SOLUBILITY

If you have a full glass of water, you can dissolve a certain quantity of table salt, then, you mix and diluted it however, if you keep adding salt, there will come a time when it starts to accumulate in the bottom of the glass. We therefore conclude that in a given quantity of solvent can dissolve a limited quantity of solute, under certain conditions.

The maximum quantity of solute that can dissolve in a solvent at a given temperature and pressure is known as solubility:

Below are some data of solubility of certain substances.

* These substances are gases, and data were taken at a pressure of 1 atm, with 1 L of H2O.

It could be from here to do a concentration unit, a solution that contains less solute possible is called unsaturated solution, while one that contains exactly the maximum quantity of solute in the solvent is called saturated solution. Take an example with ammonium sulfate at 20 ° C:

If you have 100 g of H2O, unsaturated sulfate solution it can be made by mixing between 0.1 and 36.3 g of salt, because it has not reached the maximum or solubility. A saturated solution is one in which, in the 100 g of water are mixed 36.4 g of sulfate.

As seen in the table before, the solubility of a substance changes with respect to temperature. Along with pressure, temperature is a factor in the solubility.

FACTORS AFFECTING SOLUBILITY

Temperature

A very clear way that you can see how temperature affects the solubility of a substance is through a solubility curve. These curves represent the x-axis temperature, usually in ° C, while the y-axis expresses the solubility. Here’s an example of a solubility curve for various salts:

Image taken from: http://www.pascack.k12.nj.us/70271723212442/lib/70271723212442/SolubilityCurves.jpg

According to this graph, we can see that most salts dissolve more and more with increasing temperature. There are few cases in which temperature decreases with increasing solubility. Such is the case of gases. In the table before, it indicates that the ammonia and oxygen solubility decreases rapidly with an increasing temperature. This is because when the temperature increases the kinetic energy of gas particles increases, and therefore tend to mix less and less.

Determine from the curve the approximate solubilities of the following salts under the described conditions.

  1. KNO3 – 50 ºC
  2. KCl – 80 ºC
  3. KClO3 – 60 ºC
  4. Pb(NO3)2 – 20 ºC

Pressure

Pressure is another important factor that affects the solubility of gases. When the external pressure to the solution increases, the solubility increases (figure a), but if the external pressure decreases, the gas tends to be less soluble (Figure b).

Reflection:
Water is the most abundant solvent on earth. The amount of oxygen in water is called dissolved oxygen and is an indicator of the quality of it, because it is necessary for the life of fish and other aquatic life. Oxygen enters a body of water by photosynthesis of aquatic plants and the transfer of oxygen from air and water border.

Thermal pollution reduces the quality of water due to temperature increases. The load of hot water in a lake, river or other body of water by factories and power plants decreases the solubility of oxygen. The lower level of oxygen can not sustain life, as shown in the image and greatly increases the effects of toxic and organic pollutants.

To reduce thermal pollution measures should be taken before releasing hot water into the environment. The water can be discharged into lakes, canals or containment to allow cooling. Also, many facilities use cooling towers to dissipate heat in the air.

Article taken from: DINGRADO, L. GREEG, K. HAINEN, N. WISTROM. C. Chemistry, Matter and Change. Mc Graw Hill 2003. Page 457

Image taken from: http://www.eldiario24.com/nota.php?id=207686

COLLIGATIVE PROPERTIES

We have studied the factors affecting the solubility of a substance. Now we will see that there are certain properties of solutions that rely solely on the amount of solute in the solvent. These are the colligative properties. Four, and studied one by one.

Vapor pressure depression
when preparing a solution with a solute that it will not turn into a gas, vapor pressure of the same decline. The vapor pressure can be understood as the number of molecules of a liquid on the surface thereof in the form of gas. To better understand the concept, note the figure below:

Figure a) shows a solvent that is pure, and therefore has a certain vapor pressure. Figure b) is evidence that adding a solute, the vapor pressure of the fluid decreases.

Boiling point elevation

The presence of a nonvolatile solute (which will not become a gas) will also affect the boiling point, causing the same increase. You can take the same figure above to understand that property.

Freezing point depression

By lowering the temperature of a liquid, its molecules are rearranged in a more organized pattern known as the solid state. When a solvent is mixed with a nonvolatile solute, said solute prevents this organization takes place, causing the freezing point of the solvent decreases. Figure a) shows that at a given temperature, the solvent becomes a solid. Figure b) expresses the effect of solute on the solvent.

Osmosis
Osmosis is a trend that has a solution to “diluted”, reduce its concentration as it passes through a semi- waterproof membrane. This property is best explained through a chart:

This property is widely used in various cellular processes, in which this principle is used for the transfer of certain nutrients and waste disposal.

ACTIVITY

- Mini-Lab: MEASURING THE FREEZING POINT.
Measurement: The colligative property of freezing point depression can be seen in a simple laboratory research, where you will measure the temperature of two vessels and their contents.
Materials: Two 10-ounce cups, Crushed ice, table salt, water, mixers, plastics, liquid meter, two thermometers, spoon.
Procedure:

  1. Fill two glasses of 10 oz. with crushed ice. Add 50 mL of cold tap water to each cup.
  2. Shake the contents of each beaker with a stirrer until the two cups have a constant temperature, about one minute.
  3. Measure the temperature of each cup with a thermometer and record the readings.
  4. Add 7 tablespoons of table salt to one cup and still shaking. Part of the salt is dissolved.
  5. When the temperature of each cell is constant, taking readings.
  6. To clean throws the contents of the glasses for soda, with water.

Analysis:
- Compare your readings of ice water and salt water. How do you explain the observed temperature change?

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