Water Flashcards
Define polar molecules
Molecules with charged regions
Define dipolar molecules
Molecules with two separate charges regions (e.g. water)
Why is water a dipole?
because oxygen has a greater electronegativity than hydrogen so the electrons are drawn away from the hydrogens slightly, causing each water molecule to have slightly positive and slightly negative regions
What is électronégatives
The pull of an atom of the shared pairs of electrons which form the covalent bonds
Describe cohension
The negative and positives parts of water molecules attract each other, forming hydrogen bonds between them
Why is water liquid at RT
- most compound with molecules as small as water are gases at RT
- water is liquid because of the hydrogen bonding
- one water molecule can form hydrogen bonds with up to four other water molecules
Describe water as the universal solvent
- polar and ionic substances -> have an electrostatic charge; are attracted to charges on water molecules; they dissolve (hydrophilic)
- non-polar substances -> have no electrostatic charge; do not dissolve, hydrophobic
- when a salt (ionic compound) dissolves in water, the ions separate and layers of water molecules form around them, preventing them from clumping back together; they stay in solution
List some hydrophilic substances
- gases (CO2, O2)
- waste products (ammonia)
- amino acids
- monosaccharides
- ATP
Describe how water forms a ‘skin’
- water molecules have a much stronger attraction to other water molecules than they do to molecules in the air
- water forms hydrogen bonds with others surrounding it, but not with air at air-water interfaces
- surface tension -> caused by the unequal distribution of hydrogen bonds -> causes water surface to contract and form a ‘skin’
- water has a greater surface tension than most liquids
What is surface tension
A measure of how difficult it is to stretch or break a liquid surface
Describe how ice floats
- water is most dense at 4°C; temperature at the bottom of large bodies of water remain fairly constant at 4°C, even if the surface does not
- in liquid water, the hydrogen bonds are constantly forming and reforming
- when water freezes, each molecule forms 4 bonds with other molecules, forming a rigid lattice where the molecules are further apart; water expands when it freezes
- ice is less dense than liquid water; it floats
- the many hydrogen bonds make ice strong
What is adhesion
The attraction between molecules of water and molecules of different substances caused by water molecules forming hydrogen bonding with other polar molecules
How does adhesion allow capillarity
Molecules of water can enter and move along very narrow spaces
Describe the specific heat capacity of water
- cohesive properties of water means it has a much higher BP than expected (because of the very high volume of hydrogen bonds)
- lots of heat energy is need to raise its temperature; lots of heat energy must be lost to lower its temperature
- water is resistant to temperature changes
Describe the latent heat of vaporisation of water
- water has a high latent heat of vaporisation
* when water evaporates from a surface, it draws heat energy out of the material underneath, creating a cooling effect
