Page 41 - Curriculum Visions Dynamic Book

Uses of cadmium’s low melting point
Cadmium melts at 320°C, a very low melting point for a metal. This property is used in a number of ways.
A special kind of solder containing cadmium is used to join pieces of aluminium. Like all solders, aluminium solder must be an alloy
of metals that have a low melting point. Aluminium solder is made with 40% cadmium, 50% lead and 10% tin.
Alloys containing cadmium are also used
in fire extinguisher systems. Such alloys are combinations of low melting point metals such as cadmium, lead and bismuth. These alloys have the special property that the melting point of the alloy is much lower than the melting points of any of the metals used
One important application of an alloy using cadmium is in the fire-detector nozzles of automatic sprinkler systems used in places like hotels and shops. The nozzles are sealed with an alloy containing cadmium that melts
at 70°C. The heat from a fire will raise the temperature of the air near the ceiling, causing the alloy to melt and allowing the water in
the pipes to be sprinkled automatically over the fire.
 Cadmium is a dense metal that is used to shield nuclear reactors and as the moderating material for the control rods inside the reactor core.
precipitate: tiny solid particles formed as a result of a chemical reaction between two liquids or gases.
solution: a mixture of a liquid and at least one other substance (e.g. salt water). Mixtures can be separated out by physical means, for example by evaporation and cooling.
Separating cadmium and zinc salts
Zinc and cadmium are similar in their behaviour, so it can be difficult to separate them. One way of doing this is to add sodium hydroxide to the mixture of cadmium and zinc salt solution. Hydroxides of zinc and cadmium are formed, but whereas zinc hydroxide is soluble in excess alkali, cadmium hydroxide is insoluble and so forms a precipitate.
Cadmium in batteries
Often it is more convenient to recharge
a battery rather than to throw it away. Cadmium is used with nickel to make leakproof and explosion-proof rechargeable batteries.
Rechargeable batteries are not widely used because they cannot supply the current that can be achieved from a disposable primary (dry) cell, and the charge quickly leaks away, meaning that the cells have to be recharged quite often, even if they have not been used.
The most common replacement for zinc dry cells uses nickel and cadmium. The nickel–cadmium cell produces 1.2 volts, slightly lower than the 1.5 volts of a carbon– zinc primary cell. This can mean that, unless the appliance is specifically designed for a nickel–cadmium cell, the performance of the appliance might be slightly below that obtained by using normal dry cells. Thus the brightness of a flashlight will be less using rechargeable batteries than ordinary zinc–carbon cells because the bulb is designed for cells of 1.5 volts. On the other hand, motors in portable screwdrivers are designed for nickel–cadmium batteries and so suffer no loss of performance.
41
41