Shipwrecks, Corrosion and Conservation Summary Slides PART 4 – Jack Dengate

The effect on solubility of gases and salts Temperature: Solubility of gases decreases as the temperature increases. The solubility of salts generally increases as the temperature rises. (There are a number of exceptions to this generalisation. The solubility of some salts remains the same as the temperature increases, while the solubility of others decreases.)

The effect on solubility of gases and salts Pressure: Solubility of a gas increases as the pressure of that gas in contact with the solution increases. The solubility of salts is largely unaffected by increasing the applied pressure.

Dissolved Gases The atmospheric gases most dissolved in the oceans are carbon dioxide, nitrogen and oxygen, in that order.

Dissolved Gases The concentration of most gases in the oceans is lower than their concentration in the atmosphere. Acidic carbon dioxide is an exception. The proportion of carbon dioxide in seawater when compared with oxygen and nitrogen is much higher than its proportion in the atmosphere when compared with the same two gases.

Dissolved Gases CO 2 molecules can react with water molecules to form ions which are more soluble in water. The hydrogen ions produced lower the pH of seawater. CO 2 (g) + H 2 O(l)  HCO 3 – (aq) + H + (aq)

Dissolved Gases The rate of gas movement from the atmosphere, the use and production of gases by living organisms, and chemical reactions can all affect the amount of gas present at lower depths. The concentration of oxygen in seawater is greatest near the surface because wave action increases contact between water and air. The concentration then decreases to a minimum at depths between 500m to 1000m because of the respiration of marine organisms. Below 1000m the oxygen concentration steadily increases as the depth increases.

Great Depth Corrosion Water at the bottom of oceans will be about 4 0 C because salt water has its maximum density at this temperature. Water colder than this is of lower density and will rise upwards. The rate of corrosion of metals will decrease as the temperature decreases. (You will need to draw on your knowledge of the effect of temperature on the rate of reactions to make this prediction).

Great Depth Solubility The solubilities of all gases tend to increase with depth because of the increasing pressure. Carbon dioxide reacts with water to form hydrogen carbonate ions and carbonate ions leading to increased solubility when compared with oxygen and nitrogen. CO 2 (g) + H 2 O(l) ⇌ H 2 CO 3 ⇌ H + + HCO 3 ⇌ 2H + + CO 3 2– As ocean depth increases the pH of the water changes from basic (pH 8) to neutral to acid. The increased solubility of acidic CO 2 with depth is the main cause of the drop in pH.