# Why does magnesium hydroxide solution solidify after passage of time?

I have been trying to make a suspension of magnesium hydroxide in water. Around 40% magnesium hydroxide w/w. Initially it is fluid, but as time passes, it solidifies. What is causing this?

• Gravity, time and cohesion. It is common process for all suspensions and happens in large spatial and time scale in nature. Oct 3, 2021 at 17:25
• Is your container open to the air? Oct 3, 2021 at 17:35
• @Poutnik, I can imagine it settling down, which I have seen in many suspensions, but never it creating a massive solid mass with no separate water layer Oct 3, 2021 at 17:36
• @Loong: yes. When I put it in a bottle, it still solidfies, but takes longer. Will a hydroxide get oxidised? What would be the oxidative product? Is this some sort of cross-linkage? I’d be grateful if you (or anyone) can point me to a book/reference explaining this. Oct 3, 2021 at 17:39
• @Maurice, thanks! That sounds logical. Will test the solidified mass for carbonate anion and see… Oct 3, 2021 at 20:52

The solidification of a $$40$$ % suspension of $$\ce{Mg(OH)2}$$ in water is probably due to the action of $$\ce{CO2}$$ from the air, which reacts with $$\ce{Mg(OH)2}$$ producing $$\ce{MgCO3}$$ which is insoluble in water. Remember that $$58~ g$$ $$\ce{Mg(OH)2}$$ produces $$84~ g$$ $$\ce{MgCO3}$$ and $$174~ g$$ lansfordite $$\ce{MgCO3⋅5H2O}$$, as described in the Handbook of Chemistry and Physics. This is enough to absorb all the water from your suspension. Remember also that $$\ce{Mg(OH)2}$$ is practically insoluble in water : $$0.9~ mg/L$$.

Ref.: 1. Charles D. Hodgman, et al., Handbook of Chemistry and Physics, The Chemical Rubber Publishing Co. Cleveland, $$1961$$, p. $$523$$ and $$600$$.

1. Wikipedia. Lansfordite is a hydrated magnesium carbonate mineral with composition: $$\ce{MgCO3·5H2O}$$. Landsfordite was discovered in $$1888$$ in a coal mine in Lansford, Pennsylvania. It crystallizes in the monoclinic system (space group P21/c) and typically occurs as colorless to white prismatic crystals and stalactitic masses. It is a soft mineral, Mohs hardness of $$2.5$$, with a low specific gravity of $$1.7$$

Milk of magnesia (~8% Mg(OH)2 + additives) can be shaken to make a homogeneous dispersion that settles slowly - even without additives - over a period of hours. As the Mg(OH)2 settles and becomes more concentrated, it settles more slowly until after many hours or days, the dispersion settles to a somewhat stiff paste which can be tilted without flowing, but not strong enough to be called solid. A more concentrated dispersion made by stirring powder into water will settle in the same way, most probably into a thick paste with some supernatant water. More concentrated suspensions (or slurries), with additives, are available commercially up to 60% Ref 1). Just a little more than 60% Mg(OH)2 might not even be possible to be stirred into a homogeneous fluid.

Magnesium hydroxide has a layered crystal structure (like clays)

(green is Mg, red-gray is OH) in which hydroxyl groups are available for hydrogen bonding to water.

A more complete description of the experimental conditions would be helpful. For instance, how strong is the solid that forms? How much clear liquid appears above the solid phase (so that the actual concentration of Mg(OH)2 in the solid phase can be estimated)? While the formation of an apparent solid could be due to atmospheric CO2, this path seems unlikely in the closed bottle. Forty grams of Mg(OH)2 plus sixty grams of H2O would require thirty grams (15.3 L) to completely convert the Mg(OH)2 into Lansfordite (or maybe this happens only on the surface - but the closed bottle still poses a problem - where does the CO2 come from?).

Intense homogenization separates many layers, but they eventually come together again by gravitational settling, perhaps even reforming crystals. Magnesium hydroxide is prepared from seawater by precipitation with NaOH, over a period of settling for many hours; some new crystallization can be observed by x-ray diffraction (Ref 2).

I think the problem will go away if you restir the solid, breaking down any crystallization that might have helped stiffen the suspension. Or shake the (closed) bottle vigorously. These things happen.

Ref 2. Marian Turek and Witold Gnot, Ind. Eng. Chem.Res. 1995, 34, 244-250