NDA GK Questions 2026

Polyvinyl chloride has linear chains held to one another only by weak forces, so it softens when heated and hardens when cooled and can be melted and moulded again and again; that reversibility is what makes a plastic thermoplastic, and it is also what allows such plastics to be recycled…

Vulcanisation is the heating of raw rubber with sulphur, which forms short cross-links between the long isoprene chains. Raw rubber is soft, sticky when warm and brittle when cold; the vulcanised product is strong, far more elastic and stable over a wide range of temperature.

Nylon-6,6 is a polyamide formed when hexamethylenediamine condenses with adipic acid, with a molecule of water eliminated at each linkage; the name records that each monomer contributes a chain of six carbon atoms.

Teflon is polytetrafluoroethene, made by the addition polymerisation of tetrafluoroethene. The fluorine atoms shield the carbon chain so completely that the surface is chemically inert, water repellent and almost frictionless, and it holds its properties to high temperature, which is why it coats frying pans and is used for gaskets…

Bakelite is made by condensing phenol with formaldehyde, and on moulding its chains join to one another by strong covalent cross-links into a single rigid network.

Polythene, also written polyethene, is formed by the addition polymerisation of ethene, in which the double bond of each molecule opens and the units join into a long chain with nothing eliminated.

Silk is the protein fibroin, spun by the silkworm, so it is a polymer made by a living organism and is therefore natural, like cellulose, starch, wool and natural rubber.

In this cell hydrogen is fed to the anode and oxidised, oxygen is fed to the cathode and reduced, and the two combine to give water, so the cell produces electricity for as long as the gases are supplied and leaves no polluting exhaust.

Galvanisation is the coating of iron or steel with zinc, usually by dipping the article in molten zinc, and it protects the iron twice over: the layer keeps out air and moisture, and because zinc is the more reactive metal it corrodes in place of the iron even where the…

Pure alumina melts at a very high temperature, so it is dissolved in molten cryolite, which is sodium aluminium fluoride, usually with a little fluorspar added; this brings the working temperature down sharply and makes the melt a good conductor.

The lead-acid cell has a spongy lead negative plate and a lead dioxide positive plate dipped in dilute sulphuric acid, and as the cell discharges both plates turn to lead sulphate while the acid becomes weaker, which is why its state of charge can be judged from the specific gravity…

In the Leclanche dry cell the outer container itself is made of zinc and acts as the anode, being oxidised to zinc ions as the cell works, which is why an old cell often leaks when the container is eaten through.

Its reaction is reversible, so passing a current through it in the opposite direction converts the products back into lead and lead dioxide and the cell can be used again; such a cell is called a secondary or storage cell, and the car battery is the common example.

In the chlor-alkali process a concentrated solution of sodium chloride is electrolysed; chloride ions are oxidised at the anode and come off as chlorine gas, hydrogen is set free at the cathode, and sodium hydroxide remains in the solution, which is why the industry yields three useful products at once.

The metal ions in the plating bath carry a positive charge, so they move towards the negative electrode and are reduced to metal there; the article must therefore be the cathode for the coating to build up on it.

One faraday is the charge carried by one mole of electrons, obtained by multiplying the charge on a single electron by the Avogadro number, and it comes to about 96,500 coulombs; passing one faraday through an electrolyte liberates one gram equivalent of the substance at an electrode.

The anode is the electrode joined to the positive terminal of the supply, so negatively charged ions, the anions, travel to it and give up electrons there; loss of electrons is oxidation.

When cells are joined in parallel all their positive terminals are connected together and all their negative terminals together, so each cell is across the same two points and the voltage of the combination is the voltage of one cell, that is 12 volts.

The working rule for charging a storage battery is that the charging current should be about one-tenth of the ampere-hour capacity of the battery, which is called the C by 10 rate.

More than four-fifths of the craft is of Indian origin, and it is propelled by three diesel engines that drive waterjets, which gives it the agility needed in shallow waters.