Physical Science

Experiment 4-4

Background Information

We live on a planet mostly covered with water, so water and water containing solutions are the most common solvents for us.  But other solvents exist with different properties.  Some of the oldest alternative solvents are alcohols.  Alcohol is not a single substance, but a large collection of related substances.  Two of the longest known are grain alcohol and wood alcohol.

Grain alcohol is produced in nature by yeast and other primitive organisms in a process called fermentation.  Chemists call grain alcohol ethanol (C₂H₅OH) using a scheme that describes the number of carbon atoms in each molecule: (eth- = 2), the kind of bond between carbons (-an- = single bonds holding atoms together), and the chemical functional group (-ol = the -OH atoms which impart certain properties to the molecule).  Ancient societies found that grain alcohol was useful as beverages.  For most of human history wine, beer and other beverages containing ethanol were the few water containing solutions safe for human consumption.  Only in the last two centuries have we begun to understand why:  All life needs energy.  Humans have a complex metabolism that extracts nearly all available energy from our food, leaving primarily water (H₂O) and carbon dioxide (CO₂) as waste products.  But yeast use a more primitive metabolism which only extracts a small portion of the energy from food, leaving most energy remaining in the ethanol which is one of their waste products.  In high enough concentrations ethanol is poisonous to life and so serves as a disinfectant and preservative.  In lower concentrations ethanol can be used by the human body as a food since our metabolism can extract much of the remaining energy.  Drinking beverages lacking ethanol but often contaminated by disease causing micro-organisms could result in illness and death.  During storage of alcoholic beverage, ethanol kills any contaminating micro-organisms keeping the beverage safe to drink.  Once consumed the ethanol in beverages is diluted and provides a source of food energy.  By the twelfth century it was discovered that ethanol could be concentrated by heating since its boiling point (79°C) was less than that of water.  When concentrated it could burn in lamps, heaters, and recently gasohol.  In the last century humans developed other means of sanitizing and preserving beverages,  Now society is often more concerned about the hazards caused by ethanol's impairment our judgement and delayed muscle responses in emergencies due to nerve impairment.  The value of ethanol has changed from the preservative of our beverages, to its use as a fuel for transportation.

Wood alcohol was first made from wood.  Ancient Syrians found that heating wood sealed away from the air resulted in a variety of tars and liquids.  One of the liquids, now called methanol by chemists (CH₃OH) (meth- = 1 carbon), was a useful solvent and fuel.  While methanol has the same density as ethanol and acts similarly as a solvent and fuel, it is highly poisonous even in low concentrations, causing hundreds of deaths annually for people who are unaware of the differences.

Rubbing alcohol is another poisonous solvent often used as a disinfectant.  Chemists called it 2-propanol or sometimes isopropanol (C₃H₇OH) (prop- = 3 carbons).  2-propanol may be added to ethanol to make it undrinkable.  Ethanol with such a poisonous additive is labeled denatured alcohol.

There are many other kinds of alcohol with more carbons and a wider variation of properties.  There are also many other solvents, most of which are organic which means they contain carbon.

A word of caution:  For many people with little chemical training, the following experiment seems like a nearly trivial investigation.  Perhaps that is because we have all been mixing substances since we were little children.  But our very familiarity with the process may distract many from a careful study.  Yet chemists know that a solubility study like this provides one of the most useful clues about a substance's chemical bonding.

Experiment

Purpose:  This experiment compares as solvents water, available alcohols ad perhaps other organic compounds.

• ethanol (denatured alcohol)
• isopropanol (available as rubbing alcohol in a 50% solution with water)
• methanol (available as lacquer thinner at paint and hardware stores)
• water
• table sugar (sucrose)
• table salt (sodium chloride: NaCl)
• cooking oil (mixtures of triglycerides extracted from plants)
• additional or alternate substances: candle wax, moth flakes, Vaseline (petroleum jelly), nail polish remover, paint thinner, etc.
• one or more container such as a test tube; it can be cleaned and reused.
• stir stick

Procedure:

1. Make a table to record the test of each solvent with each solute.  Also mix each solvent with each of the other solvents.
2. The critical aspect of this investigation requires using a tiny amount of solute (a tiny fraction of a cm³, a pinch).  This is NOT an experiment in which more is better.  Almost always using too much solute will result in false insoluble results.
3. Add a much larger volume of solvent (perhaps 20 cm³).
4. Attempt to dissolve by shaking or stirring for a minute or longer.
5. Watch for any sign of the volume of the solute decreasing.  If two liquids are mixed, the absence of a meniscus between the layers is a sign of dissolving.  Be cautions of the small amount of solute going unnoticed by sticking to the sides of the container.
6. Classify the results of each test as soluble, slightly soluble, or insoluble.

Record your results in your science journal.  Write a Formal Report if you need to earn credit.

Discussion of Observations and Conclusion:  Are there patterns where several substances behave similarly?  If there are patterns, how many different general types of solvents and solutes are there?  Later when you have more knowledge of chemical bonding, you may wish to revisit your careful observations to determine the bonding.