Brass gets discoloured in air because of the presence of which of the following gases in air ?
Answer: A
Brass tarnishes because of hydrogen sulphide in the air.
Traces of \(\mathrm{H_2S}\) react with the copper of the alloy to form a thin layer of black copper sulphide. The same gas is what blackens silverware.
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The hottest part of the gas flame is called the
Answer: D
The hottest part of a gas flame is the non-luminous zone.
There the air supply is plentiful, combustion is complete, and the flame reaches about 1,500 °C while burning pale blue. The luminous yellow zone is cooler because combustion is incomplete, its light coming from glowing carbon particles, and the innermost dark zone is unburnt gas.
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What is the simplest form of matter?
Answer: A
An element is the simplest form of matter.
It cannot be broken down into anything simpler by chemical means, since it consists of atoms of one kind only. An atom is a particle rather than a form of matter, and molecules and compounds are built from elements.
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The isotope atoms differ in ?
Answer: D
Isotopes differ in the number of neutrons.
They have the same atomic number, so they are the same element with the same chemistry, but different mass numbers. Their atomic masses therefore differ too - but the neutron count is the underlying cause, not the effect.
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In known elements, the maximum number is of
Answer: A
Metals are the most numerous class of known elements.
Of the 118 elements, about 90 are metals, some 17 non-metals and 7 or 8 metalloids. Everything below and to the left of the metalloid staircase in the periodic table is metallic, including all the d- and f-block elements.
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Which of the following is an example of chemical weathering?
Answer: D
All the above are chemical weathering.
Oxidation turns iron minerals to rust and crumbles the rock; carbonation dissolves limestone as \[\mathrm{CaCO_3 + H_2O + CO_2 \rightarrow Ca(HCO_3)_2}\] which is how caves form; and acid rain accelerates both. Each alters the rock's chemical composition, unlike physical weathering by frost or heat.
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The pH of pure water is nearly 7. It will increase on the addition of
Answer: A
The pH rises on adding ammonia.
Ammonia is a weak base: \[\mathrm{NH_3 + H_2O \rightleftharpoons NH_4^+ + OH^-}\] The extra hydroxide lowers \([\mathrm{H^+}]\) and so raises the pH above 7. Cane sugar and common salt are neutral, and hydrochloric acid would lower the pH.
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The number of waves in n x 10th Bohr's orbit are
Answer: B
The number of waves equals \(n\), the number of the orbit.
De Broglie's standing-wave condition for a stable orbit is \[2\pi r = n\lambda\] so an orbit of principal quantum number \(n\) accommodates exactly \(n\) complete wavelengths, whatever the multiplier in front of it.
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The number of moles of solute present in 1 kg of a solvent is called its
Answer: A
This is the molality of the solution.
\[m=\frac{\text{moles of solute}}{\text{mass of solvent in kg}}\] Because it is defined by mass rather than volume, molality does not change with temperature - which is why the colligative-property formulae use it. Molarity is moles per litre of solution and does vary with temperature.
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In addition to nitric acid, another acid which may render iron passive is
Answer: C
Chromic acid also renders iron passive.
Strong oxidising acids build a thin, adherent, impervious film of oxide on the iron surface, and that film prevents further attack - which is why concentrated nitric acid can be carried in steel tankers. Acetic, oxalic and citric acids are weak and non-oxidising.
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