Читать книгу Astrobiology - Charles S. Cockell - Страница 59

We might also think about the consequences of the phase diagram for life more directly. Have a look at the melting curve in Figure 3.17 – that is the curve between the solid and liquid regions of water above the triple point.

The line has a negative gradient – it goes from right to left as we follow it upwards in pressure from the triple point. This is unusual. Most materials have a positive gradient (Figure 3.19). This is caused by the fact that solid water (ice) is less dense than liquid water. For most substances, if you take a point in the liquid portion of the diagram near to the melting curve and imagine pressurizing the material (i.e. moving up the y axis), it will transition into a solid. In other words, if you pressurize the liquids of most substances, they get denser and solidify.

Figure 3.19 A simple schematic of a phase diagram for a “typical.” substance. It illustrates the positive melting curve gradient.

However, with water, if we pressurize its solid phase, the density increases, and it turns into liquid water. In other words, now take a point on the water phase diagram in Figure 3.17 in the solid region near the melting curve and move up the y axis. It turns into liquid. The reasons for this behavior lie in the hydrogen-bonded networks of water (review Figure 3.15). In liquid water, the molecules all intertwine and move around in their fluid state. However, when frozen, the molecules move into a more regular aligned network, crudely illustrated in just two dimensions in Figure 3.15. In this state, the molecules are more widely spaced apart than in the liquid in which the molecules are all jumbled together and more closely packed. The consequence is that ice is less dense than liquid water.

The biological consequences of this behavior of water are important because it means that when water freezes, it becomes less dense and thus floats on the surface of a lake. This allows large multicellular organisms to remain active and alive when the outside temperatures have dropped to below a temperature required to freeze water – the floating ice insulates the water below from freezing. Here we can now see how knowledge of the atomic structure of matter allows for an understanding of phase diagrams.