Properties of seawater

Properties of water

1.Polarity and hydrogen bondingcohesion

good solvent many molecules dissolve in H2O

2. lower density as a solid ice floats!

3.high heat capacity water stores heat heats & cools slowly

Salinity is an important part of ocean chemistry

1. Salinity is a measure of the amount of dissolved salts in water

2. Salinity is not homogenous (uniform) across the Earth’s oceans

3. Both salinity and temperature affect the density of seawater

Seawater is a mixture of pure water and chemical compounds

On average, Seawater is 96.5% pure

water…

…and 3.5% compounds including dissolved salts

Remember from chemistry that when salts dissolve they form ions

Cl-Na+

Sodium Chloride(A salt)

Water

Na+ Cl-

Sodium ion Chloride ion

What’s in the water?

7 primary chemicals make up almost all (~99%) the salts in seawater:

– Chloride (Cl-): 55%– Sodium (Na+):  30.6%– Sulfate (SO4

2-):  7.7%– Magnesium (Mg2+):  3.7%– Calcium (Ca2+): 1.2%– Potassium (K+):  1.1%– Bicarbonate (HCO3

-): 0.4% Can you come up with an acronym to remember them

all?

How do scientists figure out how much salt is in the water?

The Rule of Constant Proportions = the major ions of seawater are present in a fixed proportion to each other– This means that although salinity may vary, the

ratio of any one of the 7 primary ocean salts to each other will not change

– If we have a liter of seawater that has a 35 grams of total salt per liter, how many grams of calcium are there?

Salinity is a measurement of how much salt is dissolved in the water

Common unit of salinity is Parts per thousand (PPT)

What does ppt really mean?– Imagine you have a bag of 1000 m&ms – Blue m&ms represent pure water– Red m&ms represent salts– To represent 35ppt (average salinity of seawater), 35

m&ms would be red and 965 would be blue.

The Composition of Seawater at 35ppt SalinityThe Composition of Seawater at 35ppt Salinity

Factors that influence salinity

What factors do you think might influence salinity?

Three primary factors influence salinity:1.  Freshwater input - High rates of freshwater input (river inflow to the sea; melting ice) will decrease salinity2.  Evaporation - High rates of evaporation will increase salinity3.  Precipitation - High levels of rainfall will decrease salinity

Salinity is variable across the ocean

Ph

oto

: N

AS

A

Why is salinity important?

Salinity is one factor that controls the density of ocean water

What happens when water at different depths has different densities?– Layers of water will form– Formation of layers is part of the reason we have

ocean currents

Gases in SeawaterGases in Seawater

Many gases are also dissolved in seawater including Many gases are also dissolved in seawater including oxygen, carbon dioxide, and nitrogenoxygen, carbon dioxide, and nitrogen

Gases dissolve at the sea surface from the Gases dissolve at the sea surface from the atmosphere (gas exchange) and vice versa.atmosphere (gas exchange) and vice versa.

Gases dissolve better in cold water than in warm.Gases dissolve better in cold water than in warm.

Transparency

Transparency is an important property of seawater. Why do you think this is?Sunlight can

penetrate into water Different

wavelengths travel to different depths

Conditions vary with depthConditions vary with depth

What do you think happens to each of the What do you think happens to each of the following with increasing depth?following with increasing depth?– OxygenOxygen– Carbon dioxideCarbon dioxide– TemperatureTemperature– LightLight– Pressure Pressure

Pressure Increases with Pressure Increases with Increasing DepthIncreasing Depth

Pressure is also greater Pressure is also greater the deeper the depth.the deeper the depth.

Water at the bottom has Water at the bottom has the weight of the water the weight of the water above it pushing down above it pushing down on iton it

1 atm = 14.7 lbs per square inch