by Ron Kurtus (revised 14 September 2005)
A chemical solution is the result of dissolving a substance in a fluid solvent, where the materials do not react chemically and are typically either both polar or both non-polar molecules.
The solution's molecules or ions mix homogeneously and do not separate by mechanical means. Changing the temperature is usually used to separate the ingredients of solutions.
Questions you may have include:
- What are the types of solutions?
- What happens when something dissolves?
- What are other characteristics of solutions?
This lesson will answer those questions.
Types of solutions
Combination of materials to create a solution includes liquids, solids and gases that dissolve in a liquid or in a gas. The main requirement is that items must be both be either polar or both non-polar for them to readily mix and form a solution. It is possible for polar and non-polar materials to form a solution in some special cases.
(See Polar and Non-Polar Molecules for more information.)
Liquid in a liquid
Alcohol and water molecules will mix evenly together to form a solution. This is because they are both polar molecules. The molecules are spread throughout the solution, but yet they do not chemically combine to form another substance.
One plus one less than two
It is interesting to note that you can combine a liter of water with a liter of alcohol, but you do not get 2 liters of the mixture. Instead you only get about 1 1/2 liters. (The same would hold for measuring in quarts or pints). The reason that 1 + 1 is not equal to 2 in this case is that the molecules mix in between each other and fill in spaces that would normally be empty in the liquid by itself.
Solid in a liquid
Some solids can dissolve in a liquid to form a solution. Common examples are mixing salt in water or sugar in water.
Sugar water still tastes sweet, although you cannot see the sugar crystals, which have dissolved into the water.
When salt dissolves in water, the NaCl molecules split into ions, with positive Na+ ions and negative Cl- ions. This is because H2O is a polar molecule and Na has only one electron in its outer orbit and would be "more comfortable" with a complete outer shell, and Cl is missing one electron from completing its outer shell.
Gas in a liquid
A gas can dissolve in a liquid. Since ammonia (NH3) is a polar gas, it will readily dissolve in water (H2O) to form a solution.
Although Carbon Dioxide (CO2) is a non-polar molecule, it does dissolve in water to form a solution under special circumstances. Carbon Dioxide is dissolved in water under pressure in carbonated drinks. When the pressure is released by opening the bottle or can, the CO2 gas quickly separates from the water in the form of bubbles.
Liquid in a gas
An example of a liquid dissolving in a gas is water dissolving into air to form humidity.
Gas in a gas
The major gases in air, Nitrogen (N2), Oxygen (O2) and Carbon Dioxide (CO2) are non-polar molecules and thus will combine to form a solution. Air is a solution where molecules are evenly spread throughout.
The amount of a material and the rate at which is will dissolve in a solvent depends on temperature and pressure.
Once the amount of material reaches its maximum for a given temperature and pressure, it has reached its saturation point. After that, no more of the material will dissolve in the solvent. For example, sugar will dissolve in water until you reach the saturation point for that temperature. After that any sugar you add will not dissolve but simply collect at the bottom of the container.
Dissolving liquids and solids
Liquids and solids are generally more soluble at higher temperatures.
Salt in water
That means that more salt will dissolve faster in warm water than in cold. Less salt will dissolve in colder water. If you just about reach the saturation point for salt in water at a given temperature and then lower the temperature of the water, the excess salt will then precipitate or "un-dissolve" and appear at the bottom of the container.
Water in air
Likewise, a certain amount of water can dissolve in air at a given temperature. The percentage allowed at a given temperature is called the relative humidity. The saturation point for adding water to air at a given temperature means the relative humidity is 100%.
If the relative humidity is very high and temperature decreases or cools down, less water vapor can be dissolved in the air, so then the water will precipitate in the form of rain. The dew point is the temperature at which rain will occur for a given relative humidity.
Your body cools off through perspiration. If the humidity and temperature are high, this liquid will not readily dissolve in the air and you will be sweaty and uncomfortable. If the temperature is high but the humidity is low, your perspiration will evaporate into the air and you won't feel as uncomfortable.
Gases are more soluble as the pressure increases. This means that Carbon Dioxide will readily dissolve in water or a sugar-water solution under high pressure. Once that pressure is released--such as when you pop the top on your soft drink bottle--less CO2 can be dissolved in the liquid, so your carbonated bubbles appear.
Other characteristics of solutions include how they separate, their conduction of electricity and their concentration.
Concentration of a solution
The concentration of a solution is the percentage of one molecule with respect to the other. When the amounts are equal, the concentration is 50%.
By law, a liquor bottle must state the concentration or percentage of the amount of alcohol to water. They use the term proof, which is twice the concentration percentage, such that 200-proof is the maximum. 80-proof liquor has 40% alcohol in it.
Methods to separate solutions include exceed the saturation point, evaporation and boiling.
Exceed saturation point
The material will separate from the solvent when the solution is more than supersaturated. Thus was previously seen with lowering the temperature of a salt-water solution to precipitate out excess salt.
When water evaporates from a solution, the concentration of the other material becomes greater until it reaches the saturation point, at which it will precipitate out.
Distillation is heating a solution of two liquids with different boiling points. As you reach the lower boiling point of two, the more volatile material will vaporize first, leaving the other material. Some of the less volatile material will also vaporize, so you don't get a 100% pure substance.
One example is boiling an alcohol-water solution to vaporize the alcohol before the water. The boiling point of water is 100° C (212° F) and that of alcohol is 78.5° C (173° F). The vapor will be richer in alcohol. Repeated distillations can result in a 95% alcohol solution.
Boiling a solution of a solid dissolved in a liquid can result is the solid precipitating out. For example, boiling off the water in a salt solution will soon result in the concentration of salt being so great that the it will precipitate out and settle at the bottom.
Some water solutions conduct electricity and are called electrolytes. This is usually the case when the atoms break into ions, because it allows for the free movement of electrons. A common example of this is a salt-water solution.
A solution is the result of dissolving a substance in a fluid solvent, where the materials do not react chemically and are either both polar or both non-polar molecules. The solution's molecules or ions mix homogeneously and do not separate by mechanical means. Instead, changing the temperature usually separates them.
Look for the solution
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