Properties of seawater

Properties of water 1.Polarity and hydrogen bonding cohesion good solvent many molecules dissolve in H 2 O 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 (SO 4 2- ): 7.7% – Magnesium (Mg 2+ ): 3.7% – Calcium (Ca 2+ ): 1.2% – Potassium (K + ): 1.1% – Bicarbonate (HCO 3 - ): 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 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 salinity 2. Evaporation - High rates of evaporation will increase salinity 3. Precipitation - High levels of rainfall will decrease salinity

Salinity is variable across the ocean Photo: NASA

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 Seawater Many gases are also dissolved in seawater including oxygen, carbon dioxide, and nitrogen Many gases are also dissolved in seawater including oxygen, carbon dioxide, and nitrogen Gases dissolve at the sea surface from the atmosphere (gas exchange) and vice versa. Gases dissolve at the sea surface from the 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 depth What do you think happens to each of the following with increasing depth? What do you think happens to each of the following with increasing depth? – Oxygen – Carbon dioxide – Temperature – Light – Pressure

Pressure Increases with Increasing Depth Pressure is also greater the deeper the depth. Pressure is also greater the deeper the depth. Water at the bottom has the weight of the water above it pushing down on it Water at the bottom has the weight of the water above it pushing down on it 1 atm = 14.7 lbs per square inch