Why would Al(OH)₃ dissolve when excess NaOH is added?

You have a white solid, aluminum hydroxide, that stubbornly refuses to dissolve in water. Then you add a strong base, sodium hydroxide, and watch as your solid disappears into a clear liquid. This behavior can be confusing and might disrupt a chemical process if you do not anticipate it.

Aluminum hydroxide dissolves in excess sodium hydroxide because it is an amphoteric substance. This means it can react with strong bases. The added NaOH provides hydroxide ions that react with the solid Al(OH)₃ to form a new, soluble complex ion called sodium tetrahydroxoaluminate.

This chemical property is not a strange quirk; it is the absolute foundation of my industry. At my production plant here in Henan, we use this exact reaction every single day on a massive scale. It is the core principle of the Bayer process, which we use to extract pure aluminum hydroxide from raw bauxite ore. Understanding that Al(OH)₃ has this dual personality is the key to mastering its use, whether in refining or in a laboratory. Let’s explore exactly how this works.

Why does aluminium hydroxide dissolve in excess sodium hydroxide?

You believed aluminum hydroxide was a stable, insoluble solid. Watching it vanish into a solution of sodium hydroxide can seem like a magic trick, making you question how stable it really is.

Aluminum hydroxide dissolves in excess NaOH because it acts as a Lewis acid and reacts with the hydroxide ions. This reaction forms a soluble complex ion, [Al(OH)₄]⁻, which pulls the solid Al(OH)₃ precipitate out of its solid form and into the liquid solution.

The key word here is amphoteric. An amphoteric substance¹ can react with both acids and bases. Aluminum hydroxide is a classic example. When you put it in an acid, it acts as a base and neutralizes the acid. When you put it in a strong base like NaOH, it flips its role and acts as an acid. Sodium hydroxide (NaOH) in water provides a large concentration of hydroxide ions² (OH⁻). These ions are chemically aggressive. They attack the solid Al(OH)₃ precipitate³. The aluminum hydroxide molecule accepts one more hydroxide ion, changing from the neutral, insoluble Al(OH)₃ into a negatively charged, highly soluble complex ion⁴, [Al(OH)₄]⁻. The full reaction is: Al(OH)₃(s) + NaOH(aq) → Na[Al(OH)₄](aq). This is not just a textbook curiosity. It’s how we purify our raw material. We use hot, concentrated NaOH to dissolve the aluminum compounds from bauxite, leaving behind impurities like iron oxide⁵ (which is not amphoteric) as a solid red mud.

Why do some precipitates dissolve in excess NaOH?

You follow a procedure and create the solid precipitate you wanted. Then, you add a little more of your reagent, NaOH, and your product suddenly dissolves back into the liquid. This can feel like you have ruined the experiment.

Certain metal hydroxide precipitates dissolve in excess sodium hydroxide because they are amphoteric. These special hydroxides can react like an acid when faced with a very strong base (NaOH), forming a new, soluble complex ion that pulls the solid back into solution.

This phenomenon only happens with a specific group of metals. Most metal hydroxides, like iron(III) hydroxide or magnesium hydroxide, will precipitate³ out and stay as a solid no matter how much excess NaOH you add. But the hydroxides of a few key metals have this special amphoteric property. For a buyer and distributor like you, Mr. Park, knowing which ones do this is very valuable practical knowledge. The most common amphoteric hydroxides are formed from aluminum (Al), zinc (Zn), lead (Pb), and tin (Sn).

Here is a simple table to show the difference:

This selective dissolving is an incredibly powerful tool. It allows chemists and engineers to separate amphoteric metals from non-amphoteric metals in a mixture. This is not a failure of the reaction; it is a feature you can use to achieve higher purity.

Which hydroxides dissolve in excess NaOH?

You need to separate different metals from a single solution. Adding a base seems like a good method to make them all solid, but you discover that some just will not stay precipitated.

The hydroxides that dissolve in excess sodium hydroxide are the amphoteric ones. The most common a chemical buyer will encounter are aluminum hydroxide (Al(OH)₃), zinc hydroxide (Zn(OH)₂), lead(II) hydroxide (Pb(OH)₂), and tin(II) hydroxide (Sn(OH)₂).

Let’s make this very practical. Imagine you have a solution that contains both aluminum ions⁶ (Al³⁺) and iron ions (Fe³⁺). This is very similar to the problem we solve when refining bauxite. You want to get the aluminum by itself.
First, you add a controlled amount of NaOH. Both metals will precipitate³ as solids: white Al(OH)₃ and reddish-brown Fe(OH)₃. They are now mixed together as solids.
Next, you add a large excess of NaOH solution⁷ and stir. A chemical separation⁸ begins to happen. The amphoteric aluminum hydroxide reacts with the excess base and dissolves back into the liquid, forming the colorless, soluble [Al(OH)₄]⁻ ion. The iron hydroxide, which is not amphoteric, does absolutely nothing. It remains a reddish-brown solid.
Finally, you can simply pour the mixture through a filter. The solid iron hydroxide is trapped by the filter paper, while the clear liquid containing all of the dissolved aluminum passes through. You have successfully separated the two metals. This is not just a lab trick; it is a fundamental technique used on an industrial scale across the world.

What happens when excess base is added to NaOH?

This question might sound a bit strange. Sodium hydroxide (NaOH) is itself a strong base. So what happens if you add another base to it? The outcome can be confusing.

This question usually means "what happens when excess NaOH is added to a precipitate that was formed by NaOH?" In this case, if the precipitate is an amphoteric hydroxide like Al(OH)₃, it will dissolve. If it is not amphoteric, nothing further happens.

Let’s clarify the process, because the order matters. Typically, you start with a solution of metal salts⁹, perhaps aluminum chloride (AlCl₃). When you begin to slowly add a NaOH solution, you provide the OH⁻ ions needed to form the solid aluminum hydroxide precipitate. The solution becomes cloudy and a white solid appears. This reaction is: Al³⁺(aq) + 3OH⁻(aq) → Al(OH)₃(s).
At this point, you have used just enough base to form the precipitate. If you continue to add excess NaOH, you trigger the second, amphoteric reaction we have discussed. The solid Al(OH)₃ reacts with the extra OH⁻ ions and dissolves back into the solution: Al(OH)₃(s) + OH⁻(aq) → [Al(OH)₄]⁻(aq).
So, adding a base (NaOH) first creates the precipitate, and then adding an excess of the same base makes it disappear. This is a very different outcome compared to a non-amphoteric metal like magnesium. For magnesium, adding NaOH creates the Mg(OH)₂ precipitate, and adding more NaOH does nothing at all. The solid just stays there.

Conclusion

Aluminum hydroxide dissolves in excess NaOH because it is amphoteric. This key property allows it to form a soluble complex, a fundamental reaction we use for industrial-scale purification, not a chemical flaw.