Question

Helium is added to the oxygen supply used by deep sea divers because
A. It is less soluble in blood than nitrogen at high pressure
B. It is lighter than nitrogen
C. It is readily miscible with oxygen
D. It is less poisonous than nitrogen

Answer 1

Hint: Scuba divers while going down into the ocean, and the external pressure on their bodies rises. To balance this it is crucial to raise the pressure of the air they breathe from tanks so that their chests and lungs do not fail.

Complete Step by Step Solution:
Scuba divers carry a tank containing the air that they breathe and whose pressure must be raised when they go down into the ocean. This tank contains a mix of nitrogen and oxygen. But our bodies don't resort to pressurised air as we generally respire air under normal atmospheric conditions.

Due to high air pressure in the lungs gases such as nitrogen and oxygen become increasingly soluble in the blood. Oxygen is metabolised but nitrogen builds up throughout the body.

When divers want to rise from the water, they do so by coming to the surface level too fast, numerous bubbles of nitrogen form like a pressurised bottle of cold drink are suddenly emptied. When nitrogen gas forms bubbles, it collects and saturates the muscles and blood, resulting in pain which is called bends.

Helium is added to the breathing mixtures to decrease the proportions of nitrogen and oxygen so that it is less than air.
- It permits the gas mixture to be inhaled and exhaled safely on deep dives at increased pressure.
- It is less soluble in the blood and hence does not accumulate much as nitrogen giving less threat to divers and is thus utilised for deep dives.
So, Helium is added to the oxygen supply used by deep sea divers because it is less soluble in blood than nitrogen at high pressure.
So, option A is correct.

Note: This works according to Henry's Law which states that with the increase in the pressure, the solubility of gas increases. So, nitrogen from the compressed air remains in the bloodstream and other tissues. For deep diving, along with Bends, surplus nitrogen can lead to diminished mental function which is called nitrogen narcosis.