Seawater - Fun Facts Etc.

Question 1

At what temperature (C) does water reach it’s maximum density?

Answer 1

At 3.98 degrees C (Caldwell 1978)

Question 2

How much greater is the volume of ice (solid fase of water) compared to the maximum density of liquid water?

Answer 2

Appr. 10 % greater, which is why ice floats in water.

Question 3

How cold does a lake have to be for surface freezing to occur?

Answer 3

The entire lake must be cooled to appr. - 4 degrees C for freezing to begin

Question 4

What is density in Ocean salt water a function of?

Answer 4

Temperature, Salinity and Pressure.

That means that the temperature at which the maximum density of salt water occurs decreases with both Salinity and Pressure.

Question 5

What is the specific heat capacity of water and why is it so high?

Specific means relative to mass

Answer 5

It takes 1 calorie to heat 1 gram of water by 1°C per definition.

1 calorie equals 4.180 joules/g/°K

Specific heat capacity varies slightly with temperature and pressure.

The specific heat capacity for ethanol is 0.58 calories/g/°C. This difference is caused by the stronger hydrogen bonds in water.

Question 6

What does the high specific heat capacity of water mean for the transport of energy in Ocean currents?

Answer 6

Oceans are slow to warm and slow to cool. That means that large amounts of heat energy can be transported poleward from the tropics in Ocean currents.

Question 7

How much heat must be added to force water evaporation?

Answer 7

2257kJ/kg or 540 calories/g

Question 8

How much heat must be removed to allow ice formation?

Answer 8

334kJ / kg or 80 calories/g

Question 9

What is the boiling point of water?

Answer 9

100 °C at 1 atmosphere pressure

Question 10

What is the boiling point of water at 2000 m depth (200 atmospheres)?

Answer 10

330 °C

E.g. in deep sea thermal vents. This explains why magma-heated water can emerge without exploding into steam.

Question 11

Does water evaporate at sub-boiling temperatures?

Answer 11

Yes, and the evaporation is more rapid when the temperature difference between air and water is bigger.
This is how OceanS, lakes puddles, wet sand, plant transpiration all pump water vapor into the atmosphere and create clouds which enhance reflection of sunlight into space and which cause rainfall that varies geographically and seasonally and from year to year.

Question 12

Who worked out the proportions of the “conservative ions” in seawater? And when?

Answer 12

Recognised by Forchhammer in 1864 and carefully analysed by William Dittmar in 1884. The latter did this using samples collected from the world’s oceans on the Challenger Expedition (1873-1876).

Question 13

Conservative ions in salt water that vary and why they vary

Answer 13

Calcium content varies somewhat with depth, due to dissolution of shells made from CaCO3 under high pressure.

Bicarbonate varies with the amount of CO2 in solution (Ocean acidification)

Question 14

How can Salinity be measured in salt water?

Answer 14

Either by determining the any one of the dissolved ions and then by calculating the rest of them. The proportions of the major salts are near-constant.
Chloride can be measured using a silver nitrate titration.

You can also measure the overall electrical conductivity of the water.

Question 15

What is the overall average of Ocean Salinity and how is it expressed?

Answer 15

The overall average of Ocean Salinity is expressed on the Practical Salinity Scale, a number that relates itself to the grams of salt per kg of seawater.

The average is S = 35.

Question 16

What are the major dissolved ions I’m seawater? (And bonus for knowing the amount g/kg)

Answer 16

Cations: 
Na+  10.78
Mg2+ 1.28
Ca2+ 0.41
K+      0.40
Sr2+  0.008

Anions:
Cl-          19.35 g/kg
SO4 2-    2.71
HCO3 -   0.126
Br-           0.067
B(OH)4 3-   0.026
F-            0.001

Question 17

Ions that vary non-conservatively

Answer 17

Nitrate NO3- can be taken up by photosynthesizing algae and bacteria.
It varies from almost unmeasurable in the surface layers of oligotrophic central gyres to 45 mikroMolar in the deep North Pacific.

Question 18

Cells and tissue fluid of much marine life is isosmotic with seawater. What does that mean?

Answer 18

It means that the solute and water concentrations are the same inside and outside their cells.

Question 19

What problem do freshwater plants and animals have that marine organisms usually don’t? How do they solve the problem?

Answer 19

They have a constant influx of water through any porous cell surface due to the difference in osmolarity and therefore have to actively pump out water to avoid cell rupture and death.
They need some salts and dissolved organic matter in their cells to function.

Protists have specialised organelles for this while metazoans have kidneys at several levels of complexity to perform this function for the body as a whole.

Question 20

Fish evolution and osmosis. Where did fish evolve? And how do they osmoregulate in this original habitat?

Answer 20

Fish evolved in freshwater. The scales and skin is impermeable and the only place an influx of water can take place is over the gill membranes which obviously need to be exposed to the water for gas exchange.
They have very efficient kidneys, which pump out the excess water.

Question 21

What happened when fish colonized other habitats?

Answer 21

Fish colonized estuaries and the ocean and the osmotic problem was reversed, now water was moving out of the gills instead of in.
Several solutions evolved:

Sharks and rays came to tolerate large tissue concentrations of urea, giving them osmotic equivalence with the sea.

Bony fishes developed a system based on swallowing water and excreting excess salts both via the kidneys and from desalination glands on the gills.

Fish that move between salt and freshwater must shift between the modes of swallowing water and excreting salt and constant peeing. E.g. salmon, shad, eels.

Question 22

Many seabirds lose water over their lungs. How do they handle osmoregulation?

Answer 22

They drink to replace water lost and excrete salts through glands in their nostrils.

Question 23

How do marine mammals handle osmoregulation?

Answer 23

They do not have many cell membranes exposed to the water. They avoid drinking. Instead they retain water from their prey and from their metabolic reactions.
The kidneys are specialized to manage the balance of tissue electrolytes (salts)

Question 24

What pH is seawater in the surface buffered at? What works as a buffer?

Answer 24

Between 7.9 and 8.4.

The near-surface average is 8.1

Buffers are carbonate (95 % of the buffer effect) and borate components.

Question 25

What does σ t describe? (Sigma t)

Answer 25

A measure of seawater’s density if it is brought to the surface without heat exchange or salinity exchange, just decompression.

σ t = 1000 (ρ -1)

ρ= actual density, often a number like 1.02437 for which σ t = 24.37

Sigma t is shorthand for the modest but critically important variations of density due to salinity and temperature, not depth.