2.2 Water Flashcards
Molecular structure of water
polarity of water + type of bond
- Water consists of two hydrogen atoms and one oxygen atom joined by covalent bonds.
- The oxygen atom is more electronegative than the hydrogen atoms. This means that oxygen has a greater pull on the electron cloud that exists between the atoms.
- Therefore, oxygen acquires a slightly negative charge (δ-), leaving each hydrogen atom with a slightly positive charge (δ+).
Note: the polarity of water is the main reason for its important biological properties such as solvency, cohesion and adhesion.
Hydrogen bonds
Because of the dipolarity of water molecules, there are hydrogen bonds formed between water molecules.
- The partial positive hydrogen atoms of one molecule are attracted to the partial negative oxygen atoms of other water molecules.
Hydrogen bonds are weak interactions between water molecules and are responsible for many important properties of water such as cohesion and adhesion.
Hydrophilic
- The polarity of water molecules allows them to attract other polar or charged compounds and form hydrogen bonds with them. This means that most polar or charged compounds can be dissolved in water.
- Such compounds are said to be hydrophilic or ‘water-loving’. Sugars and most salts are good examples of hydrophilic compounds.
Hydrophobic
- Fats and oils are non-polar substances. There is no attraction between them and water molecules; instead they repel each other.
- Example: The waxy cuticle on the leaf has strong hydrophobic properties. It causes water to form droplets to minimise contact between the leaf and the water.
Cohesion
This is the tendency of water molecules to stick to each other due to the dipolarity causing hydrogen bonding between them.
Each water molecule can potentially form four hydrogen bonds with other water molecules in a tetrahedral arrangement.
- Although hydrogen bonds are weak bonds, the presence of a large number of hydrogen bonds in water gives the cohesive forces great strength.
- Cohesion causes surface tension, which makes the water surface behave like a strong elastic membrane allowing small organisms or objects denser than water can float.
Benefits of cohesion to living organisms
- Allows water to be pulled up from the roots to the leaves of plants.
- Permits insects, such as pond skaters, to walk/float on the surface of water to catch their prey.
Adhesion
This is the interaction that water molecules have with other (different) molecules (e.g. a glass surface) and explains why water molecules stick to other polar compounds by forming hydrogen bonds.
- Adhesion causes capillary action, which is defined as the movement of water molecules and all the things that are dissolved in it within thin spaces without relying on gravity.
- This property of water molecules is essential to allow water to be transported up the stems of plants.
Benefits of adhesion to living organisms
- Adhesion of water molecules to the cell wall of xylem vessels helps water move against gravity from the roots to the leaves.
- Capillary action generated by adhesive forces assists the pumping action of the heart to help blood move through blood vessels.
Water solvent properties
Water is commonly referred to as the universal solvent due to its capacity to dissolve a large number of substances
- Water can dissolve any substance that contains charged particles (ions) or electronegative atoms (polarity)
Modes of transport of biological molecules
Water soluble substances
- Sodium chloride (NaCl) is an ionic compound and its components (Na+ and Cl–) may be freely transported within the blood
- Oxygen is soluble in water but in low amounts – most oxygen is transported by haemoglobin within red blood cells
- Glucose contains many hydroxyl groups (–OH) which may associate with water and thus can freely travel within the blood - its also a polar molecule
- Amino acids will be transported in the blood in an ionized state (either the amine and/or carboxyl groups may be charged)
Modes of transport of biological molecules
Water insoluble substances
Lipids (fats and cholesterol) are non-polar and hydrophobic and hence will not dissolve in water
- Therefore they form complexes with proteins (lipoproteins) in order to move through the bloodstream
- Hydrophilic portions of proteins, cholesterol and phospholipids will face outwards and shield internal hydrophobic components
Water as a solvent - benefits to living organisms
- Water dissolves mineral ions in the soil and transports it along xylem vessels from the roots to all parts of the plant.
- Water in blood plasma dissolves a range of solutes and gases, which makes it possible for blood to transport nutrients and gases around the body
- Water’s versatility (its ability to exist as a liquid, solid and vapour) makes water a universal solvent and medium for numerous biochemical reactions.
Thermal properties of water
Water has the capacity to absorb significant amounts of heat before changing state
- This is due to the extensive hydrogen bonding between water molecules – the H-bonds need to be broken before a change in state can occur and this requires the absorption of energy (heat)
Water has a high specific heat capacity.
- Specific heat is the amount of heat that must be absorbed or released for 1g of a substance to change its temperature by 1°C.
- This means that a lot of heat must be absorbed in order for the temperature to change.
Water as a coolant
- A large amount of energy is needed to turn liquid water into water vapour.
- This means that water has a high latent heat of vaporisation , which can be defined as the amount of heat 1g of a liquid absorbs to change its state, from liquid to gaseous.
- When water in sweat evaporates on the surface of our skin, it cools down the surface by taking away energy in the form of heat.
Thermal properties of water vs methane
- Water is polar
- Methane is non-polar
This means that water absorbs more heat before changing state - Water has a higher specific heat capacity and a higher latent heta of vapourisation
- Water has a higher melting and boiling point
