Calcium hydroxide

Athanor · official

CaH₂O₂ · 74.09 g/mol

IUPAC name: calcium dihydroxide

Also called slaked lime, hydrated lime, E526

Inorganic

Open in Athanor

Drawn from its SMILES with RDKit

DangerCauses severe skin burns and eye damage · Causes skin irritation · Causes serious eye damage · May cause respiratory irritation

Names and identifiers

IUPAC name: calcium dihydroxide
Synonyms: slaked lime, hydrated lime, E526
+ 38 more
Slaked lime, Hydrated lime, Lime hydrate, Calcium hydrate, Lime water, Calcium hydroxide (Ca(OH)2), Hydralime, Slaked-lime, Lime milk, Calcarea caustica, INS NO.526, PF5DZW74VN, E-526, CHEBI:31341, Hydroxide, Calcium, 215-137-3, O2 Patch, RefChem:54625, KO2, Ca(OH)2, DTXCID5014410, CHEBI:35150, DTXSID601045142, 620-122-3, Bell mine, Biocalc, Calbit, Calcium dihydroxide, Caldic 1000, Calvit, Caustic lime, DTXSID7034410, Kalkhydrate, Kemikal, Kentoku K 100, Limbux, Milk of lime, NICC 3000, Rhenofit CF
Molecular formula: CaH₂O₂
Empirical formula: CaH₂O₂
Molecular weight: 74.09 g/mol
Exact mass: 73.96807
CAS number: 1305-62-0
PubChem CID: 6093208
Canonical SMILES: [OH-].[OH-].[Ca+2]
Isomeric SMILES: [OH-].[OH-].[Ca+2]
InChI: InChI=1S/Ca.2H2O/h;2*1H2/q+2;;/p-2
InChIKey: AXCZMVOFGPJBDE-UHFFFAOYSA-L
Molfile

Names, InChI and the identifiers from PubChem, retrieved 2026-10-14.

