-green, and so on (a). The supply of oxygen also matters: with plenty of air a Bunsen burner or LPG flame burns blue because combustion is complete, while with limited air it burns yellow and sooty from unburnt carbon (b).

The temperature of the surroundings does not decide the colour of the flame, so options including (c) are wrong, and options with only (a) or only (b) are incomplete.

Answer: B

The formula is AB₃.

Element A (atomic number 13) is aluminium, with configuration 2, 8, 3: it loses 3 electrons to form \(A^{3+}\). Element B (atomic number 17) is chlorine, 2, 8, 7: it gains one electron to form \(B^{-}\). Balancing the charges needs three B ions for each A ion:

\[ A^{3+} + 3B^{-} \rightarrow AB_3 \]

This is aluminium chloride, AlCl₃. A₃B reverses the ratio, AB₂ would need a 2+ ion, and AB a 1+ ion.