Water

Question 1

Where can water be found and how much does it make up in living organisms? Give six examples.

Answer 1

• Body cells are made up of 65% water
• Most water in the human body is found in the blood plasma and intracellular fluid
• Some plants and animals are composed of 90% water
• Most biochemical reactions take place in water
• Water is a universal solvent
• Water is used in processes such as photosynthesis and produced in cellular respiration

Question 2

What elements form water and how are they bonded?

Answer 2

Two hydrogen atoms and one oxygen atom are bonded by covalent bonds.

Question 3

Explain the partial charges of the elements which compose water.

Answer 3

• Oxygen atoms are more electronegative than hydrogen atoms and hence have a slight negative charge (δ-).
• Oxygen has a greater pull on the electron cloud that is present between the atoms
• Hydrogen atoms have a slight positive charge (δ+). When they form an ion they form a 1+ ion
• Small delta is used to show the partial charges
• Negative and positive attract

Question 4

What is polarity?

Answer 4

• The dipole inter-molecular forces between the slightly positively-charged end of one molecule to the negative end of another or the same molecule.
• Basically the unequal sharing of electrons within a molecule, oppositely charged (no uniform charge)
• Water has an overall charge of 0 but is polar

Question 5

What does dipole mean in the context of bonding?

Answer 5

• A dipole moment occurs when on end of the bond is positive and the other end is negative.
• A bond is polar if the atoms have significantly different electronegativities (more than 0.4)
• Polarity also links to the electronegativity (the difference is 1.4 in water)
• These means the electrons in hydrogen are more attracted to the electrons in oxygen because oxygen is more electronegative.
• Hence water is polar

Question 6

Explain the type of bonding between whole water molecules itself.

Answer 6

• Hydrogen bonds form as the partial positive hydrogen atoms of one molecule are attracted to the partial negative oxygen atoms of other water molecules.
• Hydrogen bonds are weak attractions and are responsible for many properties

Question 7

What is hydrophilicity?

Answer 7

• Water-loving
• The polarity of water molecules allows them to attract other polar or charged compounds and from hydrogen bonds with them
• Polar or charged compounds can be dissolved in water
• Hydrophilic compounds include glucose and fructose

Question 8

What is hydrophobicity?

Answer 8

• Water-hating
• Fats and oils are non-polar substances and there is no attraction between them and water molecules, they repel each other
• E.g. oil and waxy cuticle

Question 9

What are the 4 properties of water?

Answer 9

Cohesion, Adhesion, thermal and solvent

Question 10

What is cohesion in relation to water properties and what is surface tension? Explain the strength of the bonds and the area to volume ratio too.

Answer 10

• The tendency of water molecules to stick to each other due to the hydrogen bonding between them. A maximum of 4 hydrogen bonds can be formed.
• Hydrogen bonds are weak, but a large number in one area gives the cohesive forced great strength.
• This gives rise to high surface tension which makes the water surface behave as a strong elastic membrane (some organisms can therefore float on water)
• Surface tension is also responsible for water droplets adopting a spherical shape, which reduces the area to volume ratio.

Question 11

What is adhesion in relation to water properties and what is capillary action?

Answer 11

• The interaction that water molecules have with different molecules (e.g. glass surface)
• Water molecule stick to other polar compounds by forming hydrogen bonds.
• The forces of adhesion are responsible for capillary action which is the movement of water molecules within thin spaces without relying on gravity
• Capillary action is essential for water transport in plants

Question 12

What are the thermal properties and what is high specific heat capacity?

Answer 12

• They hydrogen bonds are responsible for the thermal properties such as the high specific heat capacity
• To break apart the hydrogen bonds, high amount of energy is needed. The temperature in aquatic ecosystems do not change rapidly therefore the areas are stable
• Specific heat capacity: amount of energy needed for 1 gram of a substance to increase by 1 degrees celsius

Question 13

What is specific heat?

Answer 13

The amount of heat that must be absorbed or released for 1g of substance to change its temperature by 1°C

Question 14

What function does what have as a coolant and what is the latent heat of vaporization?

Answer 14

• Water has a high latent heat of vaporization which is the amount of heat 1g of a liquid absorbs to change its state from liquid to gaseous.
• Water requires a lot of energy.
• When water in sweat evaporates off of skin, energy in the form of heat is removed

Question 15

What are the solvent properties?

Answer 15

• Water has the ability to dissolve ionic compounds, many polar compounds (glucose).
• Most reactions rely on water to dissolve the reactants for reactions to carry on.
• Some substances cannot dissolve in water such as fats which are known as fat soluble or water insoluble

Question 16

Name properties of water and one benefits to living organisms for each one.

Answer 16

• Cohesive properties: allows water to be pulled up from the roots to the leaves
• Adhesive properties: capillary action assists the pumping action of the heart to help blood move through blood vessels
• Thermal properties: evaporation of sweat from the body for a cooling effect
• Solvent properties: water is plasma dissolves a range of solutes an gases, so that blood can transport nutrients around the body

Question 17

What are the 6 biological molecules that are found in blood?

Answer 17

Glucose, Amino acids, Fats, Cholesterol, Oxygen, Sodium chloride

Question 18

What are the modes of transport for glucose and amino acids?

Answer 18

Glucose:
- It is polar and soluble in water and is therefore transported in the blood
- The concentration in the blood is strictly maintained through homeostasis (osmotic potential)
Amino acids:
- Are polar and most are soluble in water but their solubility varies depending on the size of the R group
- With a hydrophilic R group, easy transport in blood
- With hydrophobic R group, transported in lower concentrations in the blood, less soluble

Question 19

What are the modes of transport of fats, cholesterol, oxygen and sodium chloride?

Answer 19

Fats:
- Are non-polar, insoluble in water and transported in lipoproteins (layer surrounding the fat)
Cholesterol:
- Requires the help of transport lipoproteins to be transported in blood because it is hydrophobic
Oxygen:
- Is non-polar, but because of its small size is soluble in water and can be transported in the blood
Sodium chloride:
- Because it is an ionic compound it is transported in the form of Na+ and Cl- ions. These ions are soluble

Question 20

What are the similarities in properties of water and methane?

Answer 20

• They both contain covalent compounds
• They are both small in size and have similar molecular mass
• Contain hydrogen bonds

Question 21

What are the difference between water and methane?

Answer 21

• Water consists of two hydrogen atoms attached to an oxygen atom where as methane has one carbon atom bonded to 4 hydrogen atoms
• Methane is a greenhouse gas (waste product)

Question 22

What are the 7 different thermal properties between water and methane?

Answer 22

Physical state at 20°C 
- Water: liquid
- Methane: gas 
Polarity
- Water: polar
- Methane: non-polar
Solvent 
- Water: excellent 
- Methane: not a solvent 
Melting point of methane: -161°C, boiling point -182°C 
Specific heat capacity
- Water: 4.2 J/g/°C
- Methane: 2.2 J/g/°C
Latent heat of vaporization (become a gas)
- Water: 2257 J/g
- Methane: 760 J/g