Which is more acidic, Al(OH)₃ or Be(OH)₂?

It seems contradictory when a chemical we sell as an antacid, a base, is discussed in terms of its acidity. This confusion can lead to mistakes in advanced applications.

Aluminum hydroxide, Al(OH)₃, is more acidic than beryllium hydroxide, Be(OH)₂. Both compounds are amphoteric, but the higher charge of the aluminum ion (Al³⁺) polarizes the O-H bond more strongly, making it easier to release a proton and thus act as an acid.

In my line of work, we are laser-focused on the basic properties of aluminum hydroxide. It has to be pure and effective at neutralizing acid¹ for our pharmaceutical clients. But sometimes, a particularly sharp buyer like Mr. Park from Korea will ask a question like this. It shows he has a deep chemical understanding and is thinking about our product’s behavior in more complex systems. Answering these questions correctly is key to building trust and showing that we are true experts in our material, not just salespeople.

Is Al(OH)₃ basic or acidic?

You know aluminum hydroxide is the key ingredient in many antacids, so it must be a base. But then you hear chemists talk about its acidic properties, which sounds like a complete contradiction.

Aluminum hydroxide is amphoteric, meaning it can act as both a base and an acid. It behaves as a base when reacting with acids, but it behaves as an acid when reacting with strong bases.

This dual personality is what makes it so useful. When you take it as an antacid, it performs its role as a base perfectly. It reacts with the strong hydrochloric acid² (HCl) in your stomach to form harmless aluminum chloride and water. But if you were to put our aluminum hydroxide powder in a strong basic solution, like sodium hydroxide³ (NaOH), it would play the opposite role. It would react with the strong base, and in that reaction, it is acting as an acid. This happens because the central aluminum atom is electron-deficient and can accept an electron pair from the hydroxide ion of the strong base. This amphoteric⁴ nature is why Al(OH)₃ is not only a pharmaceutical ingredient but also a key raw material in chemical synthesis⁵ and catalyst production⁶.

The Two Faces of Aluminum Hydroxide

• As a Base: Al(OH)₃ + 3HCl (acid) → AlCl₃ + 3H₂O
• As an Acid: Al(OH)₃ + NaOH (strong base) → Na[Al(OH)₄] (sodium aluminate)

Is Be(OH)₂ acidic?

You may not be familiar with beryllium hydroxide, as it is less common. It is easy to assume it would just act as a simple base, like many other metal hydroxides on the periodic table.

Yes, in practice, beryllium hydroxide is considered acidic because it is also amphoteric. Like aluminum hydroxide, it reacts with both acids and strong bases. Its ability to react with strong bases means it exhibits acidic character.

The reason for this special behavior lies with the beryllium ion, Be²⁺. Beryllium is a very small atom. Packing a +2 charge into such a tiny space creates a very high "charge density⁷." Think of it as a lot of positive power concentrated in a small area. This strong positive field pulls on the electrons in the oxygen-hydrogen (O-H) bonds of the hydroxide groups. This pulling effect weakens the O-H bond. A weaker O-H bond means that under the right conditions, like in the presence of a strong base, the molecule can more easily release a hydrogen proton (H⁺). The chemical definition of an acid is a substance that can donate a proton. Because Be(OH)₂ can do this, it is considered amphoteric with significant acidic character.

Why is Al(OH)₃ more acidic than Be(OH)₂?

We have established that both hydroxides are amphoteric. So when we put them head-to-head, which one has the stronger acidic personality, and what is the scientific reason for the difference?

Al(OH)₃ is more acidic because the aluminum ion (Al³⁺) has a higher positive charge (+3) than the beryllium ion (Be²⁺). This greater charge creates a stronger pull on the O-H bonds, making them weaker and more likely to release a proton.

To truly understand this, we need to compare the central metal ions directly. The key factor is charge density, which we discussed earlier. It is a combination of the ion’s charge and its size (ionic radius). A higher charge packed into a smaller space leads to a higher charge density and stronger acidic character. The Al³⁺ ion has a larger charge (+3) than the Be²⁺ ion (+2). At the same time, their ionic radii are quite similar. This means the Al³⁺ ion has a significantly higher charge density. This "super-charged" state allows it to polarize, or pull electrons from, the surrounding O-H bonds much more effectively than Be²⁺ can. This makes the hydrogen atoms in Al(OH)₃ easier to remove, making it the more acidic of the two compounds.

Ion Properties and Acidity

Conclusion

Both compounds are amphoteric, but aluminum hydroxide is more acidic than beryllium hydroxide. This is due to the higher +3 charge of the aluminum ion, which weakens its O-H bonds more effectively.