Sian Taylor BY1: Water Sian Taylor Learning objectives: - Understand the importance of water, linking the properties of the substance to its uses.

Water is the unilateral solvent Water has the highest heat capacity of any liquid Water gets more dense as it gets colder and colder Water is a polar molecule Water is made of 2 atoms Water is made of 2 elements

Water – essential for life! The most abundant component in organisms (about 80% of cell contents) (a) It is metabolic reactant (used for lots of important reactions e.g. hydrolysis reactions) (a) It is a solvent i.e. dissolves things (most reactions take place in solution, so it provides the medium for which all most biochemical reactions take place) (a) It transports substances around (animals and plants) (b) Involved in controlling temperature. Evaporation causes cooling on a surface. Its importance stems from its properties, which are the result of its structure….

Water is a polar molecule Water is formed when two hydrogen atoms combine with an oxygen resulting in a stable sharing of electrons It has a triangular shape Although the electrons are shared it is not equal. Oxygen has a greater “pull” on the electrons than the hydrogen atoms. Will oxygen be slightly positive or negative? + _ Small negative charge Small positive charge As the shared electrons are pulled close to the oxygen atom this leaves the hydrogen atoms with a slight positive charge. The oxygen is left with a slight negative charge. Molecules that carry uneven distribution of charges are known as Polar molecules

Water forms hydrogen bonds because it is polar As each water molecule has slight positive and negative regions hydrogen bonds can form between molecules The negative charged oxygen atom will be attracted to positively charged hydrogen atoms on other water molecules Hydrogen bonds give water many of its useful and unique properties

Water is a solvent because it is polar It can weaken the attraction between ions of opposite charges in other substances (e.g. salt) because it has both positive and negative charges itself and so can attract both ions. Once the ions have separated the water molecules prevent them from rejoining by clustering around them! So water is a great solvent. It can dissolve ionic substances and polar molecules (e.g. glucose) This is very important because chemical reactions take place more easily in solution! The ions are free to move around and react (compared to when they are held together in a solid) Because they dissolve in water, it also acts as a transport medium (blood in animals, water transports minerals in xylem and sucrose in phloem in plants)

Water – thermal properties High specific heat capacity Specific heat capacity is the amount of heat needed to raise the temperature of 1g by 1ºC Needs a lot of energy to raise the temperature because the H-bonds between molecules restrict their movement Water is therefore good at maintaining its temperature irrespective of fluctuations in the temperature of surrounding environment Why is this important? Keeps aquatic environments stable Keeps effective temperature range for enzymes to work well

High latent heat of vaporisation: It takes a lot of heat energy to change it from liquid to vapour state Why is this good? Cooling things (think about sweating!) Water – thermal properties cont.

Water – Surface tension and cohesion Water acts as if it has a skin (molecules held together) It has the highest surface tension of any liquid except mercury Water molecules "stick together" due to their hydrogen bonds (individually weak, but when they are lots they form a strong lattice) so water has high cohesion. This explains why long columns of water can be sucked up tall trees by transpiration without breaking It also explains surface tension, which allows small animals to walk on water

Water – Density Most liquids decrease in volume and increase in density as the temperature drop How can we tell that water different? With water it is most dense at 4ºC. Ice is less dense than liquid water and so will float! Aquatic organisms can survive if the surface of the water freezes The ice acts as insulation for the water below Other important properties? Water is transparent – therefore light can pass through and plants below the surface can photosynthesise

Water – Properties Polar High surface tension Cohesion and adhesion of molecules Density Universal Solvent High specific heat capacity Capillary action due to cohesion Transparent Complete the activities at the different stations to explore these properties

Property of water Ice is less dense than water High surface tension Strong cohesive properties and high tensile strength Colourless with a high transmission Liquid at room temperature In order to evaporate it must absorb a large amount of energy (high latent heat of vaporisation) Water can absorb a lot of energy for only a small rise in temperature (high specific heat capacity) Significance for life Conditions are stable in cells and aquatic environments Can be used for cooling organisms by evaporation of for example sweat The high surface tension of water means that it can form a habitat on the surface of the water Ice forms an insulating layer over water Can be used for transport and a medium for reactions Light can pass through cells for photosynthesis Water can be pulled through plants in a column as the water molecules are held together by H-bonds Answers

Water – as a metabolite Chemical reactions take place in cells All these reactions together are called metabolism The chemicals involved are called metabolites Water is a metabolite in many reactions, either as a reactant or as a product of reaction e.g. It is involved in photosynthesis, digestion and aerobic respiration When water reacts with a chemical to break it into smaller molecules the reaction is described as hydrolysis (hydro = water; lysis =break down) If water is formed by a reaction, the reaction is described as a condensation reaction