How does the temperature of water affect the amount of dissolved oxygen it contains?

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THE EFFECTS OF TEMPERATURE ON DISSOLVED OXYGEN:

PURPOSE:

How does the temperature of water affect the amount of dissolved oxygen it contains?

Variable Temperatures (oC): 5, 10, 15, 20, 25, 30, 35.

HYPOTHESIS:

The higher the temperature of the water, the less dissolved oxygen it will contain.

INTRODUCTION:

Dissolved oxygen (microscopic bubbles of oxygen gas in aqueous solution, DO) is essential for the healthy functioning of all freshwater ecosystems. If more oxygen is consumed than is produced, DO levels decline and some sensitive animals may move away, weaken and even die. Therefore, DO presence in an ecosystem is a positive sign whereas low DO levels are an indication of severe pollution.

Gases are usually more soluble at colder temperatures. For example, oxygen is more soluble in cold water than in hot water. The decrease in oxygen solubility with increased temperature has serious consequences for aquatic life. Power plants that discharge hot water into rivers can kill fish by decreasing the dissolved oxygen concentration. With global industrialization, the conditions of the world have been altered drastically in a short time. Pollutants have been introduced, rivers have been diverted, and forests have been cut down. In general, the higher the DO levels, the more stable the freshwater ecosystem and thus the greater the capacity for this ecosystem to sustain life. Much of the DO in water comes from the atmosphere. Streams with a high kinetic energy and a tumbling water action promote the mixing of atmosphere oxygen, a non-polar compound, with water, a polar compound.

A high DO level in a community water supply is good because it makes drinking water taste better. However, high DO levels speed up corrosion in water pipes. For this reason, industries use water with the least possible amount of dissolved oxygen.

DO levels in streams fluctuate significantly during the day, especially if the freshwater ecosystem supports extensive plant life. DO levels are at their lowest during the early morning, tend to rise during the day and peak in the afternoon. Temperature has a significant influence on DO levels. The concentration of DO in natural water and wastewater is a function of the temperature of the air and water, the degree of hardness of the water, and the demand for oxygen in the body of water. The solubility of oxygen increases with decreasing water temperature (oxygen solubility in water is inversely proportional to temperature).

If you were to put the presence of DO into an everyday layman context then it would prove useful to observe a pot of water being heated. One can notice that bubbles form on the walls of the pot prior to reaching the boiling point. These cannot be filled with only water vapour because liquid water will not begin to vaporize until it has reached its boiling point. One can assume that this gas is oxygen, or at least a mixture of gases from the air, because bubbles of this sort form in water from virtually every source. When these bubbles form, they eventually grow to a sufficient size to leave the surface of the pot and escape to the air and therefore the dissolved gas in the liquid has decreased. This seems to support my hypothesis that dissolved oxygen will decrease when temperature is increased.

PROCEDURE:

When testing for dissolved oxygen, the concerns for safety involve water hazards and exposure to chemicals.

Precautions to follow include:

1. Cover all abrasions and if possible, wear good quality latex gloves.

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2. Wash hands frequently and always wash hands prior to eating.

3. Wear a protective smock, apron or lab coat, and surgical or rubber gloves when working in the laboratory to protect clothes and skin.

4. Read all labels carefully and know what to do in case of a spill.

The WINKLER TEST is used to determine the concentration of dissolved oxygen in water samples. An excess of manganese (II) salt, iodide (I-) and hydroxide (OH-) ions is added to a water sample causing a white precipitate of Mn(OH)2 to form. This precipitate is then oxidized by the dissolved oxygen in the ...

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