Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)
3. Household Chemistry
What is antifreeze?
Antifreeze is anything that prevents water from freezing when the temperature drops below the freezing point. It works by lowering the freezing point of water.
The freezing point is the temperature at which water freezes as fast as it melts. Below the freezing point, molecules of water move slower and will be captured by the ice. Above the freezing point, ice will melt as the molecules of water move faster.
Because heat is the random motion of molecules, there will always be some water molecules that are moving fast enough to be liquid and some that are moving slow enough to be solid ice, no matter what the temperature. But if the temperature is too warm or too cold, you may not notice, since only a tiny amount will be in the “wrong” form and only for a tiny amount of time.
The balance between melting and freezing is easy to upset. If salt is added to ice, the salt will dissolve in the water on the surface of the ice. The salt molecules (or ions) mix with the water. So now, if the liquid part is half salt molecules and half water molecules, only half as many water molecules will hit the ice as they jostle around. This means the freezing rate will be half what it used to be. But the melting rate is unchanged, so the ice melts. The balance between freezing and melting has been upset.
To get the balance back, the temperature has to be lowered. This is why salt water freezes at a lower temperature than fresh water. Salt is thus an antifreeze.
Alcohol will do the same thing as salt. The alcohol molecules mix with the water, so fewer water molecules hit the ice. Alcohol is also an antifreeze. It is more expensive than salt, but it does not cause metals to corrode like salt does, so it is a better choice for use in a car.
An even better antifreeze than alcohol is ethylene glycol. If half the water is replaced with alcohol, the freezing point is lowered by 32° Celsius (57° Fahrenheit). If half the volume is ethylene glycol, the freezing point is lowered by 34° Celsius (62° Fahrenheit). This small improvement in freezing point is not as important as two other features of ethylene glycol: it is not as flammable as alcohol and it raises the boiling point of water.