Physical properties

TemperaturePressureDensity

Appearance: Soft, white crystalline powder[6]
8 values
From Hazardous Substances Data Bank (HSDB), the most authoritative source here.
Value As written Source
– Calcium hydroxide is an odorless white granules. Sinks in water. (USCG, 1999) [1] CAMEO Chemicalscites U.S. Coast Guard. 1999. Chemical Hazard Response Information System (CHRIS) - Hazardous Chemical Data. Commandant Instruction 16465.12C. Washington, D.C.: U.S. Government Printing Office.
– Dry Powder; Other Solid; Liquid; Other Solid; Liquid; Liquid; Wet Solid; CBI; Dry Powder; Wet Solid; Large Crystals; Wet Solid; Other Solid; Dry Powder; Gas Vapor [2] EPA Chemical Data Reporting (CDR)
– COLOURLESS CRYSTALS OR WHITE POWDER. [3] ILO-WHO International Chemical Safety Cards (ICSCs)
– White, odorless powder. [4] Occupational Safety and Health Administration (OSHA)
– White, odorless powder. [Note: Readily absorbs CO2 from the air to form calcium carbonate.] [5] The National Institute for Occupational Safety and Health (NIOSH)
– Rhombic, trigonal, colorless crystals [6] Hazardous Substances Data Bank (HSDB)cites Lewis, R.J. Sr. (ed) Sax's Dangerous Properties of Industrial Materials. 11th Edition. Wiley-Interscience, Wiley & Sons, Inc. Hoboken, NJ. 2004., p. V2: 675
–shown Soft, white crystalline powder [6] Hazardous Substances Data Bank (HSDB)cites Lewis, R.J. Sr.; Hawley's Condensed Chemical Dictionary 15th Edition. John Wiley & Sons, Inc. New York, NY 2007., p. 217
– White powder [Note: Readily absorbs CO2 from the air to form calcium carbonate] [6] Hazardous Substances Data Bank (HSDB)cites NIOSH. NIOSH Pocket Guide to Chemical Hazards. Department of Health & Human Services, Centers for Disease Control & Prevention. National Institute for Occupational Safety & Health. DHHS (NIOSH) Publication No. 2010-168 (2010). Available from: https://www.cdc.gov/niosh/npg
Melting point: 580 °C (1076 °F) (decomposes) (loses water)[6]
4 values
4 of 4 sources agree on it (most common value).
Value As written Source
580 °C 1076 °F (Decomposes) (Loses H2O) (NIOSH, 2024) [1] CAMEO Chemicals
580 °Cshown 1076 °F (decomposes) (loses water) [6] Hazardous Substances Data Bank (HSDB)cites NIOSH. NIOSH Pocket Guide to Chemical Hazards. Department of Health & Human Services, Centers for Disease Control & Prevention. National Institute for Occupational Safety & Health. DHHS (NIOSH) Publication No. 2010-168 (2010). Available from: https://www.cdc.gov/niosh/npg
580 °C 1076 °F (decomposes) (loses H₂O) [4] Occupational Safety and Health Administration (OSHA)
580 °C 1076 °F (Decomposes) (Loses H2O) [5] The National Institute for Occupational Safety and Health (NIOSH)
Boiling point: Reported, but no value passed the checks.
The value and why
Decomposes (NIOSH, 2024) (could not be read; source [1])
Density: 2.24 at 20 °C (68 °F) (USCG, 1999) - Denser than water[1]
5 values
4 of 5 sources agree on it (most common value).
Value As written Source
–shown 2.24 at 68 °F (USCG, 1999) - Denser than water [1] CAMEO Chemicalscites U.S. Coast Guard. 1999. Chemical Hazard Response Information System (CHRIS) - Hazardous Chemical Data. Commandant Instruction 16465.12C. Washington, D.C.: U.S. Government Printing Office.
– Relative density (water = 1): 2.2 [3] ILO-WHO International Chemical Safety Cards (ICSCs)
– 2.24 [4] Occupational Safety and Health Administration (OSHA)
– 2.343 @25 °C [7] PAC Chemical Database, U.S. Department of Energy
– 2.24 [5] The National Institute for Occupational Safety and Health (NIOSH)
1 more value was reported but failed the checks (unreadable or out of range), so it is not shown.
Solubility in water: Solubility in water: none[3]
5 values
From ILO-WHO International Chemical Safety Cards (ICSCs), the most authoritative source here.
Value As written Source
– 0.2 % at 32 °F (NIOSH, 2024) [1] CAMEO Chemicals
– Solubility in water is decreased by the presence of fixed alkali hydroxides. [6] Hazardous Substances Data Bank (HSDB)cites Osol, A. (ed.). Remington's Pharmaceutical Sciences. 16th ed. Easton, Pennsylvania: Mack Publishing Co., 1980., p. 1258
– In water, 1730 mg/L at 20 °C [6] Hazardous Substances Data Bank (HSDB)cites PMID:2205893 Shiu WY et al; Rev Environ Contam Toxicol 116: 15-187 (1990)
–shown Solubility in water: none [3] ILO-WHO International Chemical Safety Cards (ICSCs)
– (32 °F): 0.2% [5] The National Institute for Occupational Safety and Health (NIOSH)
Vapor pressure: Reported, but no value passed the checks.
The 3 values and why
0 mmHg (approx) (NIOSH, 2024) (no temperature given; source [1])
0 mmHg (approx) (no temperature given; source [4])
0 mmHg (approx) (no temperature given; source [5])
Autoignition temperature: Reported, but no value passed the checks.
The value and why
Not flammable (USCG, 1999) (could not be read; source [1])

Value	As written	Source
–	Calcium hydroxide is an odorless white granules. Sinks in water. (USCG, 1999)	[1] CAMEO Chemicalscites U.S. Coast Guard. 1999. Chemical Hazard Response Information System (CHRIS) - Hazardous Chemical Data. Commandant Instruction 16465.12C. Washington, D.C.: U.S. Government Printing Office.
–	Dry Powder; Other Solid; Liquid; Other Solid; Liquid; Liquid; Wet Solid; CBI; Dry Powder; Wet Solid; Large Crystals; Wet Solid; Other Solid; Dry Powder; Gas Vapor	[2] EPA Chemical Data Reporting (CDR)
–	COLOURLESS CRYSTALS OR WHITE POWDER.	[3] ILO-WHO International Chemical Safety Cards (ICSCs)
–	White, odorless powder.	[4] Occupational Safety and Health Administration (OSHA)
–	White, odorless powder. [Note: Readily absorbs CO2 from the air to form calcium carbonate.]	[5] The National Institute for Occupational Safety and Health (NIOSH)
–	Rhombic, trigonal, colorless crystals	[6] Hazardous Substances Data Bank (HSDB)cites Lewis, R.J. Sr. (ed) Sax's Dangerous Properties of Industrial Materials. 11th Edition. Wiley-Interscience, Wiley & Sons, Inc. Hoboken, NJ. 2004., p. V2: 675
–shown	Soft, white crystalline powder	[6] Hazardous Substances Data Bank (HSDB)cites Lewis, R.J. Sr.; Hawley's Condensed Chemical Dictionary 15th Edition. John Wiley & Sons, Inc. New York, NY 2007., p. 217
–	White powder [Note: Readily absorbs CO2 from the air to form calcium carbonate]	[6] Hazardous Substances Data Bank (HSDB)cites NIOSH. NIOSH Pocket Guide to Chemical Hazards. Department of Health & Human Services, Centers for Disease Control & Prevention. National Institute for Occupational Safety & Health. DHHS (NIOSH) Publication No. 2010-168 (2010). Available from: https://www.cdc.gov/niosh/npg

