2.2 Water

Question 1

What is special about water that allows it to have all of its special properties?

Answer 1

Water molecules are polar. A water molecule is formed by two covalent bonds between an oxygen molecule and a hydrogen molecule. The nucleus of an oxygen atom is more electronegative and therefore is more attractive to the electrons, the oxygen will therefore get a bigger share of the electron and be partially negative and the hydrogens will be partially positive. Then the partially positive (dipole) part of one water molecule can form dipole dipole bonds or hydrogen bonds with the partially negative part of another.
Although hydrogen bonds are weak intermolecular forces, water molecules are small so there are many of them per unit volume and therefore many bonds too. This gives water its unique properties.

Question 2

What is cohesion?

Answer 2

Cohesion refers to the binding together of two molecules of the same type. Water molecules are cohesive, this is very useful for water transport in plants, water is sucked through the xylem at low pressure. This could only work if the water molecules were not separated by the suction.

Question 3

What is adhesion?

Answer 3

Adhesion refers to waters ability to bind to other molecules of different types. Water can form hydrogen bonds to other polar molecules. This can be seen in capillary tubing and is useful in the transpiration stream als.

Question 4

What are the thermal properties of water?

Answer 4

• High specific heat capacity - Hydrogen bonds restrict the motion of water molecules and increases the temperature of water required to break water molecules apart because extra attractive forces have to be broken. Because of this water remains a fairly constant temperature or is very very slow to heat up or cool down. This means that it is good for aquatic organisms.
• High latent heat of vaporisation - when a molecule evaporates it separates from other molecules in a liquid and becomes a vapour molecule. The heat needed to do this is called the latent heat of vaporisation. This means that water has a cooling effect because considerable amounts of energy are needed to evaporate it. This makes it a good coolant.
• High boiling point

Question 5

What are the properties of water?

Answer 5

• High latent heat of vaporisation
• High boiling point
• Coolant
• Adhesion
• Cohesion
• Solvent

Question 6

Explain water as a solvent?

Answer 6

Water has important solvent properties. The polar nature of the water molecule means that it forms shells around charged and polar molecules preventing them from clumping together and keeping them in solution.

Question 7

What are hydrophilic substances?

Answer 7

Polar substances, they will dissolve in water. Or ionic substances.

Question 8

What substances will not dissolve in water?

Answer 8

Non-polar or neutral charges. This is because bonds form between the water molecules but not between the non-polar molecules, so when random movements bring them together they stay together pushing the non-polar molecules all together.

Question 9

Compare methane and water?

Answer 9

Methane is non polar and so has a lower specific heat capacity, lower boiling and melting point etc.

Question 10

Give an example of water as a coolant?

Answer 10

Sweat - Sweat is created by glands in the skin, there it spreads out and because of its high latent heat of vaporisation takes a lot of heat energy to evaporate, this energy is absorbed and taken away therefore cooling the skin. Sweat secretion is controlled by the hypothalamus.
Also panting in dogs makes water evaporate from their mouths.

Question 11

How is everything transported in the blood plasma?

Answer 11

• sodium chloride has Na+ and Cl- ions so has charges and can easily dissolve.
• animo acids have both negative and positive charges so can be dissolved.
• glucose is polar
• oxygen is a non-polar molecule but because of the small size of the molecule it dissolves in water but only sparingly and water becomes saturated with oxygen at relatively low concentrations also if the temperature rises its solubility decreases so this is why we have haemoglobin.
• fats are entirely non-polar are are larger than oxygen and insoluble in water. They are carried in the blood inside lipoprotein complexes. These are groups of molecules with a single layer of phospholipids on the outside and fats inside.
• Cholesterol molecules is also transported inside lipoprotein complexes.

Question 12

What happens to oxygen as the temperature increases?

Answer 12

As temperature rises the solubility of oxygen decreases.