What can be done to water to make it freeze at a lower temperature than normal water?

Question

An aircraft flying at 40,000 feet can be heard to break the sound barrier when it achieves a speed of how many miles per hour?

Answer 1

Add impurities
Water is peculiar because it has a rigid solid structure that is less dense than its liquid state (which is why ice floats in liquid water). This lowers the freezing temperature of the water – you have to remove more thermal energy from the solution before you can lock these ions in solid water.