* Physical Properties are characteristics of a substance that can be observed or measured without changing the substance's chemical composition. Examples include color, density, melting point, boiling point, etc.

* Mixtures are combinations of substances where each substance retains its own properties.

When salt (sodium chloride) dissolves in water, it forms a mixture. The salt particles become surrounded by water molecules, but they don't fundamentally change their chemical makeup.

The physical property of salt that stays the same in a salt-water mixture is its chemical composition.

Here's why the other options change:

* Color: Salt is white, but the salt-water mixture appears clear.

* Density: Salt is denser than water. The density of the mixture will be somewhere between the two, depending on the concentration of salt.

* Melting Point: Salt has a specific melting point. In a salt-water mixture, the freezing point of water is lowered (meaning it takes a lower temperature for the mixture to freeze), but the salt itself doesn't melt at its normal temperature.

Let me know if you'd like more examples of physical properties!