Solubility Curves

Lesson Plan - Get It!

Audio:

When there is high humidity outside, it can feel muggy and sticky.

Humidity is based on how much water can be found in the air. High humidity means that the air is saturated with water vapor. Water vapor is found floating in the air, much like when solutes are added to solvents to create a chemical solution. The water vapor would represent the solute, and the air would be the solvent. In chemical solutions, water is the universal solvent because it can dissolve so many things!

Before you continue on, if you missed or would like to review the previous two lessons in our Mixtures and Solutions series, find them in the right-hand sidebar under Related Lessons.

Solutions can be unsaturated, saturated, or supersaturated, based on the amount of solute dissolved per unit volume of solvent.

• In an unsaturated solution, more solute can be dissolved. The solvent is not full of solute, like a warm cup of tea ready for a spoonful of sugar.

• A saturated solution can hold no more solute per unit volume of solvent. If extra solute is added to the solution, it will sink to the bottom of the cup and sit undissolved. This can happen if you add too much sugar to your tea.

• A supersaturated solution has more solute than the solvent can hold. This happens when the solution is heated to increase the amount of solute that can be dissolved. When solute is added to a supersaturated solution, the solute precipitates out of the solution as a solid. The solid solute particles sit at the bottom of the cup.

Watch Supersaturated Solution, from dchummer, to observe how this process occurs. Sketch an image of what occurs when solute is added to a supersaturated solution:

The solubility of different substances can be visualized in a solubility curve. Solubility curves compare the solubility values for substances at different temperatures for the same solvent. Most solubility curves are presented with water as the solvent, because water can dissolve so many substances. The temperature is graphed on the x-axis, and the solubility for most substances increases as the temperature increases. Notice how each substance is graphed in a different color in the image below. This allows you to analyze the solubility of many substances at once:

_{Image by TheKiteGuy, via Wikimedia Commons, is available under the CC0 1.0 Universal Public Domain Dedication.}

Solubility curves can be used to determine if a solution is unsaturated, saturated, or supersaturated. Take sodium chloride, the pink line, in the graph above. When 40 grams of sodium chloride are dissolved into 100 grams of water, a saturated solution is formed. This is the maximum amount of solute the solution can hold at 100 degrees Celsius. If you added 50 grams of sodium chloride, you would create a supersaturated solution. If you only added 30 grams of sodium chloride, it would be an unsaturated solution. An easy way to remember it is in the context of the solubility line.

• If the amount falls on the line, it is saturated solution.
• If above the line, the solution is supersaturated,
• while below the line represents an unsaturated solution.

Each compound and substance has a unique solubility value.

• Did you know that gases actually decrease in solubility as the temperature increases?

Solutions can be unsaturated if there is less solute than the solvent can hold. Supersaturated solutions have more solute than the solution can hold, and the solute can even precipitate out as a solid. Solubility curves can be used to communicate how solutes can dissolve into solvents at different temperatures.

• How do you think the temperature of the air would impact the amount of water that could be dissolved?
• How might this impact the humidity in an area?

In the Got It? section, you will analyze a solubility curve using an interactive.