Question

The international standards of weight and measures are made of:
A. gold silver alloys
B. platinum-iridium alloys
C. copper-gold alloys
D. platinum-iron alloys

Answer 1

Hint: Alloy is a mixture of two or more metals or metalloids that are mutually soluble in the molten state; distinguished as binary, ternary, quaternary etc. depending on the number of metals within the mixture. It can also be referred to as a combination of two or more metals that are mutually soluble in each other in the molten state.

Complete step by step answer:
Alloys are made with those metal elements that resist oxidation, tarnish and corrosion during heating, casting or soldering and when used intraorally. Examples include an alloy of gold and platinum. Good metallic surfaces retain their surface in dry air.
There are precious metal alloys. Precious metals are metals containing primary elements of the platinum group. Precious metal alloy is an alloy predominantly composed of elements considered precious, i.e. gold, the six metals of the platinum group and silver. The elements in the platinum group include osmium, iridium, palladium, ruthenium and rhodium. They are considered to be noble metals.
Now let’s consider the options. Gold-silver alloys do not have much hardness since both of them do not have much hardness.
Platinum is a bluish white metal. Its hardness is similar to copper. It is very soft. When it is mixed with iridium, its hardness is increased. Also it reduces wear. It is much stiffer and more resistant to chemicals. Thus the international standards of weight and measures are made of platinum-iridium alloys.

Hence the correct option is B.

Note:
When alloys are made, the properties of both metals get mixed with each other. Platinum-iridium alloys are also resistant to high temperature electric sparks. They are also used for electrical points. This is also used for international standards for length measurement. This is made from alloy made of $90\% $ platinum and $10\% $ iridium.