Burning Money

Title: Burning Money

Author(s): Patricia Gonzalez

Principles Illustrated:

*process of combustion

*the flammability of alcohol

*high specific heat of water versus alcohol

*the special qualities of the material used to make currency

*Solids can dissolve in liquids; solid, liquids or gases can combine to form solutions

*The two liquids in a solution can be separated because they have different properties

Standards:

Questioning Script

Prior knowledge & experience:

Root question:

Target response:

A combustion reaction occurs between alcohol and oxygen, producing heat and light (energy) and carbon dioxide and water.

C₂H₅OH + 4 O₂ -> 2 CO₂ + 3 H₂O + energy

When the bill is soaked in an alcohol-water solution, the alcohol has a high vapor pressure and is mainly on the outside of the material (a bill is more like fabric than paper, which is nice, if you've ever accidentally washed one). When the bill is lit, the alcohol is what actually burns. The temperature at which the alcohol burns is not high enough to evaporate the water, which has a high specific heat, so the bill remains wet and isn't able to catch fire on its own. After the alcohol has burned, the flame goes out, leaving a slightly damp dollar bill.

In this demonstration, the liquid used is a 50/50 mixture of isopropyl alcohol and water. Methanol or ethanol can be substituted for the isopropyl alcohol. Some salt in the solution will help to make the flame more visible. Other combustible materials, such as paper, can also be used in place of the 20$ bill. A dollar bill provided by someone in the audience is especially effective. Note carefully the purity of the alcohol before mixing. Rubbing alcohol as sold in drug stores is often 30% or more water. One could repeat the demonstration with varying mixtures of alcohol and water. Too much water will prevent the alcohol from burning, and too little water will allow the cloth or paper to char.

This demonstration illustrates the variation in the temperature required to support combustion in different substances. The alcohol burns at a temperature below the kindling temperature of the cotton. In addition, the heating and vaporization of the water removes heat and prevents the cloth from burning.

The water in the alcohol-water mixture evaporates as the alcohol burns, keeping the temperature of the paper below its ignition temperature (approximately 230°C).

The flame from the paper soaked in alcohol alone should be visible but the flame from a burning alcohol-water mixture is often difficult to see. This is why the sodium chloride is added, to give an orange-yellow colour to the flame. The demonstration looks even more impressive in subdued lighting.

Also, dissolving salt in the water will keep the water and alcohol in separate layers. This will allow the bill to be coated with the water and a layer of alcohol. The alcohol will burn, but the bill will not.

Alcohol is a nonpolar solvent. Common salt will not dissociate in alcohol.

The process of dissolving, called dissolution, is relatively straightforward for covalent substances such as ethanol. When ethanol dissolves in water, the ethanol molecules remain intact but form new hydrogen bonds with the water. When, however, an ionic compounds such as sodium chloride (NaCl) dissolves in water, the sodium chloride lattice dissociates into separate ions which are solvated (wrapped) with a coating of water molecules. Nonetheless, NaCl is said to dissolve in water, because evaporation of the solvent returns crystalline NaCl.

The solubility of one substance dissolving in another is determined by the balance of intermolecular forces between the solvent and solute and the entropy change that accompanies the solvation. Factors such as temperature and pressure will alter this balance, thus changing the solubility. Solubility may also strongly depend on the presence of other species dissolved in the solvent, for example, complex (chemistry) forming anions (ligands) in liquids. Solubility will also depend on the excess (or deficiency) of a common ion (common-ion effect). To a lesser extent, solubility will depend on the ionic strength of liquid solutions.

Common Misconceptions:

Photographs and Movies

References

http://www.sciencemuseum.org.uk

http://chemistry.about.com/od/demonstrationsexperiments/ss/burnmoney.htm

http://www.instructables.com/id/NonBurning-Money/

http://www.practicalchemistry.org/experiments/money-to-burn,49,EX.html