Chemistry Questions and Answers

Answer: D

X is a non-metal, Y is acidic and Z is acidic in nature.

Y turns blue litmus red, so Y is an acid. Oxides that dissolve in water to give acids are oxides of non-metals, so X is a non-metal. Sulphur fits the description:

\[ \begin{aligned} SO_{2} + H_{2}O &\rightarrow H_{2}SO_{3} \\ 2H_{2}SO_{3} + O_{2} &\rightarrow 2H_{2}SO_{4} \end{aligned} \]

Oxidising the acid Y gives another acid, Z (here sulphuric acid). Metal oxides give bases with water, which would turn red litmus blue, so options A and B are wrong. Option C wrongly makes Z basic.

Answer: A

Sodium chloride is formed through displacement reaction.

Chlorine is more reactive than bromine, so it displaces bromine from sodium bromide solution:

\[ Cl_{2} + 2NaBr \rightarrow 2NaCl + Br_{2} \]

The solution turns orange-brown as bromine is set free. Pure sodium cannot be obtained this way; sodium is far too reactive and is extracted by electrolysis of molten sodium chloride (B). This is not a decomposition, since two substances react (C). A reaction does take place, so D is wrong.

Answer: B

The statement that is not true is The temperature of middle zone and the non-luminous part of the flame is same.

A candle flame has three zones. The outermost non-luminous (blue) zone has the most air, so combustion there is complete and it is the hottest part. The middle luminous (yellow) zone has partial combustion and is moderately hot; unburnt carbon particles glow and give the light. The innermost dark zone around the wick holds unburnt wax vapour and is the least hot.

So the middle zone is cooler than the outer zone, and the two temperatures are not the same. Statements A, C and D describe the zones correctly.

Answer: C

Bujho should use Anode-Impure copper; Cathode-Pure copper; Solution-Copper sulphate.

In electrolytic refining, the impure metal is made the anode and a thin strip of pure metal the cathode, with a solution of a salt of the same metal as electrolyte. Copper dissolves from the impure anode as Cu²⁺ ions, travels through the copper sulphate solution and is deposited as pure copper on the cathode; impurities fall to the bottom as anode mud.

Water alone (A) does not supply copper ions or conduct well. A carbon anode (B) does not dissolve, so the impure copper is never refined. Reversing the electrodes (D) would deposit copper on the impure strip.

Answer: D

The answer is A-coal; B-high; C-high; D-carbonization.

About 300 million years ago, dense forests in low-lying wetlands were buried under soil. As more layers piled up, the dead vegetation was compressed under high pressure and subjected to high temperature deep in the earth, and was slowly converted into coal. Because coal consists mainly of carbon, this slow process is called carbonisation.

Petroleum and natural gas formed from tiny marine organisms, not from land vegetation (B, C). Oxidation is combining with oxygen, the opposite of what happens to buried plants cut off from air (A, C). Low temperature (A) is also wrong.

Answer: A

The pH paper would turn bluish with Milk of magnesia.

Universal indicator turning yellowish orange shows a slightly acidic soil sample. To make it turn blue, the mixture must become basic. Milk of magnesia is a suspension of magnesium hydroxide, a base, which neutralises the acid and raises the pH.

Lemon juice and vinegar are acids and would make the paper turn more red. Common salt is a neutral salt and would not change the colour towards blue.

Answer: A

The metal that burns if kept in air is sodium.

Sodium is so reactive that it reacts vigorously with oxygen and moisture in the air and can catch fire at room temperature. For this reason it is stored under kerosene, as NCERT's Class 8 text describes.

Magnesium burns with a dazzling white flame, but only when it is ignited by heating; kept in air it merely tarnishes. Zinc and aluminium form a thin protective oxide layer and do not burn on standing in air.

Answer: C

Magnesium hydroxide is used as an antacid.

Mg(OH)₂ is a mild base, sold as milk of magnesia. Taken by mouth it neutralises excess hydrochloric acid in the stomach, relieving acidity and indigestion:

Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O

It is neither an oxidizing nor a reducing agent in this use, since no electrons are transferred in neutralisation, and it has no role in fermentation.

Answer: B

The gas that burns with a pop sound is hydrogen.

When a burning splinter is brought near a test tube of hydrogen, the hydrogen burns rapidly with oxygen of the air, producing a characteristic 'pop' and forming water. This is the standard laboratory test for hydrogen, for example after a metal reacts with dilute acid.

Oxygen does not burn itself but makes a glowing splinter relight. Nitrogen dioxide and sulphur dioxide do not burn with a pop.

Answer: B

The one that is not a product of petroleum is CNG.

Compressed natural gas is natural gas, mainly methane, compressed under high pressure. Natural gas is a separate fossil fuel found in gas fields or above petroleum deposits; it is not obtained by refining petroleum.

Kerosene, paraffin wax and bitumen are all fractions separated from petroleum in a refinery, along with LPG, petrol, diesel and lubricating oil, as NCERT's list of petroleum products shows.