Chemistry Questions and Answers

Answer: B

The incorrect observation is that it is an endothermic reaction.

Potassium reacts violently with cold water, forming potassium hydroxide and hydrogen:

\[ 2K + 2H_2O \rightarrow 2KOH + H_2 \]

The reaction is strongly exothermic: so much heat is released that the hydrogen catches fire with a pop. So the reaction being violent, hydrogen being evolved and a pop sound being heard are all correct observations.

Answer: B

Phenelzine is a monoamine oxidase inhibitor (MAOI), one of the older classes of antidepressant drugs, and it is used to treat depression, especially depression with atypical features. Naproxen is a non steroidal anti inflammatory drug used for pain and inflammation, phenacetin is an old analgesic and antipyretic that was withdrawn because of kidney toxicity, and morphine is an opioid analgesic for severe pain. None of the other three is a treatment for depression. The answer is option B.

Answer: A

Molarity is moles of solute per litre of solution. The volume is 500 ml = 0.5 litre, so

The value 0.15 M comes from forgetting to convert millilitres to litres by the right power of ten, 0.002 M comes from dividing the volume by the moles, and 2 M would need only 1.0 mol in the same volume.

Answer: D

Cartridge brass is the 70/30 brass, 70 percent copper and 30 percent zinc. It has an excellent combination of strength and cold ductility, which is why it is used for cartridge cases and deep drawn parts. The mixtures with tin or aluminium in the first and third options are tin brass and aluminium brass type compositions with only 60 percent copper, and Cu 50 percent, Zn 50 percent is a hard, brittle alloy outside the useful alpha brass range.

Answer: C

Naphthalene balls come from coal tar. Fractional distillation of coal tar gives naphthalene in the middle oil fraction, and the solid is pressed into mothballs.

Coke is the solid residue left when coal is carbonised, and petroleum and diesel are fuels, so naphthalene is not a product obtained from them in this way.

Answer: D

Litmus solution is purple in colour. It turns red in an acid and blue in a base (alkali), so a solution that is neither acidic nor basic, which is neutral, leaves it purple, its own colour. Blue means the solution is basic, red means it is acidic, and yellow is not a colour litmus gives in either case. Litmus is a natural dye obtained from lichens. The answer is purple, option (D). Red litmus paper is turned blue by a base and blue litmus paper is turned red by an acid, which is the usual way in which the paper is used for testing.

Answer: B

The incorrect statement is Slaked lime slowly creates a layer of calcium nitrate on the walls during white washing.

The layer formed is calcium carbonate, not calcium nitrate. Slaked lime (calcium hydroxide) applied to the wall reacts slowly with carbon dioxide in the air:

\[ \mathrm{Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O} \]

which gives the shiny white finish a few days after whitewashing. Air supplies no nitrate for this reaction.

The other statements are correct. Calcium oxide (quick lime) reacts vigorously with water, releasing heat, to give slaked lime. Silver chloride turns grey in sunlight as it decomposes to silver and chlorine, which is why it is used in black and white photography. Decomposition reactions need energy, as heat, light or electricity, to break the reactants down.

Answer: A

The correct matching is A-4, B-3, C-1, D-2.

• Litmus, the acid-base indicator, is extracted from lichens.
• Nitrogen, an unreactive gas, is flushed into packets of chips to keep out oxygen and stop the oil going rancid.
• Calcium hydroxyapatite is the mineral of tooth enamel, the hardest substance in the body.
• Hydrogen peroxide is used as an antiseptic, an anti microbial agent.

Options pairing nitrogen or litmus with anything else break one of these standard textbook links.

Answer: D

In increasing order of pH the answer is (1), (3), (4), (2).

1. Gastric juice: pH about 1.5-2, due to hydrochloric acid.
2. Lemon juice: pH about 2-2.5, due to citric acid.
3. Pure water: neutral, pH 7 at 25°C.
4. Milk of magnesia (magnesium hydroxide suspension): basic, pH about 10.5.

Gastric juice is more acidic than lemon juice, so it comes first. Milk of magnesia, an antacid, is the only base and comes last.

Answer: D

The lightest metal is Lithium.

Lithium has a density of about 0.53 g/cm³, roughly half that of water, so it floats on water (and oil). It is the lightest of all metals and of all solid elements, which is why it is used in light rechargeable batteries.

Magnesium (about 1.74 g/cm³) and aluminium (about 2.7) are light structural metals, but much denser than lithium. Platinum (about 21.4) is among the densest metals.