![]() ![]() Some, like the water strider, use it to cruise along the surface of ponds. Water's surface tension has to be dealt with by those organisms that interact with a liquid-gas interface. The most obvious is the weight that the surface can support. ![]() We can measure the strength of surface tension in various ways. This molecular hand-holding leads to water's high melting and boiling points as well as its high surface tension. Even if one is broken, however, the water molecule remains linked to multiple neighbors via H-bond-type electrostatic interactions. In the liquid state, molecules jostle one another and change their H-bond-type electrostatic interaction partners constantly. ![]() To remove a molecule from its neighbors, four H-bond-type electrostatic interactions must be broken, which is relatively easy since they are each rather weak. Because of this arrangement, each water molecule can interact through H-bond-type electrostatic interactions with four neighboring water molecules. These sites of potential H-bond-type electrostatic interactions are arranged in a nearly tetragonal geometry. Each water molecule can take part in four hydrogen bonding interactions with neighboring molecules - it has two partially positive Hs and two partially negative sites on its O. So why is water different? Well, in addition to the presence of polar covalent bonds, we have to consider the molecule's geometry. In particular water stands out as dramatically different from the rest of the molecules, with significantly higher (> 70✬) melting and boiling point than its neighbors. When we look at their melting and boiling temperatures, we see rather immediately how the presence of polar bonds influences these properties. All five compounds have the same number of electrons, ten. In contrast NH 3 (ammonia), H 2O (water), and FH (hydrogen fluoride) have three, two and one polar bonds, respectively, and can take part in one or more intra-molecular H-bond-type electrostatic interactions. For example, neither CH 4 (methane) and Ne (neon) contain polar bonds and cannot form intra-molecular H-bond-type electrostatic interactions. These temperatures mark what are known as phase transitions: solid to liquid and liquid to gas.Īt the macroscopic level, we see the rather dramatic effects of bond polarity on melting and boiling points by comparing molecules of similar size with and without polar bonds and the ability to form H-bonds. The temperature at which a liquid changes to a solid is known as the melting point. While liquids flow and assume the shape of their containers, because neighboring molecules are free to move with respect to one another, solids maintain their shape, and neighboring molecules stay put. As more and more molecules interact, neighbors become permanent - the liquid has been transformed into a solid. ![]() Similarly, starting with a liquid, when we reduce the temperature, the interactions between molecules become longer lasting until such a temperature is reached that the energy transferred through collisions is no longer sufficient to disrupt the interactions between molecules 163. The molecules are said to be a gaseous state and the transition from liquid to gas is the boiling point. If they happen to collide with one another, they do not adhere the bond that might form is not strong enough to resist the kinetic energy delivered by collision with other the molecules. As we increase the temperature of the system, the energetics of collisions are now such that all interactions between neighboring molecules are broken, and the molecules fly away from one another. The molecules are moving with respect to one another, there are interactions between the molecules, but they are transient - the molecules are constantly switching neighbors. Let us start at a temperature at which the sample is liquid. Here we are are considering a pure sample that contains extremely large numbers of the molecule. Two important physical properties of molecules (although this applies primarily to small molecules and not macromolecules) are their melting and boiling points. ![]()
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