Why hot water cannot hold oxygen greatly?

Question

High temperatures more than 85° F or 29.4° C in shallow ponds; very warm water does not hold enough oxygen. The warmer the water, the less dissolved oxygen it can hold. As the water temperature increases, the less oxygen it can hold. Why?

Answer 1

Because the water melecules are moving very fast

Answer 2

Oxygen particles need to be close together, as you probably know, heat makes molecules expand, therefore the molecules in hot water cannot hold the oxygen because it escapes through the gaps in big particles, In cold water it it fairly stable because the molecules are closer together, thus, holding the oxygen in tight.

Danny

Answer 3

Dissolving stuff is all about entropy.

For solids dissolving:

entropy is almost always favourable as the solid is highly ordered and is going to a less ordered state in solution (the enthalpy for the process can either be +ve (e.g. AgNO3)or -ve(e.g.LiCl)).

For gases it is the other ways round gases being very disordered have a high entropy and dissolving them reduces the entropy. The consequence is that the enthapy will always have to be favourable.

The overall interaction between entropy and enthalpy is given by the Gibb's free energy.

G = H -TS

so as the temperature increases G becomes less favourable for gases and more favourable for solids

Answer 4

Good question, Aziz.  Usually we tell students that as the temperature of a solution increases, the solubility decreases, and leave it at that.  But you ask why should the solubility decreases.

As a substance is dissolved in water it either absorbs heat (endothermic) or gives off heat (exothermic).  In the case of most gases, the process involves the attraction between the water molecules and and the gas molecules resulting in a lower energy and a release of energy as heat.  In fact, we can think of this as an equilibrium between the gas still in the gas phase above the water and the dissolved gas in solution.

gas + water <==> solution + heat

According to Le Chatelier's principle if we add heat to an exothermic system in equilibrium, the equilibrium will shift in a direction that minimizes the effect of the added heat.  That will be shifting the equilibrium to the left, which decreases the amount of gas in solution, and lowering the solubility.

If we carry this discussion on to solids dissolved in water, then we see that it requires energy from the water to separate the particles in a crystalline solid.  This is an endothermic process.  When the solution is saturated the particles in solution will be in equilibrium with the undissolved solid, and the equilibrium will be endothermic (heat is added).

solid + water + heat <==> solution

According to Le Chatelier's principle, adding heat to an endothermic equilibrium will shift the equilibrium to the right causing more of the solid to dissolve, and increasing the solubility as the temperature increases.

========= Follow up ==========

Be careful, Danny, when you say that " ... heat makes molecules expand."   Heat doesn't change the volume of a molecule.  Heat causes the molecules to move around faster and  that increases the volume occupied by a quantity of gas molecules.

What Danny is describing is the relationship between T and V when it is in the gas phase, but it doesn't work the same way when the gas is dissolved in water.  The volume of the dissolved gas will be the volume of the water.  The volume of the gas will increase because the volume of the water will increase as it expands due to the temperature increase.

Answer 5

please the reason is the question you posed and nothing else thanks