Saturated and unsaturated solutions

Water is a very good solvent for many substances. This is especially true for ionic substances, z. B. Salts too. However, the salts also have different solubilities. Each salt dissolves in water in different amounts. Solubility is a substance-specific quantity that indicates the amount of a substance that dissolves in a given amount of solvent at a given temperature.

Depending on whether or not the amount of water can dissolve further amounts of salt, a distinction is made between saturated and unsaturated solutions.

Solvent and solute

In principle, you can try to dissolve any substance in a liquid (sodium chloride, octane, etc.).

Many substances are well soluble, but many others are poorly soluble.

This depends on the solvent (z. B. water, ethanol), from the substance to be dissolved (z. B. sodium chloride, octane), but also depends on the temperature and some other factors.

The following article is limited to dissolving salts in water.

When dissolving in other solvents, there are both differences and similarities compared to dissolving salts in water.

Dissolving a solid in water.

The dissolution of salts in water

Many salts dissolve well in the solvent water, even the poorly soluble salts are not completely insoluble.

The ions in the lattice are held together by electrostatic forces. When dissolving, the water molecules with their dipoles, starting at the corners and edges, attach themselves to and around the ions. This weakens the bonding forces to the other ions in the lattice. The ions go into solution as hydrated ions.

If you look at the dissolution of salts in water from this point of view, you can see all the characteristics of a chemical reaction.

Energetic consideration of the dissolving process
Energy is needed to "release" the ions from the ion lattice. This is called lattice energy.

The energy released during the formation of the hydrate shell is accordingly called hydration energy.

If the solution heats up while dissolving the salt, the amount of the hydration energy is larger than that of the lattice energy (exothermic dissolution process).

In an endothermic dissolution process the hydration energy is smaller than the lattice energy. The missing energy to dissolve the salt is extracted from the solvent (the solution cools down).

The solubility of a substance

The solubility of a salt indicates how much of the substance will dissolve in a given amount of solvent at a given temperature.

This quantity is substance specific . A distinction is made between saturated solution en and unsaturated solution en.

1. The saturated solution contains the maximum amount of solute at a given temperature. If the substance is added further, it settles as a precipitate in the form of soil.

2. Unsaturated solutions do not yet contain the maximum amount of dissolved substance. With further addition of substance, something else dissolves in the solution.

The solubility of a substance at a given temperature in a given solvent can be calculated by determining the concentration of the solute that is present in its saturated solution ( saturation concentration ).

The solubility is temperature dependent . With increasing temperature, the solubility of a salt decreases or increases, depending on whether the dissolution process is exothermic or endothermic. For example, if one heats a saturated potassium chloride solution, any precipitate present dissolves.

At the higher temperature, the solution was therefore not yet saturated. This connection can be shown in diagrams. For most salts, the solubility increases with increasing temperature.

Growing crystals

The relationship between the solubility of the salt and the prevailing temperature plays a special role in growing crystals.

To do this, first make a saturated salt solution by heating water and adding as much salt as necessary until saturation is reached. Then the solution is allowed to cool and the resulting soil is filtered off.

The container with the solution produced in this way is placed in a vibration-free location at a constant temperature. To protect the vessel from dust, it is covered with a filter paper, but it must not be closed. The amount of water decreases due to evaporation processes.
Since the solution is now supersaturated, the salt in question crystallizes out. A particularly well-formed crystal can be removed with tweezers, attached to a thread and suspended again in a saturated solution.

With time the crystal "grows. This means that the typical crystal form can be formed by the slow process. One can reach the supersaturation and thus the crystallization also during the cooling of the solution. One receives particularly well formed crystals however only with very slow increase of the supersaturation, as it is given with the evaporation of the water. If crystals were grown by cooling, the cooling process would therefore have to be controlled at a correspondingly slow rate.

The solution process starts at the nooks and crannies of the salt crystal. The formation of water envelopes shields the attractive forces between the ions. The crystal decays.

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