General Science GK Questions with Answers – Page 88

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.

A good fuel should have a moderate ignition temperature, not a very low one.

Destructive distillation means heating coal strongly in the absence of air, so that it decomposes instead of burning. The solid left behind is coke, which is almost pure carbon and is used as a reducing agent in the extraction of iron and other metals and as a smokeless fuel.

When the oxygen supply is short, the carbon of the fuel is only partly oxidised and carbon monoxide is formed instead of carbon dioxide.

It is the lowest temperature to which a combustible substance must be heated before it begins to burn, and it is a fixed property of that substance; substances with a very low ignition temperature, such as petrol, alcohol and liquefied petroleum gas, are called inflammable substances and are stored with…

Carbon dioxide is about one and a half times as dense as air, so when released it forms a blanket over the burning material and separates it from atmospheric oxygen, removing one side of the fire triangle.

Petrol and kerosene are lighter than water and do not mix with it, so water poured on an oil fire sinks below the burning layer and carries it outwards, enlarging the fire instead of putting it out.

Coal forms by the slow burial and compression of plant matter, and the longer and deeper the process goes the more carbon is left behind as moisture and volatile matter are driven off.

Liquefied petroleum gas is obtained during the refining of petroleum and from natural gas processing, and it is chiefly a mixture of butane and propane, which liquefy under moderate pressure at ordinary temperature and so can be stored in a cylinder.