Describe why hydrogen bonding in water is important Explain why water is such a good solvent (example: salt dissolving in water) Describe the difference between an acid and a base Compare adhesion and cohesion and give real world examples of each Define solute, solvent and solution LECTURE 4: Properties of Water

Polarity Polar because of an uneven distribution of electrons between the oxygen and hydrogen atoms A water molecule has both partial negative charges and partial positive charges. O HH (-) (+)

Question Think of the interaction of two magnets and how they either are attracted to each other or repel. Then draw a picture of how you think the water molecules will interact with each other.

Interactions Polar water molecules can attract each other. Hydrogen atom with partial positive charge (+) is attracted to the partial negative charge (-) of another atom (oxygen) This is known as a hydrogen bond Not as strong as covalent or ionic bonds Bonding between molecules

Properties of Water Cohesion

Attraction between molecules of the same substance (water) Water molecules can form up to four hydrogen bonds at the same time so it is very cohesive. Examples: Beads of water on leaves Raft spider Surface tension – glass of milk that is poured to very top and somehow doesn’t spill.

Properties of Water Adhesion

Attraction between molecules of different substances Ex.) Water forms “meniscus” in graduated cylinder Adhesion between water molecules and glass molecules is stronger than cohesion between water molecules

High Specific Heat Due to hydrogen bonding, water resists change in temperature Water has to absorb more heat energy to increase temperature compared to other compounds Helps to regulate cell temperatures in organisms.

Solution Solution: mixture of substances that is the same throughout Homogeneous mixture Made up of solute and solvent Solute substance that is dissolved (in solvent) Solvent substance that dissolves another substance (solute) Aqueous solutions Solutions where the solvent is water

Solutions Diagram

Table salt (NaCl) Remember that NaCl is bonded together by an ionic bond. Write out the two ions with their charges for table salt. Draw a solid piece of salt at the molecular level. Draw a water molecule, and add its partial charges. Remember water is POLAR! How do you think water dissolves table salt?

Water as a Solvent Since water is polar, it can dissolve ionic compounds and other polar molecules Can dissolve salts, sugars, minerals, and some gases

Polar vs. non-polar Hydrophilic- molecule that is attracted to, and tends to be dissolved by water Typically other polar substances Hydrophobic- molecule that is repelled by water Typically non-polar substances Example- oil

Acids and Bases Acid Compound that releases a proton (H + ) when dissolved in water Increases H + concentration in solution Base Compounds that remove H + ions from a solution Decreases H + concentration in solution

pH scale Indicates concentration of H + ions in a solution

Reading the pH scale Neutral: pH of 7 Concentration of OH - and H + are equal Water is neutral Acids: Compounds that form H + ions in a solution More H + than OH - Bases: Compound that produces hydroxide (OH-) ions in solution Alkaline More OH - than H +

Buffers Buffers prevent drastic changes in pH Buffers: weak acids or bases that can react with strong acids or bases to prevent sudden changes in pH