When impurities are added to a liquid, the freezing point will decrease. For example, pure water at standard atmospheric pressure freezes at 0°C (32° F). If table salt (NaCl) is added to the water, the freezing point will drop to a lower temperature.
In fact, the Fahrenheit scale was developed around water fully saturated with salt. A completely salt-saturated sample of water freezes at 0° F (-17.78 °C)