|MadSci Network: Chemistry|
Hello Patrick, What an excellent experiment! I will answer your question in two parts, a general answer and then some specific details. Freezing process The most fundamental way to explain how freezing points can vary is based on the equilibrium (or balance if you like) of a system (in the cases you describe, you have four systems, 1. is water + vinegar, 2. is water + soap, 3. is water + water and 4. is water + salt). When freezing a solution, there are two forces acting against one another. The first is what is often termed as the 'external pressure', this is the pressure exerted by the surrounding air and this pushes down on to the surface of the solution. The second force is called the 'vapour pressure' and this is caused by the molecules in the solution trying to push upwards (against the external pressure) and to evaporate. All solutions exist in a state of balance or equilibrium, where some of the molecules manage to escape from the solution and form vapour (evaporation), and some of the molecules of the vapour are forced back into solution by the air (condensation). This cycle repeats repeatedly, until the amount evaporating equals the amount condensing. This is referred to as being in a 'dynamic equilibrium'. If however, the conditions of the system are changed, then so will the rate of evaporation and condensation. As the temperature is lowered, so the external pressure increases (and the vapour pressure decreases), and condensation occurs more quickly than evaporation. In addition, as the temperature falls, the amount of energy in the solution decreases, in turn the molecules in the solution slow down. At a certain point (freezing point of the particular solution), the molecules cease to move freely and the solution turns from a liquid to a solid, such as water turning into ice. Solutions that freeze before water To begin with, vinegar itself is actually a mixture of water and a chemical called acetic acid (or ethanoic acid). The molecules of acetic acid form special types of bonds (called hydrogen bonds) with molecules of water. This means that the acetic acid molecules will have several water molecules clustered around them. The acetic acid holds the water, preventing it from evaporating so that the water molecules remain in the solution and reduce the vapour pressure. As outlined above, when the vapour pressure falls, the molecules in the solution are forced together (cease to move freely) and the solution freezes. Something similar happens with the soap and water solution. The two ends of a soap molecule are very different to each other, one draws molecules of water to it (called hydrophilic end, or water-liking) and the other end repulses water away (called hydrophobic end, or water-hating). The hydrophilic end of the soap molecule gathers water around it much as the acetic acid does with molecules of water. In the same way, the soap stops the water from evaporating, reducing the vapour pressure and causing the solution to freeze. Solutions that freeze after water The salt that dissolves into the water actually keeps the water molecules apart. In this respect, the salt operates in the opposite way that the vinegar and soap do. Because the water molecules are much more separated, they have more energy and evaporate much more easily. This increases the vapour pressure. To make the solution freeze, the water molecules must be held together. By taking energy away from the water molecules (by cooling the solution down), causes the vapour pressure to decrease. As these water molecules have more energy than water molecules with no salt present, more energy must be taken from them. Therefore, the temperature must fall below the normal freezing point of water to make the salt solution freeze. I hope this answers your question for you. Chris Wilson, Research Chemist, Cooper Vision, Southampton, England.
Try the links in the MadSci Library for more information on Chemistry.