2.2 Water Flashcards
What is water made of?
Water is made up of two hydrogen atoms covalently bonded to an oxygen atom (molecular formula = H2O)
How are the electrons distributed in water?
While this covalent bonding involves the sharing of electrons, they are not shared equally between the atoms
Which element in water attracts electrons more?
Oxygen (due to having a higher electronegativity) attracts the electrons more strongly
What does the unequal distribution of electrons create?
The shared electrons orbit closer to the oxygen atom than the hydrogen atoms resulting in polarity
Why is water described as being polar?
Water is described as being polar because it has a slight charge difference across the different poles of the molecule
What charges do the oxygen and hydrogen atoms have in water?
The oxygen atom is slightly negative (δ–) while the hydrogen atoms are slightly positive (δ+)
What does water’s polarity allow it to do in relation to other molecules?
This charge difference across the molecule (dipole) allows water to form weak associations with other polar molecules
How does water attract other polar molecules?
The slightly negative poles (δ–) will attract the slightly positive poles (δ+) of other molecules, and vice versa
What is a hydrogen bond and between which elements does it form?
When a δ+ hydrogen atom is attracted to a δ– fluorine, oxygen or nitrogen atom of another molecule, it forms a hydrogen bond
Why are hydrogen bonds particularly strong?
Hydrogen bonds are relatively stronger than other polar associations due to the high electronegativity of F, O and N
How does waters polarity affect its ability to form hydrogen bonds?
The dipolarity of a water molecule enables it to form polar associations with other charged molecules (polar or ionic)
Can hydrogen bonds form between water molecules?
Water can form hydrogen bonds with other water molecules (between a δ+ hydrogen and a δ– oxygen of two molecules)
What 3 properties of water are determined by its polarity?
- thermal properties
- cohesive/adhesive properties
- solvent properties
How does water’s polarity affect its thermal properties?
Water can absorb much heat before changing state (requires the breaking of hydrogen bonds)
How does water’s polarity affect its cohesive/adhesive properties?
Water will ‘stick’ to other water molecules (cohesion) and charged substances (adhesion)
How does water’s polarity affect its solvent properties?
Water dissolves polar and ionic substances (forms competing polar associations to draw materials apart)
Why can water absorb large 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)
Why is water’s high specific heat capacity useful for organisms?
Consequently, water is an excellent medium for living organisms as it is relatively slow to change temperature and thus supports the maintenance of constant conditions (internal and external)
What molecules is usually compared with water and why?
Methane (CH4) provides a good basis for comparison with water due to the many similarities between their structures:
Comparable size and weight (H2O = 18 dalton ; CH4 = 16 dalton)
Comparable valence structures (both have tetrahedral orbital formations, but water is bent due to unbonded electron pairs)
Why are there differences in thermal properties between water and methane?
The differences in thermal properties between water and methane arise from differences in polarity between the molecules
What is the difference between the bonds water and methane can form?
ater is polar and can form intermolecular hydrogen bonds (due to high electronegativity of oxygen atom)
Methane is non-polar and can only form weak dispersion forces between its molecules (carbon has a lower electronegativity)
How is water different compared to methane in terms of melting point?
Water has a significantly higher melting and boiling point
How is water different compared to methane in terms of specific heat capacity?
Water has a higher specific heat capacity (energy required to raise the temperature of 1 g of substance by 1ºC)
How is water different compared to methane in terms of heat of vaporisation?
Water has a higher heat of vaporisation (energy absorbed per gram as it changes from a liquid to a gas / vapour)
How is water different compared to methane in terms of heat of fusion?
Water as a higher heat of fusion (energy required to be lost to change 1 g of liquid to 1 g of solid at 0ºC)
How is water beneficial to humans in terms of temperature regulation?1
The evaporation of water as sweat is a fundamental mechanism employed by humans as a means of cooling down
- What does water require to change state? Where does this energy come from?
The change of water from liquid to vapour (evaporation) requires an input of energy
This energy comes from the surface of the skin when it is hot, therefore when the sweat evaporates the skin is cooled
- Why is water effective as sweat?
Because water has a high specific heat capacity, it absorbs a lot of thermal energy before it evaporates
Thus water functions as a highly effective coolant, making it the principal component of sweat
What molecules is water attracted to?
Because water is polar it will be attracted to other molecules that are polar or have an ionic charge
What is cohesion?
Cohesion is the ability of like molecules to stick together
Is water cohesive?
Water is strongly cohesive (it will form hydrogen bonds)
What is adhesion?
Adhesion is the ability of dissimilar molecules to stick together
Is water adhesive?
Water will form intermolecular associations with polar and charged molecules
What other property of water does cohesion explain?
The cohesive properties of water explain its surface tension
How can surface tension be explained by cohesion?
The hydrogen bonding between water molecules allows the liquid to resist low levels of external force (surface tension)
Why is the high surface tension of water beneficial?
The high surface tension of water makes it sufficiently dense for certain smaller organisms to move along its surface
What property of water does adhesion explain?
The adhesive properties of water explain its capillary action
How can capillary action be explained by adhesion?
Attraction to charged or polar surfaces (e.g. glass) allows water to flow in opposition of gravitational forces (capillary action)
Why is capillary action beneficial?
This capillary action is necessary to allow water to be transported up plant stems via a transpiration stream
What is water commonly referred to as which is a key factor of its usefulness?
Water is commonly referred to as the universal solvent due to its capacity to dissolve a large number of substances
What can water dissolve?
Water can dissolve any substance that contains charged particles (ions) or electronegative atoms (polarity)
Why is water able to dissolve polar/ionic substances?
This occurs because the polar attraction of large quantities of water molecules can sufficiently weaken intramolecular forces (such as ionic bonds) and result in the dissociation of the atoms
What are hydration shells?
The slightly charged regions of the water molecule surround atoms of opposing charge, forming dispersive hydration shells
What does it mean if a substance is hydrophilic? Give an example?
Substances that freely associate and readily dissolve in water are characterised as hydrophilic (‘water loving’)
Hydrophilic substances include all polar molecules and ions
What does it mean if a substance is hydrophobic? Give an example?
Substances that do not freely associate or dissolve in water are characterised as hydrophobic (‘water-hating’)
Hydrophobic substances include large, non-polar molecules (such as fats and oils)
What does the transport of molecules depend on?
The transport of essential molecules within the bloodstream will depend on their solubility in water
Water soluble substances will usually be able to travel freely in the blood plasma, whereas water insoluble substances cannot
What 4 substances are water soluble?
NaCl
Oxygen
Glucose
Amino acids
Why is NaCl able to be transported in the blood?
Sodium chloride (NaCl) is an ionic compound and its components (Na+ and Cl–) may be freely transported within the blood
Why is oxygen able to be transported in the blood?
Oxygen is soluble in water but in low amounts – most oxygen is transported by haemoglobin within red blood cells
Why is glucose able to be transported in the blood?
Glucose contains many hydroxyl groups (–OH) which may associate with water and thus can freely travel within the blood
Why are amino acids able to be transported in the blood?
Amino acids will be transported in the blood in an ionized state (either the amine and/or carboxyl groups may be charged)
What is an example of a hydrophobic substance that can’t normally be transported by the bloodstream?
Lipids (fats and cholesterol) are non-polar and hydrophobic and hence will not dissolve in water
How are lipids transported in the blood?
They form complexes with proteins (lipoproteins) in order to move through the bloodstream
What does a lipoprotein look like?
Hydrophilic portions of proteins, cholesterol and phospholipids will face outwards and shield internal hydrophobic components
