What is the difference between lime and lye

Structural formula
No structural formula available
Surname Calcium hydroxide
other names

Calcium hydroxide, calcium dihydroxide, calcium lye, portlandite, lime water (as an aqueous solution), slaked lime

Molecular formula Ca (OH)2
CAS number 1305-62-0
Brief description White dust
Molar mass 74.10 g mol−1[1]
Physical state firmly
density 2.24 g · cm−3 (20 ° C)[1]
Melting point Decomposition at 550 ° C)[1]
boiling pointnot applicable
Vapor pressure



almost not in water (1.7 g l−1 at 20 ° C[1], 0.77 g · l−1 at 100 ° C), not in ethanol

safety instructions

7340 mg kg−1[1] (Rat, oral)

As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions.

Calcium hydroxide (also: Calcium hydroxide, slaked lime, Slaked lime, Hydrated lime, Portlandite) is the hydroxide of calcium.


Calcium hydroxide Ca (OH)2 is a white, corrosive powder, which dissolves poorly (approx. 1.7 g / l) in water.

Burnt (unslaked) lime (calcium oxide) and slaked lime are very corrosive (pH value 12-13), contact with the eyes can lead to blindness!


Calcium hydroxide is formed under strong heat development (exothermic reaction) when adding calcium oxide (= quicklime, more undeleted Lime) with water. This process is also called lime slaughtering. The heat development is so strong that parts of the water evaporate (colloquially known as "smoking").

An overview of the conversion processes between different calcium compounds (Technical lime cycle) gives the following diagram:


Suspensions in water are:

  • Fat lime (Sump lime): a creamy, stiff mass - building material: lime mortar
  • Milk of lime: a whitish, milky liquid that separates itself - lime color
  • Lime water: the saturated solution, a clear liquid - is used to detect carbon dioxide

Calcium hydroxide is an intermediate product in the manufacture of chlorinated lime.

It is also used as a medicine in dentistry. Calcium hydroxide is used in construction under the name white lime hydrate (DIN 1060). Lime plasters consist of mixtures of calcium hydroxide, ground limestone and sand.

Calcium hydroxide is also part of soda lime, which is used in anesthesia machines or diving machines with rebreathing to filter out CO2 from the exhaled air.

The slaked lime is used, among other things, as an alternative to limestone in flue gas desulphurisation. The amount used is around 1.8 times less than for limestone. The gypsum (calcium sulfate) obtained in this way has a whiteness of 80 percent and can be used commercially. Due to its high reactivity, lower consumption quantities are required. The disadvantage is its significantly higher price compared to limestone.

Calcium hydroxide is also used as a pesticide in fruit growing. Here it is z. B. used as a fungicide (mushroom poison) against fruit tree cancer.

The caustic effect is the reason why slaked lime was previously used to disinfect stables (the "lime" of stables).

It is added to foods in the food industry as an acidity regulator. It is labeled in the EU as a food additive E 526 without maximum quantity restriction (quantum satis) generally approved for food.

Lime is also used to improve the load-bearing capacity of the building site. A soil with too high a water content and the resulting low load-bearing capacity, poor compactability can be improved by mixing in 2-4% MA lime. The lime binds part of the water and thus improves, among other things, the plasticity, the compressibility and the load-bearing capacity. That is why soil improvement with lime is a method for immediately achievable improvement in the ease of installation and the facilitation of construction work.


  1. abcdefGH Safety data sheet (Merck)

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