Biological molecules - Water Flashcards
define a hydrogen bond
weak forces between a slightly negatively charged atom and a slightly positively charged hydrogen.
usually between molecules eg water
define polarity
an unequal distribution of charge
why are water molecules polar
the oxygen atom attracts the electrons in the covalent bond, leaving it slightly negatively charged and the hydrogen atoms slightly positively charged
name the seven properties of water
liquid, density, cohesion, adhesion, good solvent, high specific heat capacity, high latent heat of vaporisation
explain the liquid property of water
water is a liquid at room temperature as the molecules are held together by the many weak hydrogen bonds
why is liquid an important property of water
water can:
provide an aquatic habitat for organisms
form a major component of tissues
provide a medium for chemical reactions
provide an effective transport medium
explain why solid water is less dense than liquid water
as temperature falls, molecules have less kinetic energy. this allows more hydrogen bonds to form and they break less easily.
as the liquid becomes solid, a semi crystalline structure forms. the bonds hold the water molecules slightly further apart than in liquid state
why is ice being less dense than water important
ice forms at the surface as water freezes and float, so prevents entire bodies of water from being frozen. the ice on the surface insulates the rest of the water against further temperature drops
explain the cohesion property of water
water molecules are attracted to each other due to their polarity.
why is cohesion important
water molecules are more strongly attracted to each other than they are to air, this cohesion causes surface tension.
this is what causes water droplets to form and allows pond skaters to inhabit the surface of bodies of water.
cohesive forces help to allow columns of water to move up xylem vessels
explain adhesion
water molecules are attracted to other, polar substances. water is adhesive to glass, for example
explain the importance of adhesion
forces of adhesion between water molecules and lignin in xylem vessels help with the transpiration stream
explain why water is a good solvent
ionic compounds and polar molecules will dissolve in water. the solute has slightly +ve and -ve parts. water molecules (polar) cluster around the slightly charged parts of the solute which helps to keep the solute molecules apart.
at this point they dissolve, and a solution is formed
why is water being a good solvent useful
once in solution, molecules move around and react with other molecules, which is the basis of metabolic reactions, additionally, polar molecules and ions can be transported around living things, dissolved in water
why does salt dissolve in water
salt is an ionic compound. the water molecules surround the ions, keeping them apart and dissolved. water is polar, so the +ve hydrogen atoms face cl- ions and -ve oxygen atoms face na+ ions
