MadSci Network: Chemistry

Re: What liquid is heavier than water

Date: Wed Mar 25 18:41:28 1998
Posted By: Ken Johnsen, MadSci Admin
Area of science: Chemistry
ID: 890865842.Ch


Let's start with your question to the MAD Scientist and get some terminology 

Is a gallon of cooking oil "heavier" than a cup of water? Of course it is; a 
gallon of oil weighs about 8 pounds while a cup of water only weighs about a 
quarter of a pound. The term you need to use is "denser", i.e. the weight per 
unit of volume [pounds per gallon, grams per cubic centimeter etc] On that 
basis oil is less dense than water and will float on water.

Rubbing alcohol is also less dense than water, but will not float on it because 
it is water soluble and mixes with the water rather than floating.

So, now you're looking for "either a liquid denser than water or less dense 
than oil, and which is soluble in neither water nor oil" OK?

That narrows down the field considerably. 

There are a number of candidate liquids that I will not reccommend because of 
safety, flammibility or toxicity reasons.

I'm going to suggest trichloroethylene or perchloroethylene for your third 
liquid. It is denser than water and oil and is not soluble in either.

You probably know these better as 'dry cleaning solvent'. The 'solvent' that 
dry cleaners actually ise is based on these liquids but also contain a variety 
of additives, especially detergents [to help lift the dirt off the garment].

A detergent-containing dry cleaning solvent will foul up your demonstration 
because it will emulsify your oil and water phases, making something that is 
not quite predictable, but certainly won't be be three seperate layers.

Pay a visit to your local cleaner and explain to the manager [not a clerk] that 
you'd like a small sample of his pure solvent. Show him this letter if it will 
help. The manager might be reluctant to help you, because these solvents are 
regulated by the EPA and he must meet some pretty stringent emission standards 
[i.e., he has to account for all the solvent he uses in his facility].If this 
comes up, promise to return the solvent to him when your project is finished.

In any case, do not simply throw the experiment down the drain when you're done 
because that, too, would be illegal! Your school's chemistry lab probably has a 
hazardous waste disposal container. Use it.

Ken Johnsen

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