NDA GK Questions 2026

Hydrofluorocarbons were adopted as substitutes for chlorofluorocarbons in refrigeration and air conditioning because they do not harm the ozone layer, but they turned out to be very powerful greenhouse gases, so the Kigali Amendment of 2016 brought them under the Montreal Protocol with a schedule for reducing their use.

The day marks the signing of the Montreal Protocol on Substances that Deplete the Ozone Layer in 1987 and is observed every year with a theme announced by the United Nations.

Marble is calcium carbonate, and sulphur dioxide in polluted air, along with the sulphuric acid of acid rain, reacts with it to form calcium sulphate, so the stone loses its shine, turns yellow and becomes pitted and flaky.

These gases come from burning coal and oil in thermal power stations, smelters and vehicles, are oxidised in the atmosphere and dissolve in water droplets to form sulphuric acid and nitric acid, which come down as acid rain.

The Montreal Protocol was adopted in 1987 under the Vienna Convention of 1985 and sets the timetable by which countries must stop producing and using chlorofluorocarbons, halons, carbon tetrachloride, methyl chloroform and methyl bromide.

The Dobson unit, named after the scientist who built the instrument for measuring atmospheric ozone, expresses how thick the ozone column would be if all of it were brought down to the surface at standard temperature and pressure; a hundred Dobson units correspond to a layer one millimetre thick.

About nine tenths of the atmosphere's ozone lies in the stratosphere, roughly fifteen to thirty five kilometres above the surface, where ultraviolet rays split oxygen molecules and the free atoms combine with other oxygen molecules to make ozone.

Nitric oxide and nitrogen dioxide, together written as nitrogen oxides, are air pollutants that come mainly from combustion in vehicles and power stations; they help form smog and acid rain but are not counted among the greenhouse gases of the natural atmosphere.

A primary pollutant is released directly from a source, while a secondary pollutant is formed in the atmosphere when primary pollutants react.

Each of the other three statements is correct on its own, which is exactly why the combined option is the answer.

In methane the carbon atom is joined to four hydrogen atoms by four single bonds and has no lone pair, so the four bonding pairs push one another as far apart as possible and point to the corners of a regular tetrahedron, with bond angles of about 109.5 degrees.

Rusting is a slow oxidation in which iron combines with oxygen in the presence of moisture to form hydrated iron oxide, the reddish-brown flaky substance we call rust.

Zinc is above hydrogen in the reactivity series, so it displaces hydrogen from a dilute acid: zinc and dilute hydrochloric acid give zinc chloride and hydrogen gas.

A nitrogen atom has five valence electrons and needs three more to complete its octet, so two nitrogen atoms share three pairs of electrons and form a triple bond, written N to N with three lines.

In an ionic or electrovalent bond a metal atom loses one or more electrons to a non-metal atom; the metal becomes a positive ion and the non-metal a negative ion, and the electrostatic attraction between the two holds the compound together, as in sodium chloride and magnesium oxide.

Vegetable oils contain unsaturated fatty acids with carbon-carbon double bonds, and when hydrogen is passed through the oil at about 60 degrees Celsius in the presence of finely divided nickel, the double bonds add hydrogen and become saturated, turning the liquid oil into semi-solid vanaspati.

An amphoteric oxide reacts with both acids and bases to give a salt and water. Aluminium oxide dissolves in hydrochloric acid to give aluminium chloride and in sodium hydroxide to give sodium aluminate, so it behaves as a base in one case and as an acid in the other.

Benzene has the formula C6H6 and a ring of six carbon atoms, each carrying one hydrogen. Count the sigma bonds first: six carbon-carbon bonds around the ring and six carbon-hydrogen bonds give twelve sigma bonds.

Ethylene, also called ethene, has the formula C2H4, and its two carbon atoms share two pairs of electrons, which is a carbon-carbon double bond made of one sigma and one pi bond; it is the simplest alkene.

In a double displacement reaction two compounds exchange their ions, so both reactants must be compounds. Here hydrogen and oxygen are elements combining to form a single product, water, which makes it a combination reaction and also a combustion and redox reaction, but certainly not a double displacement.