Value	As written	Source
580 °C	1076 °F (Decomposes) (Loses H2O) (NIOSH, 2024)	[1] CAMEO Chemicals
580 °Cshown	1076 °F (decomposes) (loses water)	[6] Hazardous Substances Data Bank (HSDB)cites NIOSH. NIOSH Pocket Guide to Chemical Hazards. Department of Health & Human Services, Centers for Disease Control & Prevention. National Institute for Occupational Safety & Health. DHHS (NIOSH) Publication No. 2010-168 (2010). Available from: https://www.cdc.gov/niosh/npg
580 °C	1076 °F (decomposes) (loses H₂O)	[4] Occupational Safety and Health Administration (OSHA)
580 °C	1076 °F (Decomposes) (Loses H2O)	[5] The National Institute for Occupational Safety and Health (NIOSH)

Value	As written	Source
–shown	2.24 at 68 °F (USCG, 1999) - Denser than water	[1] CAMEO Chemicalscites U.S. Coast Guard. 1999. Chemical Hazard Response Information System (CHRIS) - Hazardous Chemical Data. Commandant Instruction 16465.12C. Washington, D.C.: U.S. Government Printing Office.
–	Relative density (water = 1): 2.2	[3] ILO-WHO International Chemical Safety Cards (ICSCs)
–	2.24	[4] Occupational Safety and Health Administration (OSHA)
–	2.343 @25 °C	[7] PAC Chemical Database, U.S. Department of Energy
–	2.24	[5] The National Institute for Occupational Safety and Health (NIOSH)

Value	As written	Source
–	0.2 % at 32 °F (NIOSH, 2024)	[1] CAMEO Chemicals
–	Solubility in water is decreased by the presence of fixed alkali hydroxides.	[6] Hazardous Substances Data Bank (HSDB)cites Osol, A. (ed.). Remington's Pharmaceutical Sciences. 16th ed. Easton, Pennsylvania: Mack Publishing Co., 1980., p. 1258
–	In water, 1730 mg/L at 20 °C	[6] Hazardous Substances Data Bank (HSDB)cites PMID:2205893 Shiu WY et al; Rev Environ Contam Toxicol 116: 15-187 (1990)
–shown	Solubility in water: none	[3] ILO-WHO International Chemical Safety Cards (ICSCs)
–	(32 °F): 0.2%	[5] The National Institute for Occupational Safety and Health (NIOSH)

Each value as its source reports it, with the source's number after it (listed under References). Where several sources report a property, the row shows the value most of them agree on, the rule the app uses; open the values to see every one.

Calculated

Heavy atoms: 3
Rings: 0
Aromatic rings: 0
Fraction sp³ carbons (Fsp3): 0
H-bond donors: 0 computed here · PubChem 2
H-bond acceptors: 2
Rotatable bonds: 0
TPSA (Å²): 60 computed here · PubChem 2
Formal charge: 0
Complexity: 0 PubChem
Stereocenters: None
Double-bond geometry: None specified
Point group: C∞v of one 3D conformer with its hydrogens, by Athanor’s engine (as in the app)

Elemental analysis

H2.72%
Ca54.09%
O43.19%

Isotope pattern

Each peak is a nominal mass with its abundance-weighted exact mass, as a percentage of the tallest, from IUPAC natural isotope abundances.

Computed from the structure with RDKit 2026.09 when this page was built, beside PubChem's own computed values from its record; where the two differ, both are shown.

Predicted

log P: -0.73 Crippen (Wildman and Crippen 1999)
log S (water solubility): 0.16 ESOL (Delaney 2004), log mol/L
Synthetic accessibility: 7.69 SA score, 1 easy to 10 hard (Ertl and Schuffenhauer 2009)

Drug-likeness: Lipinski’s rule of five

Molecular weight ≤ 500: 74.09
log P ≤ 5 (Crippen): -0.73
H-bond donors ≤ 5: 0
H-bond acceptors ≤ 10: 2

Passes all four rules.

Predictions, not measurements; each names its method. A measured log P, where a source reports one, is under Physical properties.

Safety

Danger

Most widely reported: 6,644 company reports to ECHA. Source [8].

