Topic 1 Solution Formation

Unit 9 Solutions 

Topic 1

solution formation

Most of the Earth's surface is covered by blue oceans. If you have ever swum in sea water, you will find that sea water is bitter and salty. Why is this? It turns out that many substances are dissolved in seawater, and it is a mixture.

1. Solution

When sucrose is put into water, it " disappears " quickly. Where does it go? It turns out that the molecules on the surface of sucrose gradually diffuse into the water under the action of water molecules, and finally the sucrose molecules are uniformly dispersed among the water molecules to form a mixture—sucrose solution. If you put table salt (the main component is sodium chloride) into water, sodium chloride will also diffuse into the water under the action of water molecules, and finally evenly disperse among the water molecules to form a sodium chloride solution, but Sodium chloride exists in solution as sodium ions and chloride ions. Take out any part of the sucrose solution (or sodium chloride solution) for comparison, and find that their composition is exactly the same, that is, the solution is uniform; as long as the water does not evaporate and the temperature does not change, sucrose and water (or sodium chloride and water) There will be no separation, ie the solution is stable.

Such one or several substances are dispersed into another substance to form a uniform and stable mixture called a solution. A substance that can dissolve other substances is called a solvent, and a substance that is dissolved is called a solute. A solution is made up of a solute and a solvent. For example, in the above sucrose solution, sucrose is the solute and water is the solvent; in the sodium chloride solution, sodium chloride is the solute and water is the solvent.

Water can dissolve many kinds of substances and is one of the most commonly used solvents. Gasoline, alcohol, etc. can also be used as solvents, such as gasoline can dissolve oil, alcohol can dissolve iodine, and so on.

Experiments show that iodine is almost insoluble in water, but it can be dissolved in gasoline; potassium peroxide is almost insoluble in gasoline, but it can be dissolved in water. This shows that the solubility of the same substance in different solvents is different, and the solubility of different substances in the same solvent is also different.

A solute can be a solid, a liquid, or a gas. If two liquids dissolve in each other, the one with the larger amount is called the solvent, and the one with the smaller amount is called the solute. If one of them is water, the water is usually called the solvent. For example, in the solution formed by water and ethanol in Experiment 9-3, ethanol is the solute and water is the solvent. A solution that does not usually specify a solvent generally refers to an aqueous solution.

Solutions have a wide range of uses in daily life, industrial and agricultural production, and scientific research, and are closely related to people's lives.

2. Endothermic or exothermic phenomenon during dissolution

When a substance dissolves, it often changes the temperature of the solution. This shows that the dissolution process of substances is usually accompanied by heat changes. Some substances will absorb heat when dissolving, and some substances will exotherm when dissolving.

3. Emulsification

It can be seen from the experiment that after vigorously shaking the test tubes containing water and vegetable oil, a milky turbid liquid is obtained. Dispersed in this liquid are water-insoluble, small droplets composed of many molecular assemblies. This mixture of small droplets dispersed in a liquid is called an emulsion. This emulsion is unstable. After standing still, the vegetable oil gradually floats up and is divided into upper and lower layers.

In the test tubes with detergent, the situation was different. Although vegetable oil does not dissolve in water, the formed emulsion can exist relatively stably, and the liquid is no longer divided into two layers. Why is this?

It turns out that the detergent can disperse vegetable oil into countless fine droplets in water instead of aggregating into large oil droplets, so that oil and water are no longer separated, and the stability of the formed emulsion is enhanced. This phenomenon is called emulsification. The fine droplets formed after emulsification can flow with water, therefore, the vegetable oil in the test tube added with detergent in Experiment 9-4 can be washed away by water. This is also the reason why oil stains on clothes and tableware can be washed off with water added with detergent.

After studying this topic, you should know

1.One or several substances are dispersed into another substance to form a uniform and stable mixture called a solution. A substance that dissolves other substances is called a solvent; a substance that is dissolved is called a solute. Solutions have a wide variety of uses.

2.In the process of dissolving solutes, some give off heat and some absorb heat.

3.A mixture of small droplets dispersed in a liquid is called an emulsion. The emulsion is unstable, and emulsification can enhance the stability of the emulsion.