Have you ever noticed a truck salting roads during the winter? Have you ever added salt to water when cooking pasta? If so, you have already been exposed to a group of four physical phenomena called colligative properties. These characteristics are fully dependent on the number of particles dissolved in the solution -- it does not matter what those particles are. If the number of particles is significant, colligative properties can be substantial. Dissolving particles in a solution has four different colligative effects.
Lower Freezing Point
When a substance is dissolved in water or some other solvent, the freezing point is lowered. Every solvent has a different susceptibility to colligative effects. Adding about 68.5 grams of salt to 1 liter of water lowers its freezing point by 1.86 degrees Celsius (3.4 degrees Fahrenheit). When salt is added to an icy road, it dissolves into water above the ice and keeps it from freezing even though the temperature of the road is lower than 0 degrees Celsius -- the freezing point of water. This happens because solvent molecules must order themselves into a lattice when freezing, and dissolved particles interfere with the process. In addition, heat is released when the salt dissolves, which melts some of the ice below, which then allows more salt to dissolve in a kind of chain reaction.
Higher Boiling Point and Lower Vapor Pressure
On the other end of the spectrum, the boiling point of a solvent with dissolved particles is elevated. The same amount of salt as before will raise the boiling point of water by 0.5 degrees Celsius (about 1 degree Fahrenheit). This property is directly related to a third colligative property: a lower vapor pressure. Liquids boil when the vapor pressure above the liquid is higher than the atmospheric pressure, so a solvent with dissolved particles present needs more energy to overcome atmospheric pressure in order to boil. Adding salt to cooking water raises the temperature at which the food can be cooked.
The final colligative property is osmotic pressure. Osmosis is the process by which water flows from an area of high concentration to an area of low concentration. This property is a critical component of body fluids because it will determine how much water flows into or out of cells. If the extracellular fluid has too much dissolved electrolytes, water will flow out of cells. On the other hand, if the fluid has too little electrolytes, water will flow into the cell, causing it to burst. You can see the effects of osmotic pressure by dropping blood into pure water and a 10% salt solution. In pure water, the red drop coloring will get fainter and spread throughout the water like food coloring as the cells burst. In the salt solution, the drop of blood will appear to clump up as cells shrivel.
Osmotic Pressure Illustrated
If you separate a container of water into two sections by a membrane designed to allow nothing but water to pass through, you can see a change in osmotic pressure by dumping salt into one of the sections. Because of the difference in water concentration between the two sections, water will flow from the pure side to the salt side, making the water level rise above the pure section.
- Purdue University: Freezing Point Depression
- "Pharmacy College Admission Test Review"; Kaplan Publishing; 2012
- Engineering Toolbox: Boiling Points of Water at Various Elevations
- Boundless: Osmotic Pressure
- "Human Anatomy and Physiology"; Elaine N. Marieb; 2012
- Georgia State University: Freezing Point Depression and Boiling Point Elevation
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