CHAPTER 4: The Chemical Basis of Life 4-1: Water

Water expands slightly as it freezes, therefore the density of ice is less than the density of water. This causes ice to float on water. What would be the impact on lake life if ice were more dense than water? The lake would freeze from the bottom up and all organisms would freeze to death.

4-1: Water Why do we consider water a polar molecule? The bonds that hold Hydrogen and Oxygen together are polar covalent bonds (there is an uneven sharing of electrons). This is because oxygen is more electronegative (wants electrons more) than hydrogen. This uneven sharing of electrons leads to a  + (partial positive) charge on the hydrogen and a  - (partial negative) charge on the oxygen.

4-1: Water What impact does water’s polarity have on the way it interacts with a) other water molecules? cohesion – water is attracted to other water molecules This attraction between adjacent water molecules is due to hydrogen bonding

4-1: Water A Hydrogen Bond is a weak attraction between the  + (partial positive) hydrogen of one water molecule and the  - (partial negative) oxygen of an adjacent water molecule.

4-1: Water What impact does water’s polarity have on the way it interacts with b) other polar molecules? adhesion – water is attracted to other polar molecules

4-1: Water Adhesion and cohesion are important in the movement of water in plants. Tree Video As water evaporates from the leaves, it pulls up (cohesion) the water below it, which pulls on the water below it and so on all the way down to the water in the roots. Water holds on to (adhesion) the tubes in the plant

4-1: Water What impact does water’s polarity have on the way it interacts with nonpolar molecules? repulsion – water is repulsed by nonpolar molecules (ex. Oil)

4-1: Water A mixture is 2 or more elements or compounds mixed together but not chemically combined (not linked by chemical bonds). – Ex) atmosphere, soil The difference between mixtures and compounds is that in a chemical compound, atoms are held together by chemical bonds.

4-1: Water Two types of mixtures that involve water are Solutions and Suspensions A solution is mixture in which molecules are evenly distributed in a solvent. A solution is made of: a solute – thing that is disolved, and a solvent – thing doing the dissolving

4-1: Water

The characteristics of the water molecule which make it such an excellent solvent are that the  + (H) and  - (O) parts of the water molecule are attracted to other charged molecules.

4-1: Water Ionically bound compounds often dissociate (or break apart) in water. Compare and contrast the dissociation of the ions in NaCl and HCl. NaCl Na + + Cl - HCl H + + Cl -

4-1: Water Compounds that release Hydrogen ions (H + ) into solution are known as acids. Example: HCl H + + Cl - What can this H + do? A Hydrogen ion (H + ) is the most chemically reactive substance. It can attack bonds in a variety of substances.

4-1: Water Compounds that release Hydroxide ions (OH - ) into solution are known as bases Example: NaOH Na + + OH -

4-1: Water A neutralization reaction is the mixing of an acid and a base to form a salt and water. The H + and the OH - combine to form H 2 O, which is neutral. HCl + NaOHNaCl + H 2 O HBr + NaOHNaBr + H 2 O

4-1: Water pH is a measure of the amount of H + in a solution. Describe the pH scale. (See page 66) Logarithmic scale – a change of 1 on the scale indicates a ten-fold change in concentration Ranges from 0 – 14 0 – <7acidic 7 neutral >7 – 14basic

Acid Neutral Base

4-1: Water A suspension is a mixture of H 2 O and non- dissolved material. The flow of water keeps the material suspended. Ex) blood, thick soup

4-1: Water Blood is both a solution and a suspension? Blood is mostly H 2 O - some solutes are dissolved in the water (ex. Salt), it is therefore is a solution Blood also contains other suspended particles (ex. Cells, clotting factors) and is therefore a suspension