Topic 2.2 Water

2.2 (U1) Water molecules are polar and hydrogen bonds form between them.

• Water is referred to as a polar molecule because it has a negative and positive end.

• Oxygen atoms have slightly negative charge while hydrogen atoms have a slightly positive charge.

• The polar covalent bond that forms between them results in a slightly negative charge at the oxygen end and a slightly positive charge at the hydrogen end. The electrons are not shared equally between the oxygen and hydrogen atoms

• This is referred to as the polarity of water.

2.2 (U1) Water molecules are polar and hydrogen bonds form between them.

• The attraction between water molecules is a weak hydrogen bond.

• It forms when the slightly positive charge on the hydrogen atom on one polar molecule is attracted to the slightly negative atom of another polar molecule.

Source: http://www.physicalgeography.net/fundamentals/images/water.JPG

2.2 (U2) Hydrogen bonding and dipolarity explain the adhesive, cohesive, thermal and solvent properties of water.

• Water is known as the medium of life. Many organisms are comprised of 70-90% water.

• The ability of water molecules to form hydrogen bonds explains the many properties of water that make it so important to living things.

http://www.youtube.com/watch?v=aVmU3CLxvgU

Adhesive Properties of Water• Adhesion of water

molecules occurs when the water molecules are attracted to another polar molecule.

• This happens in the xylem tissue of plants. The water molecules stick to the walls of the xylem due to the adhesive properties of water.

Cohesive Properties of Water.

• Due to its polarity, water molecules are highly cohesive. They stick to one another due to the formation of hydrogen bonds.

• Some examples of cohesion are transport of water in plants, surface tension and formation of water droplets.

Thermal Properties of Water.

• One thermal property of water is high specific heat capacity. This means that it takes a lot of energy to increase the temperature of water and subsequently a lot of energy is given off when it cools. In other words water can absorb or give off a significant amount of heat and maintain a relatively stable temperature.

• Another thermal property of water is that it has a high heat of vaporization. This means that water requires a lot of energy to change from a liquid to a gas.

Solvent Properties of Water

• The polarity of water allows for many substances to dissolve in it and is an excellent solvent for polar solutes.

• Many substances found inside and outside of cells are polar such as nucleic acids, proteins and carbohydrates. (Lipids are mainly non-polar)

2.2 (U3) Substances can be hydrophillic or hydrophobic

• All substances that dissolve in H2O are hydrophilic, including… Glucose (polar) and Na+ and CL- (charged)

• Substances water adheres to are also hydrophilic (ex. Cellulose)

• Lipids are all hydrophobic (insoluble in water, non-polar, no charge)

2.2 (A1) Comparison of the thermal properties of water with those of methane.

• Methane is a byproduct of anaerobic respiration of prokaryotes that live in swamps, wetlands and the guts of animals such as cows, sheep and termites.

• Methane is a gas composed of 4 hydrogen atoms and 1 atom of carbon. It is held together by covalent bonds but unlike water, methane molecules are not polar.

• Due to these differences their physical properties are very different.

2.2 (A1) Comparison of the thermal properties of water with those of methane.

Property Methane Water

Formula CH4 H20

Molecular Mass 16 18

Density 0.46g per cm3 1g per cm3

Specific Heat Capacity 2.2 J per g per °C 4.2 J per g per °C

Latent Heat of Vaporization

760 J/g 2 257 J/g

Melting Point -182 °C 0°C

Boiling Point -160 °C 100 °C

2.2 (A2) Use of water as a coolant in sweat.

• Many organisms use vaporization as a method of cooling the body (thermoregulation). This is an example of water being used as a coolant in living things; sweating in humans.

• High temperatures can be very dangerous to living things. It can cause proteins to denature (change shape). The hypothalamus in the brain controls sweating in humans.

• Plants will increase transpiration rates when in danger of over-heating.

• Dogs will pant when they overheat.

2.2 (A3) Methods of transport of glucose, amino acids, cholesterol, fats, oxygen and sodium chloride in blood in

relation to their solubility in water.

• A watery habitat can dissolve substances that can then be absorbed by organisms.

• The watery environment of the cytoplasm allows for dissolved substances.

• When substances are dissolved in a liquid medium, metabolic reactions can take place easily and efficiently.

• Blood plasma (the liquid portion of blood) is 95% water and 5% dissolved substances

2.2 (A3) Methods of transport of glucose, amino acids, cholesterol, fats, oxygen and sodium chloride in blood in

relation to their solubility in water.

Glucose

• Glucose easily dissolves in water and is therefore transported in the blood plasma

Amino Acids• Amino acids have positive and

negative charges and their solubility in water varies depending on the R group. Despite this all 20 amino acids are soluble in water and therefore transported by the blood.

2.2 (A3) Methods of transport of glucose, amino acids, cholesterol, fats, oxygen and sodium chloride in blood in

relation to their solubility in water.

Fats• Fats are large non-polar

molecules. They are insoluble in water.

• Due to this insolubility they are carried in water inside lipoprotein complexes.

Cholesterol

• Cholesterol molecules are primarily hydrophobic and are not soluble in water.

• They are carried inside lipoproteins along with fats.

2.2 (A3) Methods of transport of glucose, amino acids, cholesterol, fats, oxygen and sodium chloride in blood in

relation to their solubility in water.

• Oxygen is a non-polar molecule but it does dissolve in water due to its small size.

• Only a small amount of oxygen can dissolve in water and it becomes saturated with oxygen at fairly low concentrations.

• Solubility of oxygen in water is also affected by temperature. As temp increases, solubility decreases.

• Blood plasma at a normal temp of 37°C can only transport a limited amount of oxygen. Blood relies on the protein hemoglobin on the surface of red blood cells to adhere to and transport oxygen.

2.2 (A3) Methods of transport of glucose, amino acids, cholesterol, fats, oxygen and sodium chloride in blood in

relation to their solubility in water.

Sodium Chloride

•Sodium chloride is an ionic compound that easily dissolves in water.

•The sodium and chlorine ions separate and are transported in the water.