2.2 - Water

Question 1

Hydrogen bonding in water

Answer 1

• Water molecules are polar, hydrogen bonds form between them
• Water molecule are formed by covalent bonds between an oxygen atom and two hydrogen atoms
• Bond involves unequal sharing of electrons – it is a polar covalent bond
• Due to unequal sharing of electrons: the hydrogen atoms have a partial positive charge, oxygen has a partial negative charge
• Positively charged particles and negatively charged particles are attracted to each other and form an ionic bond
• Water molecules partial charge means attraction is less than ionic bond but still has significant effects (hydrogen bond)
• A hydrogen bond is the force that forms when a hydrogen atom in one polar molecule is attracted to a slightly negative atom of another polar covalent molecule
• Hydrogen bonds are a weak intramolecular force, water molecules are small, large numbers of hydrogen bonds per square unit of water, unique properties, immensely important to living things

Question 2

Properties of water + cohesive properties + adhesive properties

Answer 2

• Hydrogen bond and dipolarity explain the cohesive, adhesive, thermal and solvent properties of water.
• Cohesive properties: cohesion is the binding together of two molecules that are the same type. Water molecules are cohesive, this property is important for water transport in plants, water is sucked through the xylem at low pressure. Hydrogen bonding stops the water from being separated during transport
• Adhesive properties: binding together of two different molecules. Hydrogen bonds can form between water and other polar molecules. This property is useful in leaves where water adheres to cellulose molecules in cell walls. If water evaporates from the cell walls, adhesive forces cause water do be drawn out of the nearest xylem vessel keeping the walls moist and ready to absorb carbon for photosynthesis.

Question 3

Hydrophilic and hydrophobic

Answer 3

• Hydrophilic: water loving
• Hydrophobic: water hating
• Words used to describe molecules that are chemically attracted to water.
• All substances that dissolve in water are hydrophilic, including polar molecules such as glucose and particles with positive or negative charges such as sodium and chloride ions. Substances that water adheres to are also hydrophilic
• Some substances are insoluble in water although they dissolve in other solvents, the term hydrophobic is used to describe these substances
• Molecules are hydrophobic if they do not have negative or positive charges and are nonpolar. All lipids are hydrophobic, fats and oils.
• If nonpolar molecules are surrounded by water hydrogen bonds form between water molecules but not between water and nonpolar molecules.
• If two nonpolar molecules are surrounded by water and randomly come together they act as if they are attracted to each other due to a slight attraction between nonpolar molecules.
• Nonpolar molecules tend to form bigger groups in water. The forces that cause nonpolar molecules to join together are called hydrophobic interactions

Question 4

Comparison of the thermal properties of water with those of methane

Answer 4

• Water and methane are both small molecules with atoms linked by single covalent bonds.
• Water molecules are polar and can form hydrogen bonds
• Methane molecules are nonpolar and do not form hydrogen bonds
• Water has a higher specific heat capacity, higher latent heat of vaporisation, higher melting point and higher boiling point
• Methane is liquid over a range of only 22C
• Water is liquid over 100C

Question 5

Use of water as a coolant in sweat

Answer 5

• Sweat is secreted by glands along the skin
• The sweat is carried along narrow ducts to the surface where it spreads out
• The heat needed for the evaporation of water in sweat is taken from the tissues of the skin, reducing their temperature
• Blood flowing through the skin is therefore cooled
• Solutes in the sweat, especially ions such as sodium, are left on the skin surface and can sometimes be detected by their salty taste
• Sweat secretion is controlled by the hypothalamus, it has receptors that monitor blood temperature and also receives sensory inputs from temperature receptors in the skin
• If the body is overheated the hypothalamus stimulates sweat glands to secrete up to two litres of sweat per hour
• Usually no sweat is secreted if the body is below the target temperature, though when adrenalin is secreted we sweat even if we are already cold
• This is because adrenalin is secreted when our brain anticipates a period of intense activity that will tend to cause the body to overheat.
• Other methods of cooling include: panting for dogs and transpiration for leaves

Question 6

Methods of transport of glucose, amino acids, cholesterol, fats, oxygen and sodium chloride in blood in relation to their solubility in water

Answer 6

• Blood transports a wide variety of substances using several methods to avoid possible problems and ensure that each substance is carried in large enough quantities for the bodies needs
  1. Sodium chloride
• Is an ionic compound that is freely soluble in water, dissolving to form sodium ions and chloride ions, which are carried in blood plasma
  2. Amino acids
• Have both positive and negative charges
• Because they are soluble in water but their solubility varies depending on the R-group, some of which are hydrophilic while others are hydrophobic
• All amino acids are soluble enough to be carried dissolved in blood plasma
  3. Glucose
• Is a polar molecule which is freely soluble in water and is carried dissolved in blood plasma
  4. Amino acids
• Have both positive and negative charges
• Soluble in water
• Solubility depends on R-group, some of which are hydrophilic while others are hydrophobic
• All amino acids are soluble enough to be carried dissolved in blood plasma
  5. Fats molecules
• Are nonpolar, larger than oxygen and insoluble in water
• Carried in blood inside lipoprotein complexes, these are groups of molecules with a single layer of phospholipid on the outside and fats inside
• The hydrophilic phosphate heads of the phospholipids face outwards and are in contact with water in the blood plasma. The hydrophobic hydrocarbon tails face inwards and are in contact with the fats
• There are also proteins in the phospholipid monolayer

Question 7

thermal properties of water

Answer 7

• Thermal properties include:
  • High specific heat capacity: Hydrogen bonds restrict motion of water molecules, increases in temperature require hydrogen bonds to be broken. Energy needed, the amount of energy needed to raise the temperature of water is relatively large. To cool down water must lose relatively large amounts of energy. Waters temperature is relatively stable allowing aquatic organisms to live in it.
  • High latent heat of vaporization: When a molecule evaporates it separates from other molecules in a liquid becoming a vapour molecule. The heat needed to do this is latent heat of vaporization. Considerable amounts of water are needed to evaporate water, because hydrogen must be broken. Water is a good evaporative coolant
  • High boiling point: boiling point is the highest temperature a substance can reach in liquid state, for the same reason water has a high latent heat it has a high boiling point. Water is liquid from 0-100 C.
• Solvent properties: the polar nature of water means it can form shells around charged and polar molecules, preventing them from clumping together and keeping them in solution. Water forms hydrogen bonds with polar molecules. Its partially negative oxygen pole is attracted to positively charged ions, and its partially positive hydrogen pole is attracted to negatively charged ions, so they both dissolve. Cytoplasm is a complex mixture of dissolved substances in which the chemical reactions of metabolism occur.