Is Magnesium Hydroxide Soluble in Water? Solubility, Behavior & Uses Explained

Is magnesium hydroxide soluble in water? Learn how it behaves, why it forms a suspension, and what this means for antacids, wastewater treatment, and industry.

Is magnesium hydroxide soluble in water?
If you’ve seen “milk of magnesia,” wastewater treatment chemicals, or industrial Mg(OH)₂ slurries and wondered why they look cloudy instead of clear, you’re asking the right question.

Struggling to figure out whether it dissolves, reacts, or just floats around?
Let me break it down like I would to a colleague on a quick call.

Is Magnesium Hydroxide Soluble in Water? (Short Answer)

No.
Magnesium hydroxide is only very slightly soluble in water.

It dissolves just a tiny bit—so little that the water remains cloudy, not clear.

Most of the Mg(OH)₂ stays as solid particles suspended in the water, which is why products like “milk of magnesia” look milky.

Why Magnesium Hydroxide Is Only Slightly Soluble

Low Solubility Product (Ksp)

Mg(OH)₂ has a very low Ksp (solubility product), meaning it resists dissolving.

That’s just chemistry talk for:
It almost refuses to dissolve, even when you force it.

What Actually Happens in Water

When mixed with water, magnesium hydroxide:

Dissolves a tiny amount

Releases OH⁻ ions (makes the water alkaline)

Leaves most particles undissolved and suspended

This is why it forms a white suspension, not a solution.

Why Warm or Hot Water Doesn’t Help

Some compounds dissolve more in hot water.
Magnesium hydroxide is the opposite.

It becomes even less soluble as temperature increases.

How Its Low Solubility Affects Real-World Uses

1. Antacids (Milk of Magnesia)

Low solubility = slow, gentle release.

That’s why Mg(OH)₂ is perfect for:

Neutralizing stomach acid

Acting as a mild laxative

Avoiding sudden pH spikes

If it dissolved too quickly, it would be harsh on the stomach.

2. Wastewater Treatment

In water treatment, magnesium hydroxide acts as a pH stabilizer.

Its low solubility helps it release alkalinity slowly and evenly, which prevents:

Dangerous pH spikes

Overdosing

Fluctuations in treatment systems

Operators love it for that reason.

3. Flame Retardants

Because it doesn’t dissolve, it stays intact inside plastics and releases water vapor only when heated.

That makes it ideal for:

Cable compounds

Building materials

Rubber products

Polymer blends

4. Industrial Slurries

Most industrial magnesium hydroxide products are sold as slurries, not powders.

Why?

It doesn’t dissolve anyway

Suspensions are easier to pump

Slurries prevent dust hazards

Better reactivity in neutralizing acidic streams

Its insolubility becomes an advantage.

A Quick Real-Life Example

I once asked a wastewater engineer why they switched from caustic soda to magnesium hydroxide slurry.

He said,
“Caustic soda slammed our pH up instantly. Mg(OH)₂ raised it slowly and kept it stable. That low solubility is exactly what we needed.”

That’s the power of “slightly soluble.”

FAQs: Is Magnesium Hydroxide Soluble in Water?

1. Why does magnesium hydroxide form a cloudy mixture in water?

Because most of it stays suspended as solid particles instead of dissolving.

2. Does magnesium hydroxide dissolve better in hot water?

No.
It becomes less soluble at higher temperatures.

3. Is magnesium hydroxide considered insoluble?

It’s classified as sparingly soluble—meaning it dissolves just enough to have an effect but nowhere near fully.

4. Why is magnesium hydroxide used in wastewater treatment if it doesn’t dissolve well?

Because its slow release provides stable pH control, unlike strong alkalis that react too fast.

5. What happens to magnesium hydroxide in acidic water?

It dissolves quickly, because acids break it down into magnesium salts.

Final Thoughts

So, is magnesium hydroxide soluble in water?
No—it’s only slightly soluble, and most of it remains suspended.

But that low solubility is exactly why it works so well in antacids, water treatment, flame retardants, and industrial processing.

Now you have a clear, real-world understanding of is magnesium hydroxide soluble in water and how its behavior actually benefits multiple industries.