Carbohydrate and Sulfuric Acid

Carbohydrate and Sulfuric Acid

Images of a demonstration by Eric Muller

Introduction

When you add concentrated sulfuric acid to sugar the result is interesting. The sulfuric acid dehydrates the carbohydrate sugar, removing the water and heating it into steam leaving behind a carbon foam.

Eric puts on his safety goggles. Eric pours a few hundred milliliters of granulated sugar into a beaker. He then adds enough water to dampen the sugar.Eric adds concentrated sulfuric acid to the damp sugar mixture. Eric stirs the acid into the sugar.

At first nothing happens, then the sugar turns brown, and then black. As soon as it begins to steam...Eric pulls back, a cylinder of black foam rises from the beaker. The steam smells of cooked sugar and is very irritating since it is rich in hot steaming acid droplets. The cylinder continues to grow and steam. A long black cylinder of carbon foam protrudes from the beaker.

What's Going On?

The sugar, a carbohydrate such as C6H12O8, is mixed with the sulfuric acid, H2SO4.

The sulfuric acid dehydrates the carbohydrate removing water H2O and leaving behind carbon, C. The hot steam creates bubbles in the carbon making a carbon foam. When the foam cools it is stiff. The foam should be handles while wearing gloves since it might contain sulfuric acid.

Moral:. Johnny was a chemist's son,

Johnny ain't no more,

What Johnny thought was H2O,

Was H2SO4.

Objectives

Students will distinguish between a chemical and a physical change; and use skills such as: observation, recording observations, and making inferences from observations.

Motivation for Learning

Materials

• 100 mL of granulated sugar
• Two 100 mL beakers
• Concentrated sulfuric acid

Procedure

1. Fill each beaker ½ full with sugar.
2. Add 40 mL of water to the first beaker and 40 ml of concentrated acid to the second. Stir and let them both sit.
3. Ask the students: "In which beaker do the reactants still have the same properties?" (The reaction should be carried out in a well ventilated area, under a hood or near a window.)

In beaker #1 the sugar and water are simply mixed together illustrating a physical change. The components of the mixture still retain their physical properties and could be separated back out of the mixture using those properties.

However, in the second beaker the sugar and the acid did not merely mix together. A chemical change took place, resulting in products that are completely different than the original sugar and acid. A gas (SO2) is produced along with water vapor forcing a mass of charcoal to expand out of the beaker. The two key indications of the chemical reaction are the color change and the gas produced.