3.1.7 Water

Question 1

How is water as a solvent used ?

Answer 1

Water molecules are charged, with oxygen being slightly negative and hydrogen slightly positive.
These opposite charges attract, forming hydrogen bonds that loosely bind water molecules together.
Water’s charge makes it a very good solvent, facilitating most life’s chemical reactions in aqueous solutions.
Charged or polar molecules like salts, sugars, and amino acids dissolve readily in water and are hydrophilic (“water loving”).
Uncharged or nonpolar molecules like lipids do not dissolve well in water and are hydrophobic (“water hating”).

Question 2

How does water resist changes in temperature ?

Answer 2

‒ Hydrogen bonds between water molecules can absorb a lot of energy
‒ This means that water has a high specific heat capacity - it takes a lot of energy
to heat it up.
‒ Water has a specific heat capacity of 4.2 J g-1 °C-1, which means that it takes 4.2
joules of energy to heat 1 g of water by 1°C.
‒ This unusually high heat capacity means that water does not change temperature
very easily.
‒ The constant temperature is useful for living organisms because it means water
doesn’t experience rapid temperature changes.
‒ Water is a good habitat as the temperature underwater is likely to be more stable
to on land.
‒ Water inside organisms is also fairly stable - which helps them to maintain a
constant internal body temperature.

Question 3

How is water used for strong cohesion between water molecules ?

Answer 3

‒ Cohesion is the tendency of molecules within a substance to “stick together”
‒ Water molecules are very cohesive because they are polar hydrogen bonds
‒ A strong cohesion helps water to flow, which is useful for transporting substances
- for example the xylem (tube-like transport cells) in plants relies on water being
pulled up
‒ Cohesion also gives the water a high surface tension, allowing small organisms
like pond skaters and other insects to “walk” on the surface of a pond

Question 4

How is water used for its density and freezing properties ?

Answer 4

‒ Water is unusual because its solid form is less dense than its liquid form. Below
4°c the density of water starts to decrease.
‒ Ice floats on water and insulates the water below it - reducing the chances of
large bodies of water completely freezing and increasing the chances of life.
‒ Changes in density of water with temperature set up ocean currents, which
circulate nutrients.

Question 5

How is water used in photosynthesis ?

Answer 5

‒ Water is used in photosynthesis, so it is responsible for the production of glucose.
This in turn is used in the synthesis of many chemicals.