In an experiment we did in class ethanol was poured into a test tube and we noticed that the ethanol did not collect and form a dome at the opening of the tube in the same way that water had. I know why water collected at the opening, but i'm not sure why the ethanol didn't collect as much. Our teacher was going on about the amount of bonds between ethanol molecules but i couldn't really catch what he was trying to say. could anyone help clear this up for me? Thanks!
The "dome" is caused by the surface tension of the compound as well as the "wetting" power of the liquid against the glass test tube. The "dome" really is an upside down dome, is it not. That is the meniscus for water. The surface tension (which you can read about in your text or notes) is different for different molecules. Here are some good picture of a meniscus.
http://www.google.com/images?q=picture+liquid+meniscus&oe=utf-8&rls=org.mozilla:en-US:official&client=firefox-a&um=1&ie=UTF-8&source=univ&sa=X&ei=gkqJTZTdIdLpgQevja3ZDQ&ved=0CDEQsAQ&biw=787&bih=398
Sure, I can help clarify this for you! The reason why ethanol does not form a dome at the opening of the test tube like water is because of the intermolecular forces between the molecules of ethanol.
Water molecules are held together by strong hydrogen bonds, which are relatively stronger than the intermolecular forces present in ethanol. These hydrogen bonds between water molecules cause them to stick together more strongly, creating surface tension and allowing water to form a dome-shaped meniscus at the opening of the test tube.
On the other hand, ethanol molecules are held together by weaker intermolecular forces, mainly van der Waals forces or London dispersion forces. These forces are not as strong as hydrogen bonds, hence ethanol molecules do not stick together as strongly as water molecules. As a result, the surface tension of ethanol is lower, and it does not form a dome-shaped meniscus at the opening of the test tube to the same extent as water.
To recap, the different intermolecular forces between water and ethanol molecules contribute to the differences in their surface tension, which in turn affects how much they collect and form a dome at the opening of a test tube.