Properties of Seawater

Divided into two categories:  Physical properties  Chemical properties  Both of these are important to our tanks and to marine life.

Physical Properties We’ll be looking at:  States of Matter  Heat and Temperature  Density  Light  Sound

Three States  Water is the only substance on Earth that exists in all three states.  Water can absorb a lot of energy before it changes state.  Helps keep the temps of the Earth stable, controls climate. H 2 O Vapor H 2 O Ice

The Water Cycle wrote large  329 million cubic miles of oceans is evaporated (& condensed & precipitated) each year.  The average water molecule stays in the ocean about 4100 years.

Heat and Temperature  Water has a very high heat capacity.  It takes 1 calorie to raise the temp of 1 gram of water by 1 degree C.  Water is very resistant to temp changes when heat is added or removed.

 By comparison, sand has a Heat Capacity of 0.2 cal/g˚C  This means that the sand will heat up five times faster than the water.  That’s why beaches can be too hot to stand on, but the water is nice and cool.

Ouch! Hot!

Temp Extremes Comparisons  Land  Highest Average: 122˚F  Lowest Average: -129˚F  Difference: 251˚F

 Water  Highest Average: 90˚F  Lowest Average: 29˚F  Difference: 61˚F

Density As you recall from Earth Science (especially if you had a really good Earth Science teacher), density is the mass per unit volume of a substance.

D = m/v Usually measured in g/ml. Density of pure water is 1.00 g/ml. “ I’m your Density.”

What changes density?  Temp: Density increases as the temps drop until it reaches 4˚C, when it starts to drop.  That’s why ice floats.

 Salinity: Raises density  1 L of seawater weighs about 3% more than 1 L of freshwater.

 Pressure: Higher pressure increases density.  Deeper you go, the denser the water.

Density Summary  Temp: T  = D , until 4˚C  Salinity: S  = D   Pressure: P  = D 

Light  Water scatters light as the light passes through it.  Red scatters the most.  Blue scatters the least.

Depth of light penetration  On average, light can penetrate the ocean to about 330 feet.  In the tropics, it’s about 2, 000 feet.  At coasts, it’s about 100 feet.

Sound  Sound travels at 3,345 mph in seawater.  That’s 5X the speed of sound in air.

Can you hear me now?  Sound travels fastest at warm surfaces and slows down as you get deeper.  At about 3, 300 feet, increasing density speeds it up again.

SOFAR layer  The layer at which sound moves slowest is called the SOFAR layer.  Sound here is trapped between the faster layer above and the faster layer below.

 This makes any sound produced in the SOFAR able to travel a long way.  The record is well over 3, 000 miles so far. SOFAR…so good! ;)

 Called SOFAR which is short for SOund Fixing And Ranging, because it was originally planned to be used as a way to find lifeboats.

In Oceanography Text Chapter 6 Ocean Water & Structure  Define these words from page 181.  Calorie  Heat Capacity  Refraction  Thermal Inertia  Thermostatic property  Answer Thinking Critically questions 1, 4 & 5 on page 181.

Chemical Properties We’ll be looking at:  Salinity  Dissolved Gases  pH  The Nitrogen Cycle

Salinity  Water is the best dissolving agent we know of.  It’s very, very good at dissolving salts.  Water dissolves salts best at warmer temps.

 Seawater is about 96.5% water and 3.5% dissolved stuff, mainly salts.  There are 5.5 trillion tons of salt in the ocean.  That’d cover the entire Earth to a depth of 45 feet.

 The ocean’s salinity varies from 3.3% to 3.7%.  So the average is…?  It’s normally measured in ppt, though, so the salinity is said to be 33 o / oo to 37 o / oo. Salinity (cont’d)

Types of Ions  Chloride Cl - is the most common, followed by sodium Na +.  Sulfates, magnesium, calcium, potassium and bicarbonates make up the rest.

Effects of Salinity on Water 4 Colligative Properties 1. Heat capacity decreases 2. Freezing point decreases 3. Evaporation slows 4. Osmotic pressure increases

Source of Salts Some comes from coastal erosion (esp. Na + ), but not all Underwater volcanoes (Cl - ) add others, while deposition and rock formation remove some

Forchhammer’s Principle Danish chemist Johan Georg Forchhammer, in 1865, at age 71, he noted that that even if the amount of salts is different in different parts of the world, the ratio of those salts is always the same. Principle of Constant Proportions AKA Principle of Constant Proportions

Simply put… If you have X mg of chlorine in a liter of seawater, you’ll always have 0.6X mg of sodium. So if you know the concentration of one ion you can determine the total salinity of the water. So if you know the concentration of one ion you can determine the total salinity of the water.

Residence Time This is the amount of time an ion, on average, stays in the ocean. The longest is Cl at 100, 000, 000 years. Fe is the shortest, at about 200 years.

Cl and ions like it are said to be conservative because they aren’t used by living things and cycle slowly. Fe and ions like it are said to be nonconservative because they are used by living things and cycle quickly.

Dissolved Gases Gases dissolve Gases dissolve better in cold water. In order of amounts: In order of amounts: N 2, N 2, O 2, O 2, CO 2 CO 2

Bubbles!!! Oxygen needs to be at a concentration of about 6 ppm to sustain life. Oxygen needs to be at a concentration of about 6 ppm to sustain life. Almost all the oxygen in the oceans (and on the entire Earth) comes from photosynthesis. Almost all the oxygen in the oceans (and on the entire Earth) comes from photosynthesis.

CO 2 moves quickly from the atmosphere to the ocean, but is trapped there by organisms using it for shells and skeletons.

pH Seawater is slightly alkaline with a pH of 7.8 – 8.4. Seawater is slightly alkaline with a pH of 7.8 – 8.4. Rapid photosynthesis, like in an algae bloom, increase pH. Rapid photosynthesis, like in an algae bloom, increase pH. Dead zone waters tend to have a build- up of CO 2, which lowers pH. Dead zone waters tend to have a build- up of CO 2, which lowers pH.

Nitrogen Cycle Fish and other marine organisms excrete waste, primarily in the form of ammonia (NH 3 ) and urea (CH 4 N 2 O). Nitrosomonas (nitrifying bacteria), through the process of nitrification, convert the ammonia and urea to nitrites (NO 2 - ).

The nitrites are converted to nitrates (NO 3 - ) by nitrobacter (another type of nitrifying bacteria). In the ocean, plants and algae take up the nitrates and convert them to proteins. Consumers eat the plants, excrete the nitrogen as wastes and the process starts all over again.

Nitrosomonas Nitrobacter Photosynthesis consumption Poo-poo

In Oceanography Text Chapter 7 Ocean Chemistry  Define 4 words (p 198)  Colligative properties  Conservative constituent  Residence Time  Salinity  Answer Thinking Critically questions 3, 4, 5, 6 & 7 from page 199.