Properties of Water Section 2.5

Learning Objectives Explain how hydrogen bonds between adjacent (neighboring) water molecules govern many of the properties of water. Explain how water is important to biological systems.

Importance of Water in Biological Systems Most of the mass of organisms is from water In photosynthesis water is the source of the oxygen released by plants Water is the solvent for most biological reactions Water is a reactant or product in many chemical reactions in organisms Many organisms live in water or depend on the availability of clean water

Hydrogen Bonds Form between water molecules Because water is polar Hydrogen of one water molecule forms a hydrogen bond with the oxygen of a neighboring water molecule One water molecule can form 4 hydrogen bonds

Water Has Unique Properties Due to its ability to easily form hydrogen bonds Cohesion Adhesion High Specific Heat Less Dense as a Solid Excellent Solvent

Cohesion Cohesion = Molecules being attracted to each other Water molecules form hydrogen bonds with other water molecules

Adhesion Adhesion = molecules are attracted to other types of molecules Water molecules are attracted to the sides of a glass

Capillary Action Cohesion & Adhesion work together to pull water up a small tube Adhesion between water & surface of tube pulls the first water molecule up the tube Cohesion between water molecules pulls the rest of the water molecules up behind the first one

Capillary Action

Surface Tension Water has a high surface tension Surface Tension = the attraction that molecules at the surface of a liquid have for each other Allows some organisms to walk on water

Hydrophilic Substances Dissolve in Water Hydrophilic = water loving Hydrophobic = water fearing To dissolve a hydrophilic substance water molecules form hydrogen bonds with the polar molecule or the ion Water molecules surround the polar molecule and ions & prevent them from forming bonds with other molecules

Polar Molecules & Ionic Molecules Dissolve in Water

Water Helps Maintain a Stable Temperature Kinetic Energy = energy of movement Heat = Total kinetic energy that is not usable to do work Temperature = average kinetic energy

Water has a high heat of vaporization & a high specific heat Heat of Vaporization = the amount of kinetic energy required for a water molecule to change from liquid to gas Specific Heat = the amount of kinetic energy required for 1 g of water to increase by 1 °C

Water requires a large amount of energy to increase in temperature & evaporate Because it can form so many hydrogen bonds It is hard to break all the hydrogen bonds & free one water molecule before new hydrogen bonds form

Results… Water takes a long time to heat up compared to land Land surrounded by water stays cooler than land surrounded by land When you sweat it actually cools you down Water evaporating takes a lot of heat with it, leaving less heat behind than you had before

Ice Floats Most solids are more dense than their liquid form Not Water! Hydrogen bonds hold water molecules apart & as a solid the space is “frozen” Ice is less dense than liquid water Water is densest at 4°C

Allows organisms to live in water If ice was more dense than eventually all liquid water at Earth’s surface would freeze in cold areas