Which of the following describes a chemical change: A. Table salt dissolves readily in water. B. Steel rusts when exposed to moist air. C. Ice cream melts when taken out of the freezer. D. Solid iodin

A chemical change results in the formation of new substances with different chemical properties. A physical change alters the form or appearance of a substance but does not change its chemical composition.

Let's examine each statement:

• A. Table salt dissolves readily in water. When table salt ($\text{NaCl}$) dissolves in water, it separates into ions ($\text{Na}^+$ and $\text{Cl}^-$), but it is still chemically table salt. The salt can be recovered by evaporating the water. This is a physical change.

• B. Steel rusts when exposed to moist air. Rusting is the process where iron (a component of steel) reacts with oxygen and water to form iron oxides (rust). This creates a new substance with different chemical properties. This is a chemical change.

• C. Ice cream melts when taken out of the freezer. Melting is a change of state from solid to liquid. The chemical composition of the ice cream remains the same. This is a physical change.

• D. Solid iodine, when heated rapidly, changes to a gas without melting. This process is called sublimation, which is a change of state from solid directly to gas. The chemical identity of iodine ($\text{I}_2$) does not change. This is a physical change.

Therefore, the statement that describes a chemical change is B.

The final answer is $\boxed{\text{B}}$.