MadSci Network: Chemistry

Re: My science project is growing crystals in household 'chemicals', water, . .

Date: Fri Dec 9 08:00:25 2005
Posted By: Michael Lawliss, Faculty, Science Dept , Clinton Community College
Area of science: Chemistry
ID: 1132725025.Ch

My four liquids are water, white vinegar, sprite and isopropyl alcohol(70% pure). I wanted to know how crystal growth would be affected by different liquids. My hypothesis is that the alcohol would [a]effect it the most.

I have two questions. I boiled all the liquids and then created a super-saturated liquid by adding 7 tablespoons of salt - one tablespoon at a time. I placed a cotton string tied to a paper clip and dropped the paper clip inthe liquid.

Question 1) In the glass jar with the alcohol the liquid seems to have seperated, approximately 70% is a totally clear liquid and is on the top (I am guessing this to be the alcohol) and a cloudy liquid just below the clear and just above the salt at which did not dissolve completely and is now at the bottom of the jar (I am guessing the water in the isopropyl alcohol). There appears to be some small crystalization on the string in the cloudy portion of the liquid. Is my assumption true and if so what is happening?

ANSWER: I would have to assume that what you are seeing is the seperation of the water 30% from the alcohol 70%. The salt you added is polar (that means it has a positive and negative part) and water is polar so they mix well. The alcohol (isopropyl - if you used rubbing alcohol)is just polar enough to mix with water but the salt will not mix with the alcohol. so what you are seeing is the water beeing attracted to the salt more, and is attracted to the alcohol less.

The salt water is more dense and will sink to the bottom. The Salt did not dissolve all the way because it had much less solvent to dissolve in. 2) All liquids aside from the alcohol are growing crystals - the crystals in the vinegar look longer and thinner than the crystals in the water and in the sprite although there are cube shaped crystals on the string too. What kinds of crystals am I growing in each liquid? Are they all salt or is somthing else going on? Where can I find the chemical formulas for vinegar and for sprite?

The sprite crsytals look slightly different too.

ANSWER: If the crystals are long and thin than you are growing crystals of another substance. Salt (NaCl) crystals are very square looking ( all angles are 90 degrees (like the corner of your paper) You can find the contents on the side of the package, Soda is a mixture/solution of a lot of different substances. Unless it was diet sprite the crystals you are most likely seeing are sugar.

In the vinegar they are most likely Sodium Acetate - because vinager is made of 5% Acetic Acid whic is just a Hygrogen hooked onto an acetate (C2H3O2) the Sodium (Na) from the salt took the place of the Hydrogen when the crystal was formed.

The crystals form when the water cools and when it evaporates, because the water can't "hold" them in solution (keep them dissolved).

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