Corrosive
Harmful or irritant

H314Causes severe skin burns and eye damage (22.4% of reports)
H315Causes skin irritation (45.3% of reports)
H318Causes serious eye damage (76.9% of reports)
H335May cause respiratory irritation (38.9% of reports)

Precautionary statements: P260, P261, P264, P264+P265, P271, P280, P301+P330+P331, P302+P352, P302+P361+P354, P304+P340, P305+P354+P338, P316, P317, P319, P321, P332+P317, P362+P364, P363, P403+P233, P405, P501

View all classifications (3)

[8] European Chemicals Agency (ECHA)This chemical does not meet GHS hazard criteria for 0.4% (28 of 6644) of reports.
[8] European Chemicals Agency (ECHA) · 6,644 reports · DangerH314, H315, H318, H335
[6] Hazardous Substances Data Bank (HSDB) · DangerH315, H318, H335, H402

Exposure limits

PEL-TWA (8-Hour Time Weighted Average): 15 mg/m³ (total dust), 5 mg/m³ (respirable fraction)[4]
PEL: TWA 15 mg/m3 (total) 5 mg/m3 (resp)[5]
REL-TWA (Time Weighted Average): 5 mg/m³[4]
REL: TWA 5 mg/m3[5]
TLV: 5 mg/m[3]
IDLH: N.D. See: IDLH index[5]

First aid

Excerpt from NIOSH Pocket Guide for Calcium hydroxide: Eye: Irrigate immediately - If this chemical contacts the eyes, immediately wash (irrigate) the eyes with large amounts of water, occasionally lifting the lower and upper lids. Get medical attention immediately. Skin: SOAP flush immediately - If this chemical contacts the skin, immediately flush the contaminated skin with soap and water. If this chemical penetrates the clothing, immediately remove the clothing and flush the skin with water.[1]
(General first aid procedures) Eye: Irrigate immediately - If this chemical contacts the eyes, immediately wash (irrigate) the eyes with large amounts of water, occasionally lifting the lower and upper lids. Get medical attention immediately. Skin: Soap flush immediately - If this chemical contacts the skin, immediately flush the contaminated skin with soap and water. If this chemical penetrates the clothing, immediately remove the clothing and flush the skin with water.[5]
Fresh air, rest. Refer for medical attention.[3]
Remove contaminated clothes. Rinse skin with plenty of water or shower. Refer for medical attention .[3]

Storage

Separated from strong acids.[3]
... Preserved in well-filled, tight containers, at a temp not exceeding 25 °C. (calcium hydroxide topical soln)[6]
Store under tightly capped conditions.[6]

Incompatibilities

Phosphorus boiled with alkaline hydroxides yields mixed phosphines which may ignite spontaneously in air.[6]
When chlorinated phenols are heated for analytical purposes with calcium hydroxide-potassium nitrate mixtures, chlorinated benzodioxins analogous to the extremely toxic tetrachlorodibenzodioxin may be formed.[6]
The self-heating and decomposition of /1,3,5-trinitrohexahydro-1,3,5-triazine, calcium hydroxide and water/ was studied ... In a sealed capsule and in a larger scale furnace test. A rapid exothermic decomposition reaction can be initiated at 100 °C or below, and may lead to spontaneous ignition and then deflagration or detonation.[6]
Maleic anhydride, phosphorus, nitroethane, nitromethane, nitroparaffins, nitropropane [Note: Attacks some metals].[6]

Safety data sheets on the supplier’s page: Thermo Fisher Scientific, Angene Chemical, Alfa Chemistry.

Always read the supplier’s safety data sheet before working with it.

Spectra

FTIRSpectraBase (KBr pellet)

Experimental spectra held by these sources; each link opens the source's own record.

Literature

Reading PubChem, OpenAlex and GSRS…

Bioactivity

Reading ChEMBL…

References

[1]CAMEO Chemicals · Public domain (NOAA) · via PubChem · retrieved 2026-10-14
[2]EPA Chemical Data Reporting (CDR) · Public domain (US EPA) · via PubChem · retrieved 2026-10-14
[3]ILO-WHO International Chemical Safety Cards (ICSCs) · CC BY 4.0 · via PubChem · retrieved 2026-10-14
[4]Occupational Safety and Health Administration (OSHA) · Public domain (US DOL) · via PubChem · retrieved 2026-10-14
[5]The National Institute for Occupational Safety and Health (NIOSH) · Public domain (US CDC) · via PubChem · retrieved 2026-10-14
[6]Hazardous Substances Data Bank (HSDB) · Public domain (US NLM) · via PubChem · retrieved 2026-10-14
[7]PAC Chemical Database, U.S. Department of Energy · Public domain (US DOE) · via PubChem · retrieved 2026-10-14
[8]European Chemicals Agency (ECHA) · ECHA legal notice: reuse with acknowledgement · via PubChem · retrieved 2026-10-14
[9]SpectraBase · Linked only · via PubChem · retrieved 2026-10-14
·Calculated values: computed by Athanor with RDKit 2026.09 from the structure.
·Literature on PubChem

Data last updated 2026-10-14.