Read about ordinary water and transform the word given in bold at the end of each line into a suitable form that fits the space in the same line

Zink

Read about the main constituent of brass and answer the questions to the text.

BRASS

The addition of zinc to copper to form brass is the oldest and one of the most important applications of zinc. The art of producing bronze by alloying copper with tin was developed relatively early and was one of primitive man's greatest technological advances, but due to the difficulties in­herent in the metallurgy of zinc, brasses do not seem to have been produced until the first or second century B.C., when the cementation process was practised in India and China, and also in Europe by the Romans.

A mixture of zinc oxide (as calcined calamine) and charcoal was placed in a crucible and covered by a layer of pieces of copper. The crucible was heated to 1000°C, when the zinc oxide was reduced and formed zinc vapour, which dissolved in and was retained by the copper. As the zinc content of the copper rose to approximately 30 per cent, the alloy began to melt and ran down to the bottom of the crucible, and at the end of the reaction was cast into moulds – usually made of stone slabs – and later hammered into the shapes required. The process did not involve the separation of zinc as metal. The method was the main source of brass production for many centuries. It was not capable of producing alloy containing more than 30 per cent zinc.

When methods for producing zinc were developed in India and China in the thirteenth and fourteenth centuries A.D., and the metal became generally available, some brass was made by directly alloying the two metals. This method was more controllable and enabled alloys with a higher zinc content than 30 per cent to be made if required, but the cementation process was presumably cheaper and its use did not die out for many years.

The brasses constitute an important series of alloys, since by varying the composition and heat treatment, a wide range of mechanical properties can be produced with valuable characteristic and excellent corrosion resistance. They form one of the three major outlets for zinc metal.

Zinc is a silvery white metal with a relatively low melt-Ing point (419.5°C) and boiling point (907°C). When unalloyed its strength and hardness is greater than that of tin or lead, but appreciably less than that of aluminium or cop­per and it cannot be used in stressed applications. Except when very pure, zinc is brittle at ordinary temperatures but malleable above 100°C and can then be readily rolled. When alloyed with 4 per cent aluminium its strength and hardness is increased considerably. Small additions of copper and titanium appreciably improve the creep resistance of rolled sheet and the use of this material is growing. With additions of 20-22 per cent aluminium, superplasticity can be developed, the alloys produced flowing readily at temperatures of 220°C under vacuum.

One of the most useful characteristics of zinc is its re­sistance to atmospheric corrosion, one of its main applications thus being for the protection of steelwork. The electro-negative character of zinc also leads to its use in consid­erable quantities in dry batteries.

Zinc, cadmium and mercury constitute Group IIB of the periodic table.

Zinc is divalent only and can give up the two outer electrons to form an electrovalent compound, for example, zinc carbonate ZnCO₈. It may also share those electrons as in zinc chloride ZnCl₂, in which the bonds are partly ionic and partly covalent.

8. Answer the following questions:

1. When did the production of zinc occur? 2. When was lead produced? 3. What is brass? 4. What characteristics of zinc do you know? 5. What chemical properties of zinc do you know?

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