In both cases, the answer is based on the fact that adding salt to an ice water mixture in equilibrium, lowers the freezing point or melting point of the equilibrium. When you add just ice to the ice cream maker, the ice absorbs heat from the surrounding and starts melting. At 0C equilibrium is reached and the temperature cannot go any lower.
This is not cold enough for making ice cream. Atmospheric Science. Many city and municipalities depend on rock salt to de-ice their roadways when the weather gets bad. Road salt is technically halite, which is simply the mineral form of sodium chloride, or salt. It's just a less pure version of table salt. What is the best salt for driveways? Any salt should be perfectly fine for driveway use.
However, if you're worried about exceptionally cold temperatures or potential damage to your driveway, magnesium chloride or calcium chloride road salts are gentler choices. How is road salt different from table salt? Road salt is just a non-purified version of regular salt, with more minerals in the mix.
Why do we use road salt? Salt actually lowers the freezing temperature of the water. If the temperature gets down to where the salt water can freeze, energy will be released when bonds form as the liquid becomes a solid. This energy may be enough to melt a small amount of the pure ice, keeping the process going. You can demonstrate the effect of freezing point depression yourself, even if you don't have an icy sidewalk handy.
One way is to make your own ice cream in a baggie , where adding salt to water produces a mixture so cold it can freeze your treat.
If you just want to see an example of how cold ice plus salt can get, mix 33 ounces of ordinary table salt with ounces of crushed ice or snow. Be careful! Table salt dissolves into sodium and chloride ions in water. Sugar dissolves in water, but does not dissociate into any ions. What effect do you think adding sugar to water would have on its freezing point?
Can you design an experiment to test your hypothesis? Putting salt on water isn't the only time freezing point depression occurs. Any time you add particles to a liquid, you lower its freezing point and raise its boiling point.
Another good example of freezing point depression is vodka. Vodka contains both ethanol and water. Ordinarily, vodka does not freeze in a home freezer.
The alcohol in the water lowers the freezing point of the water. Because of thermal vibrations in the ice, a large number of molecules per second become detached from its surface and enter into the water. During the same period of time, a large number of water molecules attach themselves to the surface of the ice and become part of the solid phase.
At higher temperatures, the former rate is faster than the latter and the ice melts. At lower temperatures the reverse is true. At the freezing point the two rates are equal. If salt is dissolved in the water, the rate of detachment of the ice molecules is unaffected but the rate at which water molecules attach to the ice surface is decreased, mainly because the concentration of water molecules in the liquid molecules per cubic centimeter is lower. Hence, the melting point is lower.
John Margrave, a chemistry professor at Rice University, explains. All icy surfaces in fact contain small puddles of water. Because salt is soluble in water, salt applied to such surfaces dissolves. Liquid water has what is known as a high dielectric constant, which allows the ions in the salt positively charged sodium and negatively charged chlorine to separate.
These ions, in turn, react with water molecules and hydratethat is, form hydrated ions charged ions joined to water molecules. This process gives off heat, because hydrates are more stable than the individual ions.
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