3.1.7 Water

Question 1

Describe what is meant by electronegativity

Answer 1

The power of an atom to attract electrons within a covalent bond

Question 2

Describe how the structure of water molecules, leads to hydrogen bonding

Answer 2

• A molecule of water contains 1 atom of oxygen, covalently bonded to 2 atoms of hydrogen.
• In water molecules, the oxygen atoms has got a partial negative charge, and the hydrogen atoms have got a partial positive charge.
• The opposite charges means that water molecules are attracted to each other. These forces of attraction are called hydrogen bonds.

Question 3

Describe why oxygen has a slight negative charge in water

Answer 3

• Oxygen is more electronegative than hydrogen so it attracts the electrons within the covalent bond more strongly
• As electrons are negatively charged, the oxygen therefore becomes slightly negatively charged

Question 4

Describe why hydrogen has a slight positive charge in water

Answer 4

• Hydrogen is less electronegative than oxygen so it attracts the electrons within the covalent bond less strongly than oxygen
• As electrons are negatively charged, the hydrogen therefore becomes slightly positively charged

Question 5

State the main
properties of water

Answer 5

• Metabolite
• Solvent
• High specific heat capacity
• High latent heat of vaporisation
• Cohesive

Question 6

Explain the biological
importance of
hydrogen bonding in
water

Answer 6

• Hydrogen bonds mean that water is cohesive and can be pulled up through a tube. This is useful in plants where water is pulled through the xylem vessels.
• Also, these strong cohesive
  forces enable surface tension to occur, where the surface of water acts like a skin and this is strong enough to support small organisms.
• Finally, hydrogen bonding increases the boiling point of water, without hydrogen bonding water would be a gas at room temperature!

Question 7

State 5 properties of water

Answer 7

• Metabolite
• Solvent
• High specific heat capacity
• Large latent heat of vaporisation
• Cohesive

Question 8

Why is water a metabolite

Answer 8

Water is reactive and takes part in many reactions

Question 9

Describe the importance of water being a metabolite to cells

Answer 9

Reactions such a hydrolysis, condensation and photosynthesis can occur

Question 10

Describe the importance of water being a metabolite to organisms

Answer 10

Reactions such a hydrolysis (e.g. digestion) can occur

Question 11

Why is water a solvent

Answer 11

Water is polar, and so it can dissolve other polar substances

Question 12

Describe the importance of water being a solvent to cells

Answer 12

Allows chemical reactions to occur in solution, such as in the cytoplasm
(reaction occur faster in solution)

Question 13

Describe the importance of water being a solvent to organisms

Answer 13

Allows easier transport of polar substances through a transport medium, such as the blood plasma

Question 14

Why does water have a high specific heat capacity

Answer 14

A lot of energy is required to raise the temperature of water

Question 15

Describe the importance of water having a high specific heat capacity to organisms

Answer 15

Water acts as a buffer against sudden temperature changes, therefore this reduces change of damage to the tertiary structure of proteins in organisms.

Question 16

Why does water have a high latent heat of vaporisation

Answer 16

A lot of energy is transferred when water evaporates

Question 17

Describe the importance of water having a high latent heat of vaporisation in organisms

Answer 17

Organisms can cool down when water evaporates from their surface, (e.g. sweating cools the skin)

Question 18

Why is water cohesive

Answer 18

Water molecules are attracted to each other due to hydrogen bonding

Question 19

Describe the importance of water being cohesive to organisms

Answer 19

Water can form continuous columns, which allows the transports of water in the xylem of plants

Question 20

Why does water have high surface tension

Answer 20

Hydrogen bonding between water molecules causes the surface of water to behave like an elastic sheet

Question 21

Describe the importance of water having high surface tension to organisms

Answer 21

Water can support the mass of small organisms, such as insects, allowing them to move around on the water surface

Question 22

Why is ice less dense than water

Answer 22

Hydrogen bonding causes water to expand when it freezes

Question 23

Describe the importance of ice being less dense than water to organisms

Answer 23

• A layer of ice acts as an insulator at the top of ponds/lakes, which reduces energy transfer from water underneath to the surroundings.
• This prevents water underneath from freezing, allowing survival or aquatic organisms