Amphoteric Aluminium

Materials: ★★☆ Available in most school laboratories or specialist stores
Difficulty: ★★☆ Can be done by science teachers
Safety: ★★☆ Some safety precautions required to perform safely

Categories: Acids and Bases, Chemical Reactions

Alternative titles:

Summary

This set of experiments demonstrates the amphoteric nature of aluminium. Aluminium is converted to alum crystals, which are then analyzed and tested in reactions with acids and bases to show how aluminium compounds can behave both as acids and bases.

Procedure

Start with aluminum hydroxide (Al(OH)₃).
Heat it gently in a furnace (about 250 °C for 30 minutes) or over a Bunsen burner (450 °C for a few minutes) to form Al₂O₃.
Place a small amount of the oxide into dilute hydrochloric acid (with methyl orange) and note if it reacts.
Place another small amount into dilute sodium hydroxide (with phenolphthalein) and note if it reacts.

Links

Amphoteric Hydroxides - Chemistry and Biochemistry Demo lab at OSU:

📄 Obtaining and Investigating Amphoteric Properties of Aluminum Oxide in a Hands-On Laboratory Experiment for High School Students - Journal of Chemical Education: https://pendidikankimia.walisongo.ac.id/wp-content/uploads/2018/10/22-26.pdf

Variations

Try similar acid–base tests on other oxides from the same period (e.g., Na₂O, MgO, SiO₂).

Safety Precautions

Handle hot crucibles or dishes with tongs and allow them to cool before touching.
Avoid breathing in powders; keep the area well ventilated.
Use only dilute acids and bases for testing, but rinse any spills with water.

Questions to Consider

What does it mean for a substance to be amphoteric? (It reacts with both acids and bases.)
What changes happen when aluminum hydroxide is heated? (It loses water and becomes Al₂O₃.)
How does this experiment connect to the periodic table trend for oxides across Period 3? (Oxides change from basic to amphoteric to acidic across the period.)