L5 - water

• Water is made of hydrogen and oxygen bonded together by covalent bonds
• Due to hydrogen bonding between its molecules; the density of ice is less than water due to hydrogen forming an open type lattice. High surface tension and relatively high viscosity. Can act as a solvent for many substances
• Water:

  -essential to life

  -human body is approximately 80% water and some plants and animals can be as much as 95%

  -consists of atoms of hydrogen and oxygen bonded by covalent bonds

  -two hydrogen atoms and 1 oxygen atom
• Structure of water:

  -the shared negative hydrogen electrons are pulled towards the oxygen atom

  -the other side of each hydrogen atom is left with a slightly positive charge

  -the unshared negative electrons give it a slightly negative charge

  -this makes water a polar molecule

  
• Hydrogen bonding:

  -the slightly negatively charged oxygen atoms attract the slightly positively charged hydrogen atoms of other water molecules

  -this attraction is called hydrogen bonding and it gives water some of its useful properties

  -hydrogen bonds are relatively weak interactions, which break and reform between the constantly moving water molecules
• Hydrogen bonds forming is also called cohesion. At room temperature, water forms a lattice

  
• Properties of water:

  -wide temperature range over which water is a liquid

  -water below 4*C is less dense than water above 4*C

  -high specific heat capacity

  -high latent heat of evaporation

  -polar molecule

  -reactant

  -many hydrogen bonds

  -transparent
• Water as a liquid:

  -water molecules constantly move around (like any other liquid)

  -unlike other liquids, the hydrogen bonds are continually breaking and reforming

  -the hydrogen bonds make it more difficult for water molecules to escape and become a gas so more energy is required

  -water has a low viscosity (it can flow easily)
• Water density:

  -most substances are more dense as solids than liquids

  -water becomes more dense as it gets colder until about 4*C

  -as it goes from 4*C to freezing, the water molecules arrange themselves in a less dense structure than liquid water

  -the hydrogen bonds fix the polar molecules slightly further apart than the average distance in the liquid state; producing a giant, rigid but open structure with every oxygen at the centre of a tetrahedral arrangement of hydrogens

  
• Water as a solvent:

  -good solvent for many substances found in living things

  -because it’s polar, it’s attracted to other polar substances

  -the positive and negative regions are attracted to the positive and negative solute molecules or ions

  -water molecules cluster around these charged parts of the solute molecules or ions and help them separate and stay apart. They have now dissolved and a solution has formed

  -only ions and polar molecules will dissolve but non polar molecules will not
• Water as a solvent:

  
• Cohesion and surface tension:

  -hydrogen bonds between water molecules pull them towards each other so they stick together (cohesion)

  -at an air water surface, the cohesion between water molecules produces surface tension as the hydrogen bonds also pull the molecules inwards at the surface

  -water molecules can be attracted to surfaces such as narrow tubes (adhesion)
• Cohesion: a property of water in which water molecules are attracted to each other by hydrogen bonding, allowing the molecules to move together
• Tension: a force that tends to stretch something
• Adhesion: a property of water in which water molecules are attracted to surfaces such as walls of the cells, vessels or tubes
• Solution: a liquid (the solvent) with dissolved solids, liquids or gases (solutes)
• High specific heat capacity:

  -hydrogen bonds between water molecules absorb a lot of energy

  -these hydrogen bonds give water a high specific heat capacity

  -this means water doesn’t experience rapid temperature changes

  -water absorbs/loses a relatively large amount of heat before its temperature changes
• The specific heat capacity of water has a value of 4.184 kJ kg–1 K–1
• Specific heat capacity: the energy needed to raise the temperature of 1g of a substance by 1*C
• High latent heat of vaporisation:

  -it takes a lot of energy to break the hydrogen bonds between water molecules

  -when water evaporates, heat energy, known as the latent heat of vaporisation, helps the molecules to break away from each other to become a gas

  -due to hydrogen bonds, water has a high latent heat of vaporisation

  -water absorbs a large amount of heat before it turns into water vapour
• The latent heat of vaporisation of water has a value of 2.26 MJ kg–1 K–1
• Water as a reactant:

  -water is a reactant for many reactions such as photosynthesis and hydrolysis reactions

  -its properties as a reactant do not directly draw on its polarity