Chemistry Demonstration in Lab

Making observation is the most basic skills that a chemist should acquire. The most common observation one could see when doing experiment may probably be the color change or the formation of precipitate. However, have you ever seen the reactions with those fabulous visual effects? Can you relate them to the chemical reactions?

The first reaction is Pharaoh's Snake.

By simply igniting the mercury thiocyanate Hg(SCN)2, the white powder burns with blue flame.

Here are the three equations contributing to the observations.

1. Hg(SCN)2 --> HgS + CS2 + C3N4

2. HgS + O2 --> Hg + O2

3. CS2 + 3O2 --> CO2 + 2SO2

The solid formed is porous and brittle. It is with yellow surface, black inner part and the volume is very large. With 0.01 mole (3.16g) of powder, 0.025 mole (0.56L) of gases can be formed. This accounts or the large volume of the product.

*Black salt HgS, yellow solid C3N4, volatile liquid CS2 ignites to give CO2 and SO2.

*Mercury compound and sulfur dioxide is toxic, the reaction should be carried out inside fumehood.

The second reaction is Chemical Volcano.

In the reaction, gaseous nitrogen and water are liberated as products which initiate the effect of volcano eruption with dark green solid chromium(III) oxide produced as ashes.

The decomposition of ammonium dichromate starts at 180oC, after that as the reaction is exothermic, it can self-sustain at 225oC.

This is the equation:

(NH4)2Cr2O7 → Cr2O3 + 4 H2O + N2

The final one is Belousov-Zhabotinsky reaction.

This is a complicated reaction involving 18 steps and many equilibria.

Here are some chemicals involved: sodium bromate, sulfuric acid, sodium bromide, malonic acid, ferroin & water.

The color change of the system is mainly due to the redox reaction of ferriin and ferroin.

Oxydised form (ferriin): blue

Reduced form (ferroin): red

When ferriin is in excess, it reacts with other chemicals in the solution to release bromide ions and turn to forroin slowly. However, bromide inhibits the reaction so less ferriin is formed. Then, most ferriin is reduced so the solution turns to red. Bromide ions are captured again, and ferroin is produced again. The cycle repeats so the color changes back and forth.

There are many typical demonstrations in laboratories that give spectacular visual effects. Next time when you see some special visual effects, you may think of the chemistry behind!