The correct option is An alloy in which one of the metals is mercury.
Explanation
An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal. The specific classification of an alloy often depends on its primary constituents. When mercury is one of the metals present in the mixture, the resulting alloy is distinctively termed an amalgam.
Detailed Analysis:
- An alloy in which one of the metals is mercury is Correct: An amalgam is defined as an alloy in which mercury is one of the constituent metals. Mercury is capable of dissolving many metals (such as gold, silver, sodium, and zinc) to form amalgams. For example, dental amalgam is a mixture of mercury with silver, tin, and copper.
- An alloy of iron and carbon only is Incorrect: An alloy of iron and carbon is known as steel (if carbon is up to 2%) or cast iron (if carbon is above 2%). It is not an amalgam.
- A mixture of metals and acids is Incorrect: A mixture of metals and acids typically results in a chemical reaction producing a salt and hydrogen gas. It does not form a physical mixture or solid solution characteristic of an alloy.
- A compound of sodium with mercury is Incorrect: While a compound or mixture of sodium and mercury is indeed called "sodium amalgam," this is a specific example rather than the general definition. The term "amalgam" applies broadly to alloys of mercury with various metals, not just sodium.
Key Takeaway:
The defining characteristic of an amalgam is the presence of mercury as a constituent metal in the alloy. Iron, platinum, tungsten, and tantalum are notable exceptions that do not form amalgams with mercury.