Biology,  Chemistry

Water

covalently bonded diagram of water

Water is a made up of two hydrogen atoms and one oxygen atom as shown in the water molecule below. Water can act as a buffer and has a high specific heat capacity.

Why Water Acts as a buffer?

A buffer is a solution which can withstand a change in pH. Water can do this as it breaks down into both an acid and a base. When water dissociates it forms an H+ and an OH-. H+ is acidic and OH- is basic. Being both equally acidic and basic water has the ability to act as a buffer to pH change.

Water also acts as a buffer in another way. With temperature. As mentioned before, the water has a high specific heat capacity. Meaning, it requires a high amount (4182 J/Kg degrees celsius) to change one degree of 1Kg of water.

Hydrogen bonding

diagram that shows hydrogen bonding in water by dahses between lone pairs and a hydrogen

Water has a high specific heat capacity because of hydrogen bonding .

Hydrogen bonding is a type of intermolecular force. Hydrogen bonding only occurs with atoms nitrogen, fluorine and oxygen as they are very electronegative ( likelihood of an atom to draw bonding electrons towards itself). Water is a polar molecule meaning that two ends of the molecule have a different charge. In H2O the oxygen is more electronegative than hydrogen so oxygen will be delta (slightly) negative and the hydrogen will be delta positive. The lone pair of the oxygen will be attracted to the delta positive hydrogen from another water molecule. This attraction is shown by the dashes in the image to the left. Hydrogen bonding is the strongest intermolecular.

Cohesion

Water molecules have a tendancy to stick together this is called cohesion. The water molecules have cohesion force because they form hydrogen bonds. Cohesion force allows water to travel up xylem vessels from the roots to the leaves

Signifciance of Water for organisms

Water is used for many reactions in an organism; for example, photosynthesis. Water is also used in hydrolysis and condensation reactions. Hydrolysis is a reaction that uses water to break apart a molecule. The example below shows a polymer being hydrolysed.

Codensation reactions are the opposite. When two molecules bond together, water will be formed as a waste procuct.

Condensation reactions are useful in the body as they form lipids and carbohydrates and many other macromolecules (large molecules). Hydrolysis does the opposing. The main hydrolysis reactions you will come across will be digestive ones. For example the digestion of lipids in the body. (Will be covered at a future date).

Why ice floats?

represents ice floating on water

So we have all seen ice floating but why does it float? It all comes down to hydrogen bonding. In liquid water hydrogen bonds are constantly forming and reforming . However, in the solid water (ice) the hydrogen bonds do not break. This allows the hydrogen bonds to stay together and thus making the ice less dense than liquid water. If something is less dense than another substance it will float/rise above it. Another example of this ( and a pacticle you can try at home) is oil and water. Once again water is more dense than oil so the oil floats on top of the water.

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