10T2K © Water and pH

Thing 1: Water is polar Water is a polar molecule. That is, it has a negative end (at the oxygen) and a positive end (at each hydrogen). Water is a polar molecule. That is, it has a negative end (at the oxygen) and a positive end (at each hydrogen). Because water is polar it can make hydrogen bonds with other molecules. Because water is polar it can make hydrogen bonds with other molecules.

Thing 2: Cohesion Cohesion is water making hydrogen bonds with other water molecules. Cohesion is water making hydrogen bonds with other water molecules. –Surface tension, which allows insects to walk on water, is one result of cohesion.

Thing 3: Adhesion Adhesion is water making hydrogen bonds with something other than water. Adhesion is water making hydrogen bonds with something other than water. –Capillary action, which brings water to the top of trees, is partly the result of adhesion of water to the “veins” of the trees.

Thing 4: High specific heat Water has a high specific heat. That means that it takes a lot of energy to get water to warm up. Water has a high specific heat. That means that it takes a lot of energy to get water to warm up. –That’s why the water in a swimming pool can be cold on a very hot day. –That’s also why the temperature of Earth isn’t a lot hotter—oceans and lakes absorb a lot of energy from the sun.

Thing 5: High heat of vaporization Water has a high heat of vaporization. That means that it takes a lot of energy to get water to evaporate. Water has a high heat of vaporization. That means that it takes a lot of energy to get water to evaporate. –That’s why the oceans don’t evaporate away every summer.

Thing 6: Ice floats Because of the way hydrogen bonding of water works, ice is less dense than liquid water. Because of the way hydrogen bonding of water works, ice is less dense than liquid water. –That’s why lakes and oceans don’t freeze solid every winter.

Water is a good solvent Water is a good solvent Lots of things dissolve in water. That means it is a good solvent. Lots of things dissolve in water. That means it is a good solvent. –The stuff that does the dissolving is called a solvent. –The stuff that gets dissolved is called a solute. –The combination of solvent and solute is called a solution.

The pH scale The pH scale pH is a measurement of how acidic or basic a solution is. pH is a measurement of how acidic or basic a solution is. The pH scale ranges from 0 (very acidic) to 14 (very basic). The pH scale ranges from 0 (very acidic) to 14 (very basic).

The pH Scale The dissociation of water The dissociation of water H 2 O  H + + OH - In water: In water: [H + ] = [OH - ]

The pH Scale Acid: Acid: –A substance that increases the H+ concentration of a solution [H + ] > [OH - ] Base: Base: –A substance that decreases the H+ concentration of a solution [H + ] < [OH - ]

Acids and bases An acid has a pH under 7. An acid has a pH under 7. –Anything that lowers the pH of a solution is an acid. A base has a pH above 7. A base has a pH above 7. –Anything that raises the pH of a solution is a base. Water has a neutral pH. Water has a neutral pH. –Water is neither acidic nor basic. –Water is a 7 on the pH scale.

Transpiration and evaporative cooling Transpiration and evaporative cooling Transpiration is when water vapor leaves a plant through its leaves. Transpiration is when water vapor leaves a plant through its leaves. Evaporative cooling is how sweating (perspiration) keeps us cool. Evaporative cooling is how sweating (perspiration) keeps us cool.