* Solid: At room temperature and standard pressure, magnesium chloride is a solid. This is the most common form you'll encounter.
* Liquid: Magnesium chloride melts at 714°C. So, at temperatures above this, it becomes a liquid.
* Aqueous: When dissolved in water, magnesium chloride forms an aqueous solution. This is a common way to use it, for example, in brine solutions.
* Gas: While magnesium chloride can exist in a gaseous state, this is only at very high temperatures.
So, to be most accurate, it depends on the context! If you're talking about a sample at room temperature, it's solid. If you're talking about its form when dissolved in water, it's aqueous.
