Topic 2 Solubility

Topic 2

Solubility

We already know that sucrose or table salt dissolves easily in water to form a solution. But can they be dissolved in a certain amount of water without limit?

1. Saturated solution

Adding a certain solute to a certain amount of solvent at a certain temperature, when the solute cannot continue to dissolve, the resulting solution is called a saturated solution of the solute; the solution that can continue to dissolve is called an unsaturated solution of the solute.

In experiment 9-5 , when the sodium chloride can continue to dissolve, the solution is unsaturated; when the sodium chloride solid cannot continue to dissolve and remains, the solution becomes saturated. When water was added, the remaining solid sodium chloride continued to dissolve, and the solution changed from saturated to unsaturated.

In Experiment 9-6 , the potassium nitrate added for the second time could not completely dissolve, but when the beaker was heated, the remaining solid potassium nitrate continued to dissolve, and the potassium nitrate added again could completely dissolve. This shows that when the temperature rises, the saturated solution of potassium nitrate at room temperature becomes an unsaturated solution at a higher temperature, thereby continuing to dissolve potassium nitrate.

The above experiments show that a saturated solution can become an unsaturated solution by increasing the solvent or increasing the temperature. Therefore, only when "in a certain amount of solvent" and "at a certain temperature" is specified , the " saturated " and " unsaturated" of the solution have definite meaning.

in Experiment 9-6 that when the hot potassium nitrate solution cooled, solids appeared at the bottom of the beaker. This is because during the cooling process, the unsaturated solution of potassium nitrate becomes a saturated solution; when the temperature continues to drop, too much potassium nitrate will precipitate out of the solution in the form of crystals. This process is called crystallization.

In addition to the method of cooling a hot saturated solution, evaporation of solvent is also a common method to obtain crystals. For example, using seawater to dry salt is to use the high tide to introduce seawater into the storage tank. After the seawater is clarified, it is first introduced into the evaporation tank, and the water is partially evaporated after being blown by wind and sun; Blowing and sun exposure, the seawater will gradually become a saturated solution of salt; and then exposure, the salt crystals will gradually precipitate from the seawater to obtain coarse salt, and at the same time obtain a mother liquor (called bittern) containing a large amount of chemical raw materials.

2. Solubility

Through the above experiments, we can roughly draw the following conclusions: At room temperature, the mass of sodium chloride or potassium nitrate that can be dissolved in 20 mL of water has a maximum value, and this maximum mass is what can be dissolved when forming its saturated solution the quality of. This shows that at a certain temperature, the amount of solute dissolved in a certain amount of solvent is limited. Solubility is used in chemistry to express the limit of this dissolution.

The solubility of a solid indicates the mass of a solid substance dissolved in 100 g of solvent at a certain temperature when it reaches saturation. If no solvent is specified, solubility is generally referred to as the solubility of the substance in water. For example, at 20 °C, 100 g of water can dissolve up to 36 g of sodium chloride (at this time the solution reaches saturation), we say that at 20 °C, the solubility of sodium chloride in water is 36 g.

The solubility of a substance at different temperatures can be measured experimentally

Using the solubility curve, we can find out the solubility of a substance at different temperatures; we can compare the solubility of different substances at the same temperature; we can compare the solubility of different substances affected by temperature changes; it can be seen that the solubility of substances varies with temperature The law of change; etc.

From Figure 9-12 and Figure 9-13 , it can be seen that the solubility of most solid substances increases with the increase of temperature, such as potassium nitrate, chlorinated hydrogen, etc.; the solubility of a few solid substances is slightly affected by temperature changes, Such as sodium chloride; the solubility of very few solid substances decreases with the increase of temperature, such as calcium hydroxide.

Because it is difficult to weigh the mass of gas, the solubility of gas is usually expressed by volume. The solubility of a gas commonly used refers to the volume of the gas when it dissolves in 1 volume of water and reaches saturation when the pressure of the gas is 101 kPa and a certain temperature . For example, when the pressure of nitrogen is 101 kPa and the temperature is 0℃ , 0.024 volume of nitrogen can be dissolved in 1 volume of water at most, then at 0Y , the solubility of nitrogen is 0.024 .

After finishing this topic, you should know

1.When a certain solute is added to a certain amount of solvent at a certain temperature, when the solute cannot continue to dissolve, the resulting solution is called a saturated solution of the solute.

2.the mass of a solid substance dissolved in 100 g of solvent at a certain temperature when it reaches saturation.

3.The solubility of a gas usually refers to the volume of the gas when it dissolves in 1 volume of water and reaches saturation when the pressure of the gas is 101 kPa and a certain temperature .

4.The curve showing the solubility of a substance as a function of temperature is called a solubility curve. Using a solubility curve, a lot of information about the solubility of a substance can be obtained.