In a very cold winter fish in garden ponds owe their lives to hydrogen bonding why?

Answer 1

In very cold winter conditions, the water in garden ponds can freeze, forming ice on the surface. However, due to the unique properties of water and its ability to form hydrogen bonds, the ice that forms on the surface of the pond is less dense than liquid water. This allows the ice to float on the surface, insulating the water below and preventing it from freezing completely. This insulation helps to maintain a stable temperature in the pond, ensuring that the fish are able to survive in the cold winter conditions.

Answer 2

Water is a unique molecule. Water is far more viscous than it should be, it has a far higher surface tension than it should have and it is especially unique because solid water is actually larger than liquid water.

The fact that water expands when it freezes is significant because it means that ice is less dense than water. If you drop ice cubes into water they float whereas if you drop a block of iron into liquid iron it will sink.

Ice is less dense than liquid water because there are less particles per unit area. This change in density is usually in the other direction and would normally be caused by an increase in kinetic energy for the particles. Here it is something different.

Water has a high viscosity and surface tension due to its molecular structure. Water molecules are covalently bonded and look like Mickey Mouse.

Covalent bonds are bonds where the outer electrons are shared. Hydrogen only has one electron and when the two hydrogen atoms bond to the oxygen atom (making the two ears for our Mickey Mouse model) all the negative charge that the atom has becomes concentrated around the oxygen atom (Mickey's face). This means that water is polar.

A polar molecule has a charge. Water has two weak positive charges around the hydrogen atoms and a weak negative charge around the oxygen atom.

Normally covalently bonded atoms have strong INTERmolecular bonding but very weak INTRAmolecular bonds. Think about chains. You have strong bonds between links in a chain but two different lengths of chain have no real attraction between them. Water is a simple 3 link piece of chain (whereas diamond is a massive chain-link fence by comparison). If you fill a bucket up with 1, 2 or 3 link pieces of chain and throw it up in the air they'll all fly out. If you plunge your hand in it'll go in easily. If you have longer chains or more complex ones they won't fly up and they'll resist your hand. Water sits somewhere in between with a weak force of attraction between different molecules (like little magnets for want of a better model).

So in our model water chains don't fly in the air as easily as they should (so water is a liquid at room temperature rather than being a gas). Water resists our hand being plunged into it (water has high viscosity and surface tension).

These little bits of attraction are all well and good when the molecules have enough kinetic energy to move around. As water cools the molecules lose kinetic energy and the other part of our "magnet" model comes into play... Just like with the poles of a magnet like charges repel one another and as water solidifies the inability to turn around and match up opposite charges combined with the sheer proximity of like charges due to increasing density actually PUSHES APART the water molecules leading to water being less dense as a solid than as a liquid.

Why is this important for fish?

Water freezes from the top down so unless things get REALLY cold there will always be liquid water under them. If ice was, as it should be, more dense than water then ponds would freeze from the bottom up trapping fish in ice.

The unique properties of water and other molecules containing hydrogen bonded in this way lead to many, many other reasons why life is possible including the structures of Deoxyribonucleic acid and the tertiary and quaternary structures of proteins.