An amphoteric substance in chemistry is a molecule or ion that reacts as both an acid and a base. Amphoteric oxides are compounds that react with both acids and bases to form salts and water. Numerous metals, including beryllium, zinc, tin, lead, and aluminium, can create amphoteric oxides or hydroxides. An example of an amphoteric oxide is Al2O3. The oxidation states of the oxide highly determine amphoterism.

In this article, we will learn about amphoteric oxides, their definition, examples, identification, properties, and applications along with FAQs.

Amphoteric Oxides

Amphoterism is the ability of a substance to act like both an acid and a base, depending on what it reacts with. In other words, an amphoteric substance can “switch roles” depending on the situation.

Usually, metal oxides are basic and non metals oxides are acidic. But some metal or semi-metal oxides can be amphoteric, meaning they react with both acids and bases. There are also some non-metal oxides that are neutral, meaning they don’t show acidic or basic behaviour. 

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For example:

• When an amphoteric oxide reacts with an acid, it acts as a base and forms salt and water.
• When it reacts with a base, it behaves like an acid and again forms salt and water.

This dual nature is what makes amphoteric oxides unique. Their pH can also change based on the reaction:

• With acids→ the solution becomes basic (pH> 7)
• With bases→ the solution becomes acidic (pH< 7)

Now, there’s also something called amphiprotic substances. There is a special type of amphoteric compounds that can either donate or accept a proton (H+). According to the Bronsted-Lowry acid-base theory, this is what defines an acid or base.

Some examples of amphiprotic substance include:

Water

Amino acids (because they have both NH2 and COOH groups)

Ions like hydrogen carbonate (HCO3-), dihydrogen phosphate (H2PO4-), and hydrogen sulfate (HSO4-).

These substances can behave in flexible ways in chemical reactions, making them important in both biology and chemistry.

An acid and a base both can be neutralized by an amphoteric oxide. Therefore, it is necessary to study the reactions of a compound with an acid like HCl and a base like NaOH in order to determine whether it is amphoteric. The given material must be determined if it is an oxide or not after confirming that it is amphoteric. There are tests for different anions, such as sulphate, nitrate, nitrite, chloride, etc. If every test turns up negative, the reported substance is an oxide.

The element can be heated in oxygen to produce any of the oxides. The hydrated oxides are produced when water solutions of metal trihalides react with hydroxide. As one moves down the group, the metallic character of the elements involved causes a change from acidic oxides to amphoteric to basic.

Amphoteric Oxides in Periodic Table

As we move across the periodic table, the nature of oxides (whether they’re acidic, basic, or amphoteric) changes based on the element’s position. These changes are influenced by factors like the oxidation state, atomic size, and whether the element is a metal or non-metal.

Trends in a Periodic Table(Left to Right)

• Basic oxides are usually formed by metals on the left side.
• As we move to the right, elements become more non-metallic, and their oxides become more acidic.

• Amphoteric Oxides lie in the middle–showing both acidic and basic properties.

• Sodium and magnesium oxides are basic.
• Aluminium oxide (Al2O3) is amphoteric
• The rest become increasingly acidic as we move to the right

Trends in a Group (Top to Bottom)

Going down a group, the size of the metal atom increases, and so does the basic nature of its oxides.
For example, in group 15:

Effect of Oxidation State on Acidity

• Higher oxidation states usually make metal oxides more acidic.
• Example with manganese oxides:

MnO<Mn2O3<Mn2O7

From basic to strongly acidic as the oxidation state of manganese increases.

Properties of Amphoteric Oxides

Amphoteric Oxides have some unique characteristics that set them apart. Let’s break them down into two main categories: Physical and Chemical properties.

Physical properties:

High melting and boiling points-Amphoteric Oxides are usually very stable and require a lot of heat to melt or boil.

Strong structures- They often have large covalent structures, which makes them tough and hard to break down.

Not easily soluble- Most of these oxides don’t dissolve in water.

Chemical properties:

The most crucial property of amphoteric oxides is a reaction with both acid and base.

Lead Oxide(PbO)

Reaction with an acid- PbO+2HCl→PbCl2+H2O

Reaction with a base- PbO+2NaOH+H2O→Na2(Pb(OH4))

Aluminum oxide (Al2O3)

Reaction with an acid- Al2O3+ 6HCl→2AlCl3+3H2O 

Reaction with an base-PbO+ 2NaOH+3H2)→2Na(Al(OH)4)

Zinc Oxide (ZnO)

Reaction with acid-ZnO+H2SO4→ZnSO4+H2O

Reaction with base-ZnO+2NaOH→ZnO+2NaOH+H2O→Na2(Zn(OH)4)

Stannous Oxide(SnO)

Reaction with acid-SnO+2HCl⇌SnCl2+H2O

Reaction with base-SnO+4NaOH+H2O→Na4(Sn(OH)6)

Amphoteric Oxides Applications

The applications of amphoteric oxides are quite wide due to their ability to react with both acids and bases. Some of the most common uses of amphoteric oxides are as follows:

• The electrical resistance, refractive index, and X-ray absorption of glass are all increased by the addition of PbO. It also lessens the viscosity of glass at the same time. PbO is therefore crucial for the glass industry. PbO is also utilised in the ceramics industry to create magnetically and electrically inert ceramic products.
• Zinc oxide (ZnO) can be used to improve the performance of many different goods and materials, including plastics, rubber, glass, ceramics, lubricants (including cement), ointments (including ointments and creams), paints (including sealants), adhesives (including foods), pigments (including fire retardants), batteries (including batteries), and first-aid tapes. Additionally, it is employed in the production of permanent cells, including dry cells.
• Ampholytes are amphoteric compounds that include both acidic and basic groups. Over a particular pH range, they are generally discovered as zwitterions. In isoelectric focusing, aminolytes can be utilized to keep the pH gradient stable.
• The manufacturing process for aluminium additionally makes use of aluminium oxide Al2O3. This oxide is also a popular filler in polymers since it is chemically white and inert. In addition to being a common ingredient in sunscreen, it can also be discovered in cosmetics including blush, lipstick, and nail polish. Occasionally, different types of glass are made with aluminium oxide as a component. It is used as a catalyst in both the Claus process and the dehydration of alcohol to alkenes.

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