Final answer:
The chemical characteristic of iron that causes it to corrode in moist air is a true chemical property, as it involves a chemical reaction resulting in a new substance, rust.
Step-by-step explanation:
The chemical characteristic of iron from the options provided is iron corrodes in moist air. This is a reaction where iron combines with oxygen and water to form iron(III) oxide hydrate, commonly known as rust. The density of iron, being soft when pure, and its melting point are physical properties and do not reflect how iron interacts chemically with other substances.
It's important to distinguish between chemical properties, which involve how a substance reacts chemically with others (such as corrosiveness), and physical properties, like density, which do not involve such changes. Therefore, from the options given, the fact that iron corrodes in moist air (option 1) is the chemical property because it indicates a chemical change - the production of rust.