Water

Question 1

Structure of water

Answer 1

1. Water consists of one oxygen atom joined to two hydrogen atoms by covalent bonding
2. The shared pair of electrons in each of the O-H covalant bonds is pulled closer to the oxygen because oxygen is more electronegative
3. Therefore, there is a slight negative charge on the oxygen atom and slight positive charge on both hydrogen atoms
4. This makes water a polar molecule
5. Therefore, hydrogen bonds can form between water molecules (which is what gives water its properties)
6. The slighly positive hydrogen of one water molecule is attracted to the slightly negative oxygen of another water molecule - hydrogen bonding

Question 2

Useful properties of water

Answer 2

1. Is a metabolite
2. High latent heat of vapourisation
3. Good solvent
4. High specific heat capacity
5. Strong cohesion between water molecules

Question 3

Water is a metabolite

Answer 3

1. Water is used in metabolic reactions - chemical reactions which occur within an organism to keep organism alive. This includes hydrolysis and condensation reactions. For example, a molecule of water is used in hydrolysis to break a chemical bond, e.g. when hydrolysing ATP to release energy (occurs in photosynthesis + respiration)

Question 4

Water has a High latent heat of vapourisation

Answer 4

1. Lots of heat energy is needed (by water) to break the MANY hydrogen bonds between water molecules + evaporate
2. Therefore, for an organism to lose lots of heat, only a small amount of water needs to evaporate (only a few hydrogen bonds need to be broken between water molecoles, allowing water to evaporate)
3. This provides a cooling effect for living organisms e.g. evaporation of water as “sweat” from the skin

Question 5

Water is a good solvent

Answer 5

1. Water is a polar molecule (uneven distrobution of charge across the molecule)
2. This makes it a good solvent for ionic compounds (ionic compounds can dissolve in it easily), ions + other polar molecules
3. This is because one end of water molecule is negative and other end is positive. Positive ions are attracted to negative ends of water molecule and negative ions are attracted to positve ends of water molecule
4. Water molecules totally surround ions therefore, dissolve them
5. THEREFORE, WATER IS A GOOD MEDIUM FOR METABOLIC REACTIONS E.G. IN CYTOPLASM
   + WATER ALLOWS TRANSPORT OF SOLUTES E.G. IONS IN TRANSPIRATION

Question 6

Water has a high specific heat capacity

Answer 6

1. Water has a high specific heat capacity. This means lots of energy (4.2 joules) is required to raise the temp of 1g of water by 1 °C.
2. This is due to many hydrogen bonds between the water molecules. They are strong + require lots of heat energy to break
3. Therefore, water can buffer (resist) changes in temp
4. Allows water to be a good habitat for aquatic organisms - stable temp
5. Allows water inside living organism to remain at a stable temp, allowing organisms to maintain a stable temp inside the body

Question 7

Strong cohesion between water molecules

Answer 7

1. MANY hydrogen bonds between water molecules allow for strong cohesion between water molecules - WATER MOLECULES TEND TO STICK TOGETHER
2. Helps water flow + makes it good at transporting substances:

for example,

• water travels up the xylem in columns (plants)
• water travels up blood vessels in columns (animals)

3. Strong cohesion between water molecules due to presence of many hydrogen bonds between water molecules creates high surface tension. This is where the body of water meets air + hydrogen bonds form between the top layer of water molecules (and air), this creates a “film” on the body of water and is what allows insects e.g. pond skaters to walk on surface of water

Question 8

Definition of cohesion

Answer 8

The attraction between molecules of the same type

Where molecuels of the same type stick together.