When does a substance have a definite shape?
1 answer:
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Answer:
A non-polar liquid.
Explanation:
Whether a substance dissolves quickly or not depends on how strongly the molecules (or atoms of an element) of a substance are attracted to one another. These interactions between atoms and/or molecules are called intermolecular forces, or IMFs for short. There are several different ones, and these are distinguished from <em>intra</em>molecular forces which are the bonds holding atoms in the molecule together. Attached is a nice little summary of these forces to consider. Our decision lies within the fact that we must pick the substance that experiences the strongest IMF (the one with the most energy). As it turns out, a dipole in a molecule confers some charge distribution on the molecule which makes slightly positive and negative ends. These can attract each other, and it's called dipole-dipole interactions. It can technically happen in a mixture, but let's assume we're dealing with pure substances. Dipoles can only form in polar compounds however, so a non-polar liquid (which is composed of non-polar molecules), will lack these dipoles and therefore cannot form dipole-dipole interactions between the molecules. This results in only having something called dispersion forces (which really every molecule attraction has - so this is the only one). It is very weak, and since the attraction between these molecules is weak, they will tend to come apart, and evaporate. You can think of the IMFs like glue, and a weak glue will not hold the molecules together well, and they will evaporate away.
On the other hand, polar (from dipole interactions) compounds can have general dipole-dipole interactions or hydrogen-bonding interactions (which is a special type of dipole-dipole interaction). H-bonding requires a Hydrogen bonded to either a Nitrogen, Oxygen, or Fluorine to do this. The main thing, is the non-polar ones don't have a dipole, and so they can't form a good intermolecular bond and evaporate quickly.
Water can H-bond, which is why it takes so long to dry and for it to evaporate in general. Nail polish, which is really a solution of acetone, has considerably weaker dipole-dipole bonds (compared to H-bonds), and evaporates quicker than water. Hope this helps!
Note: Figure taken from Chemistry: The Molecular Nature of Matter and Change 8th edition.
