4A_3 – Slide 1

Topic 04A:Atoms, Molecules, and

Seawater ChemistryPart III

Online Lecture:Temperature & Heat

4A_3 – Slide 2Solids, Liquids, Gases, and Temperature

○ If atoms or molecules move too far apart, electrical attraction may weaken enough for them to escape one another (for the bond to break).

○ If you wanted to transform liquid water into water vapor (a gas), what would you do?

– higher temperature = faster-movingmolecules, whichmove far enough apart to break their bonds

Heat itRockets &“escape velocity”

4A_3 – Slide 3Different “Phases” of Water

Solid LiquidGas

How is the behavior of the molecules different in each phase?

Bonds?Speed?

“Serial Monogamy”“No Long-TermRelationships”“Monogamy”

4A_3 – Slide 4Latent Heat being added and removed

Evaporating =liquid to gas

Melting =solid to liquid

Condensing = gas to liquid

Which oneis missing?

4A_3 – Slide 5Latent Heat I: Causes Substances to “Transform”

○ Latent Heat is the heat that must be:

– added to make atom/molecules move fasterand get far enough apart to break their bonds● transform from solid → liquid or liquid → gas

– lost to make atoms/molecules move slowerand get close enough together to form bondswith one another● transform from gas → liquid or liquid → solid

What is gaining heat?What is losing heat?Are bonds breaking

or forming?

Water evaporates off your skin.

A raindrop forms in the atmosphere.

added

lost

4A_3 – Slide 6Latent Heat II: It is Hard to Make Water Transform

○ Water has high latent heats compared to other substances. In other words, it takes a lot of heat to get water to transform, owing to the unusually strong bonds between molecules.

○ Liquid water is necessary for life:

– solid = no change = no life●water bonds more weakly than atoms

– gas = no connections, everything flies apart = no life ●water bonds more strongly than other molecules

“Goldilocks:”bonds aren’ttoo strong or

too weak

Oxygen is a gas at room temperature, while salt is a solid.

4A_3 – Slide 7The Water Cycle

It is important that water

can & does “transform.”The ocean

is the source of

most of our fresh water!

4A_3 – Slide 8

○ If an object gets warmer:

– faster-movingatoms/molecules

– atoms/molecules can get farther apart before electrical attraction pulls them back together

(the bonds stretch, but do not break)

●object gets larger

●object’s density decreases

Thermal Expansion/Contraction

Warm Atoms Cold Atoms

4A_3 – Slide 9Exceptional Ice

○ Water expands when it is heated and contracts when it is cooled except when it freezes:– Ice takes up more space than liquid water,

because of the “V”-shape of water molecules:●hydrogen bonds form between the O and H atoms

of different water molecules, but only in specificdirections, a very inefficient use of space

so it has a lower density

and floats!

unlike molecules in a solid, molecules in a liquid can bond in a variety of

orientations & slide past one another

EmptySpace

4A_3 – Slide 10Salt and Freezing

○ the freezing point of seawater is about 2oC (3.5oF)lower than the freezing point of fresh water

– salts atoms “get in the way”when seawater freezes,don’t fit into the ice crystal (cannot bond in the correct directions)

– oceanographers say that the salt is “rejected”by the ice:● the sea ice is

mainly fresh water● the nearby ocean water

becomes saltier

Roads

You can safely drink frozen seawater!

4A_3 – Slide 11Afternoon at the Beach

Think about a hot afternoon at the beach:Both the sand and the water have received

the same amount of heat from the Sun.Which is warmer, the sand or the water?

Hot Sand

CoolWater

4A_3 – Slide 12Heat Capacity I

○ If water gains a lot of heat, its temperature will change very little (compared to other substances).

○ If water loses a lot of heat, its temperature will change very little (compared to other substances).

– water “holds” more heat than other substances at the same temperature (because it took more heat for them to get to that temperature)

→ we say water has a high heat capacity

Sand vs. Water,Concrete vs. Grass,

and Pizza

There are many ways for atoms/molecules to move, but only some motions are effective at breaking the bonds that hold them together.

4A_3 – Slide 13Heat Capacity II

○ In what way is the climate “better”near the coast than farther inland?

Why is the climate “better?”

Heat Capacity & Latent Heat - What’s the Difference?

Not Too Hot, Not Too Cold

Very Hot in Summer,Very Cold in Winter

4A_3 – Slide 14Diffusion

○ In liquids and gases the molecules are moving from place to place, bumping and colliding and flying off again at random, then the molecules will tend to mix andspread out (tends to “even out” differences)– we call this process diffusion

Phytoplankton and Nutrients

Freshwater and Saltwater Fish

Keep adding drops of dye: same thing happens→ the molecules never stop moving

No stirring!The water is “still.”

4A_3 – Slide 15Atoms & Molecules Never Stop Moving

○ Atoms and molecules never stop moving:always wiggling in place or traveling from place to place.

○ Everyday objects “come to a stop,” because when 2 objects rub together, their atoms or molecules bump into one another. The objects slow down, but their atoms go faster (are heated).

– “organized” motion (all atoms & molecules going in the same direction) becomes “disorganized” motion

– the atoms & molecules never lose their motionMoving Forward Comes to a Stop

e.g., rub your hands together

4A_3 – Slide 16Heat Flows from Hot Objects to Cold Objects

○ Heat flows from hot objects to cold objects, because the atoms (or molecules) of the objects are colliding with one another.

○ In most collisions:

– the faster-moving (“warmer”) molecule tends to slow down (get “cooler”)

– the slower-moving (“cooler”) molecule tends to speed up (get “warmer”)

“bumping into”“hitting”

O

O

O

O

Faster ("Warmer")

Slower("Cooler")

Slowed Down("Cooler")

Speeded Up("Warmer")

#1#1

#2#2

4A_3 – Slide 17Understanding Phenomena in Terms of the Atomic Theory of Matter

○ If your answer does not mention atoms and/or molecules,then you probably do not have a very good answer.

○ Describe what are the atoms and/or molecules are doing,and explain why:

– bonding or electrical attraction:●breaking bonds? forming bonds?

– motion:●speed: slow or fast?●wiggling in place or moving around (traveling)?●colliding/bumping into one another?

Need todiscuss at leastone of these!

Don’t say…