World wide, is water in short supply?

In Oregon, is water in short supply?

How much water do you use on a daily basis

National average is ~80 gal per day (300 liters) for consumption and hygiene only

Water limited regions?

The chemistry of the water molecule

What is the chemical formula for water?

What is the formula weight for water?

What type of bond holds the atoms of the molecule together?

Hydrogen Bonding

Some molecules with hydrogen are strongly polar

+ side of molecules are attracted to - side of molecules

This attraction is known as a hydrogen bond

moderate in strength but can control properties of molecule

Properties of water due to hydrogen bonding

high specific heat

high surface tension

expands when frozen

low density of ice

Why is water unique?

Has very unique chemical properties

Expands when it freezes…why?

Expansion of water due to freezing…

Causes solid-form to be less dense than liquid form.

H2O Liquid: 1g/cm3 Iron Liquid: 7.23g/cm3

H2O Solid: 0.8g/cm3 Iron solid: 7.87g/cm3

What are some other effects?

How do fish survive in winter when lakes freeze?

Water becomes dense as it cools, drops to bottom of lake…water same

temp. from top to bottom

Top layer freezes and Ice is now less dense than water and insulates water

below

Water has a greater density than many other liquids

Oil/ chemical spills

Difficult to clean up due to surface mobility

Water doesn’t vaporize easily

Why do you think this is?

Important biologically for humans--> keeps us cool

low heat of vaporization--> remove perspiration before it could cool us

down

Has a high surface tension

Why?

Water is considered the universal solvent

Why does it dissolve many things so easily?

Water takes a lot of energy to heat

Because of Hydrogen bond, it requires a lot of energy input to change its temperature.

Water has a high specific heat

heat needed to raise the temperature of one gram of something by 1o C (J/g • oC)

What is the specific heat of water?

If we have 10 grams of water and it takes 41.84J of heat to increase the temperature

1oC, what is the specific heat of water?

Your Turn…

Use the following information and determine which of the following has the highest specific heat?

To raise the temperature of 25g of copper 1^oC it requires 9.625J of heat

To raise the temperature of 15g of ice 1^oC it requires 30.45J of heat

To raise the temperature of 21g of iron 1^oC it requires 9.45J of heat

Based on your calculations, how does water compare to iron, copper and ice?

Based on the specific heat values you determined, calculate how much heat would be

required to raise the temperature of 100g of each item by 1^oC

What happens when water is heated and why does it take so much energy.

Addition of heat--> what happens atomically

molecules absorb energy and start vibrating

hydrogen bonds begin to loosen

solid turns to liquid

molecules continue to vibrate

break hydrogen bonds and create steam

Breaking of hydrogen bonds and phase changes require large amounts of energy.

Specific heat, Oceans and temperature

Oceans act as heat reservoirs on Earth

helps to drive weather systems on earth

provides a constant source of heat and maintains a reliable temperature variation on

earth

Why doesn’t it snow very often at the coast?

What is the difference between heat and temperature?

Three different temperature scales

Fahrenheit

Celsius

Kelvin

How are these scales different from each other?

Fahrenheit

First scale created (early 1700’s)

Created by Gabriel Fahrenheit

Arbitrary points assigned to freezing and boiling of water

Water freezes at 32^o and boils at 212^o

Celsius Scale

Developed in 1749

Created by Anders Celsius

Water freezes at 0^o and boils at 100^o

Arbitrary points

Used by rest of the world (not USA)

Kelvin

Scale is based on the concept of absolute zero

coldest temperature possible

limiting factor--> absolute zero is approachable but not attainable

calculated by extrapolating experimental results

Kelvin has NO UNITS

Kelvin scale was Scientifically derived gas pressure vs. temperature

What happens when gas is heated?

Gas expands--> pressure increases

What happens when gas is cooled?

Gas contracts--> pressure decreases

Point where pressure equals zero is where T=O

this point is -273.2^oC

Comparing Celsius and Fahrenheit

both scales based on the freezing and boiling of water

1^oC is larger than 1^oF (100^oC spans 180^oF)

ratio of degrees F to degrees C is 9:5 or 9/5

the size of a degree F is 5/9 the size of a degree C

We can use this information to convert from one to the other

Converting Celsius to Fahrenheit and vise versa

T_c = 5/9 (T_f - 32^o)

T_f = ( 9/5 T_c ) +32^o

ex//

Convert 85^oC to Fahrenheit

Convert 85^oF to Celsius

Conversion to Kelven

Tk = Tc + 273.2

ex//

convert 35^o C to Kelvin

convert 35^o F to Kelvin

Thermodynamics vocabulary

Thermal contact: if heat can flow between two objects those objects are in thermal

contact

Thermal equilibrium: two objects are at the same temperature and heat ceases to flow

Two basic concepts

Heat always flows from warmer to cooler objects if they are in thermal contact

Heat transfer will stop when the temperature of the two objects are the same

How is heat transferred?

Radiation

heat travels as waves

often takes the form of infrared radiation (remember the electromagnetic

spectrum?)

heat can travel through a vacuum with this mechanism

Convection

motion of a fluid carrying heat energy

driven by density differences

often creates convection cells or heat cells

mechanism of transfer for ANY fluid (including air)

Conduction

heat transfer through a solid medium

transfer of heat on an atomic level through vibration

no physical movement of medium

Exercise:

What type (s) of heat transfer is occurring in each of the following situations

A hot iron rod sitting in a fire?

The energy of the sun heating up the ocean?

Your morning cup of coffee cooling down while it sits on a counter?

What happens when water is heated up

temperature of the water begins to rise

molecules begin moving

when T reaches 100^oC bubbles forms

water begins to convert to vapor

What happens to the temperature in this situation?

Temperature will remain at 100^oC till all the liquid is converted to water vapor

When all the water is converted, the temperature beings to rise again

The change from liquid to vapor is known as a phase change

What happens when liquid water is cooled?

Temperature of water drops

water molecules slow

when T reaches 0^oC ice begins to form

Temperature response?

T will remain at 0^oC until all the liquid is converted to ice

when all the liquid is converted the temperature will drop again