Alkaline solutions or lyes are aqueous solutions of metal hydroxides such as sodium hydroxide ( caustic soda ) or potassium hydroxide ( potassium hydroxide ). These metal hydroxides are among the alkali hydroxides . In a broader sense, the term is also used for any solution of bases . These include ammonia ( ammonia water ) and organic compounds such as amines . Alkaline solutions in the broadest sense can also be non-aqueous solutions.
Aqueous solutions are alkaline if the concentration of hydroxide ions OH - exceeds that of oxonium ions H 3 O + . The pH value is then greater than 7. Strongly alkaline aqueous solutions have a pH value greater than 10, for example normal sodium hydroxide solution has a pH value of 14.
Alkaline solutions feel slippery (soapy, greasy) on the skin. This is done by saponifying the fats on the surface of the skin . Lye has a degreasing effect on and off the skin; on the one hand through the continuous formation of soaps from fat and on the other hand through the fat dissolving properties of these soaps .
Acids and alkaline solutions have some properties in common. These solutions produce characteristic colors with indicators . The solutions are corrosive , so protective goggles must always be worn when working with alkaline solutions. They also show electrical conductivity , so there must be freely moving ions.
Strongly alkaline solutions are corrosive: they can dissolve metals such as aluminum and proteins , so skin contact with them should be avoided. Gloves and protective goggles should be worn when handling strongly alkaline solutions. They can attack glass surfaces - especially if they are left on for a long time at elevated temperatures - and lead to the clouding of glasses. The handling of caustic soda, etc. in glass vessels such as Erlenmeyer flasks is still possible as long as ground- glass stoppers are not used, as these will get stuck in longer storage.
In technology and chemical production, alkalis are used to neutralize acids.
Preparation of alkaline solutions
Production of aqueous alkali metal hydroxide solutions
The following reactions , each of which is given with an example, lead to the formation of aqueous solutions of alkali metal hydroxides:
Dissolve the corresponding hydroxide in water
This is the easiest way to put the solutions into practical use in the laboratory . However, the heat generated during dissolution ( exothermic reaction ) can be so strong that the water begins to boil, whereby the alkaline solution can splash and cause burns. Example:
- Sodium hydroxide + water Sodium hydroxide solution or caustic soda
- NaOH (s) + H 2 O (l) Na + (aq) + OH - (aq) + H 2 O (l)
Reaction of an alkali metal oxide with water
Solid alkali metal hydroxide is formed when a stoichiometrically appropriate amount of water is added, and an alkali metal hydroxide solution is formed when excess water is added. The dissolving process described in production variant 1 takes place with the lithium hydroxide formed during the reaction. Example:
- Lithium oxide + water lithium hydroxide
- Li 2 O (s) + H 2 O (l, stoichiometric) 2 LiOH (s)
- Lithium oxide + water Lithium hydroxide solution
- Li 2 O (s) + H 2 O (l, excess) Li + (aq) + OH - (aq) + H 2 O (l)
Reaction of an alkali metal with water
As with the second production variant, the amount of water added is decisive for the creation or naming of the product . Example:
- Sodium + water sodium hydroxide + hydrogen
- 2 Na (s) + 2 H 2 O (l) 2 NaOH (s) + H 2 (g)
If water is added in excess, the sodium hydroxide from production variant 1 is dissolved. Depending on the alkali metal used and the test procedure, the resulting hydrogen ignites due to the heat of reaction released. High-speed recordings of the reaction of alkali metals with water suggest a Coulomb explosion.
Electrolysis of alkali metal halogen salt solutions
By the electrolysis of brine, caustic soda is produced industrially, and it is largely prevented by a porous partition wall (diaphragm) that the brine and caustic soda mix. This enables continuous electrolysis:
- 2 Na + (aq) + 2 Cl - (aq) + 2 H 2 O (l) 2 Na + (aq) + 2 OH - (aq) + Cl 2 (g) + H 2 (g)