6.1.4. Equilibrium Involving Dissolution of Solid or Gases in Liquids

We know from our experience that we can dissolve only a limited amount of salt or sugar in a given amount of water at room temperature. If we make a thick sugar syrup solution by dissolving sugar at a higher temperature, sugar crystals separate out if we cool the syrup to the room temperature. We call it a saturated solution when no more of solute can be dissolved in it at a given temperature. The concentration of the solute in a saturated solution depends upon the temperature. In a saturated solution, a dynamic equilibrium exists between the solute molecules in the solid state and in the solution: Sugar (solution) ⇌ Sugar (solid), and the rate of dissolution of sugar = rate of crystallisation of sugar.

When we try to dissolve substances like sugar in water, there is a limit to how much can dissolve at a given temperature, leading to a point where the solution becomes saturated. At this point, the solid sugar and the dissolved sugar molecules are in a state of dynamic equilibrium. This means that sugar molecules are continually dissolving into the solution and solidifying back into sugar crystals at equal rates, keeping the total amount of dissolved sugar constant. This equilibrium illustrates that the system is active and balanced, with no net change in concentration.

Think of making lemonade. When you add sugar to the water, it starts to dissolve. If you keep adding sugar after a while, it won't dissolve anymore because the water can't hold any more sugar at that temperature, just like a sponge unable to absorb more water once it's full. If you heat the lemonade, more sugar can dissolve; however, if you cool it down again, some of that sugar will crystallize back out, demonstrating the concept of dynamic equilibrium.

When a soda water bottle is opened, some of the carbon dioxide gas dissolved in it fizzes out rapidly. The phenomenon arises due to difference in solubility of carbon dioxide at different pressures. There is equilibrium between the molecules in the gaseous state and the molecules dissolved in the liquid under pressure i.e., CO2 (gas) ⇌ CO2 (in solution). This equilibrium is governed by Henry’s law, which states that the mass of a gas dissolved in a given mass of a solvent at any temperature is proportional to the pressure of the gas above the solvent.

When you open a bottle of soda, the pressure inside is released, causing the carbon dioxide gas, previously dissolved in the liquid, to escape rapidly as bubbles. This process is an example of gas dissolving in a liquid reaching equilibrium. Henry's law helps explain this process, which tells us that the concentration of dissolved gas in a liquid is directly proportional to the pressure exerted by that gas in the atmosphere above the liquid. As the pressure drops when the bottle is opened, the dissolved gas molecules cannot be retained in the solution and escape as bubbles.

This is similar to shaking a can of soda. When you shake it, the pressure builds up inside the can. When you finally open it, all that accumulated gas escapes quickly as fizz. Just like a sponge that can hold more water under pressure, the soda can hold more carbon dioxide when under pressure, but releases that gas when the pressure is released.

We can generalize that: (i) For solid-liquid equilibrium, there is only one temperature (melting point) at 1 atm at which the two phases can coexist. If there is no exchange of heat with the surroundings, the mass of the two phases remains constant. (ii) For liquid-vapour equilibrium, the vapour pressure is constant at a given temperature. (iii) For dissolution of solids in liquids, the solubility is constant at a given temperature. (iv) For dissolution of gases in liquids, the concentration of a gas in liquid is proportional to the pressure (concentration) of the gas over the liquid.

These general characteristics describe the behavior of different types of solubility equilibria. For solid-liquid equilibria, a specific temperature allows the solid to exist together with its dissolved form without losing mass. In the case of liquid-vapour equilibria, the pressure exerted by the vapour remains constant for a particular liquid and temperature. When dealing with solids dissolving in liquids, the maximum amount of solute that can dissolve remains constant at a designated temperature. Conversely, for gases, their solubility in liquids varies directly with the pressure of the gas above the liquid, illustrating that changes in pressure affect how much gas can dissolve.

For instance, if you take a pot of water and heat it to boil, you will see the water vaporize into steam. The steam represents the liquid-vapour equilibrium while the temperature of the water remains constant until it is fully boiled. Similarly, you can think of an experiment where we dissolve sugar in water; it stops dissolving when it reaches a particular saturation point, resembling a limitation in solubility reflecting these equilibria.

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