# Can Aluminum Hydroxide (Al(OH)3) be heated to it's melting temperature before decomposing?

On the wikipedia page for aluminum hydroxide the listed melting point is 300 °C (572 °F), but in the same page it states that aluminum hydroxide decomposes at only 180 °C (356 °F). Can aluminum hydroxide be melted without decomposing?

• The decomposition of pure aluminium hydroxide to water and oxide takes place above 550 °C. What you were reading in Wikipedia refers to the use in a mixture as a fire retardant. – andselisk Jun 2 '20 at 21:51
• Thanks for the clear answer! Do you have a source for the decomposition temperature of pure aluminum hydroxide? I am trying to find good resources for chemical properties – bakingluvr Jun 2 '20 at 22:00

TL.DR: Aluminium oxide (sometimes known as alumina) is made by heating the aluminium hydroxide to a temperature of about 1100 - 1200°C. (Chemguide)

$$\ce{Al(OH)3 ->[\Delta] Al2O3 + 3H2O}$$

As @andselisk said, the temperature Wikipedia mentions is aluminium hydroxide mixture in a fire retardant which might have other compounds. Decomposition of pure aluminium hydroxide has been studied in various papers and its decomposition reaction mechanism, temperature, reaction kinetics has been exhaustively studied.

A paper examines$$\ce{^{[1]}}$$ the decomposition of aluminium hydroxide at a temperature of 973-1123 K(700 °C-850 °C).

Another paper$$\ce{^{[2]}}$$ shows aluminium hydroxide being heated from room temperature to 1200 K(926.85 °C) where the exact temperature of decomposition products i.e $$\ce{Al2O3}$$ is noted:

[...] we find the first and the smallest endothermic peak at 519 K, which is due to the partial dehydroxylation of gibbsite ($$\ce{Al(OH)3}$$) and formation of boehmite ($$\ce{AlOOH}$$). Endothermic peak at 585 K corresponds to two processes: (i) transformation of gibbsite to phase $$\ce{χ-Al2O3}$$ and (ii) additional conversion of gibbsite to boehmite. The first process is in accordance with the results obtained. Another endothermic peak at 815 K is due to decomposition of boehmite and formation of alumina $$\ce{γ-Al2O3}$$. The obtained TG curves show clearly three steps of weight loss. The weight loss in the first step (about 5 wt.%) is due to the partial transformation of gibbsite to boehmite; the second step (about 25 wt.%) corresponds to decomposition of gibbsite to boehmite, otherwise to $$\ce{χ-Al2O3}$$. The last step of around 3 wt.% relates to the formation of $$\ce{γ-Al2O3}$$. Further observations show that the total weight loss is equal to 33 wt.%. All these transformations of gibbsite and the chemical composition are confirmed by the XRD analysis.

References

1. Chemical kinetics and reaction mechanism of thermal decomposition of aluminum hydroxide and magnesium hydroxide at high temperature (973-1123 K) by Ienwhei Chen, Shuh Kwei Hwang, and Shyan Chen, Ind. Eng. Chem. Res. 1989, 28, 6, 738–742 DOI: https://doi.org/10.1021/ie00090a015
2. http://przyrbwn.icm.edu.pl/APP/PDF/131/a131z3p62.pdf
3. Bhattacharya, Indra & Das, S. & Mukherjee, P. & Paul, Subir & Mitra, P.. (2004). Thermal Decomposition of Precipitated Fine Aluminium Trihydroxide. Scandinavian Journal of Metallurgy. 33. 211 - 219. 10.1111/j.1600-0692.2004.00686.x.(DOI) (Here aluminium hydroxide is subjected to dehydration upto a temperature of 1440 °C)