What happens when maximum amount of solute is dissolved in a given amount of solvent at room temperature?

Solubility

The definition of solubility is the maximum quantity of solute that can dissolve in a certain quantity of solvent or quantity of solution at a specified temperature or pressure (in the case of gaseous solutes). In CHM1045 we discussed solubility as a yes or no quality. But the reality is that almost every solute is somewhat soluble in every solvent to some measurable degree.

As stated in the definition, temperature and pressure play an important role in determining the degree to which a solute is soluble.

Let's start with temperature:

For Gases, solubility decreases as temperature increases (duh...you have seen water boil, right?) The physical reason for this is that when most gases dissolve in solution, the process is exothermic. This means that heat is released as the gas dissolves. This is very similar to the reason that vapor pressure increases with temperature. Increased temperature causes an increase in kinetic energy. The higher kinetic energy causes more motion in the gas molecules which break intermolecular bonds and escape from solution. Check out the graph below:

As the temperature increases, the solubility of a gas decreases as shown by the downward trend in the graph.

For solid or liquid solutes:

CASE I: Decrease in solubility with temperature:

If the heat given off in the dissolving process is greater than the heat required to break apart the solid, the net dissolving reaction is exothermic (See the solution process). The addition of more heat (increases temperature) inhibits the dissolving reaction since excess heat is already being produced by the reaction. This situation is not very common where an increase in temperature produces a decrease in solubility. But is the case for sodium sulfate and calcium hydroxide.

CASE II: Increase in solubility with temperature:

If the heat given off in the dissolving reaction is less than the heat required to break apart the solid, the net dissolving reaction is endothermic. The addition of more heat facilitates the dissolving reaction by providing energy to break bonds in the solid. This is the most common situation where an increase in temperature produces an increase in solubility for solids.

The use of first-aid instant cold packs is an application of this solubility principle. A salt such as ammonium nitrate is dissolved in water after a sharp blow breaks the containers for each. The dissolving reaction is endothermic - requires heat. Therefore the heat is drawn from the surroundings, the pack feels cold.

The effect of temperature on solubility can be explained on the basis of Le Chatelier's Principle. Le Chatelier's Principle states that if a stress (for example, heat, pressure, concentration of one reactant) is applied to an equilibrium, the system will adjust, if possible, to minimize the effect of the stress.  This principle is of value in predicting how much a system will respond to a change in external conditions.  Consider the case where the solubility process is endothermic (heat added). An increase in temperature puts a stress on the equilibrium condition and causes it to shift to the right.  The stress is relieved because the dissolving process consumes some of the heat. Therefore,  the  solubility  (concentration)  increases  with  an  increase  in  temperature.    If  the process is exothermic (heat given off). A temperature rise will decrease the solubility by shifting the equilibrium to the left.

Now let's look at pressure:

Solids and liquids show almost no change in solubility with changes in pressure. But gases are very dependent on the pressure of the system. Gases dissolve in liquids to form solutions. This dissolution is an equilibrium process for which an equilibrium constant can be written. For example, the equilibrium between oxygen gas and dissolved oxygen in water is O2(aq) <=> O2(g). The equilibrium constant for this equilibrium is K = p(O2)/c(O2). The form of the equilibrium constant shows that the concentration of a solute gas in a solution is directly proportional to the partial pressure of that gas above the solution. This statement, known as Henry's law, was first proposed in 1800 by J.W. Henry as an empirical law well before the development of our modern ideas of chemical equilibrium.

Henry's Law:

Sg stands for the gas solubility, kH is the Henry's Law constant and Pg is the partial pressure of the gaseous solute.

Table: Molar Henry's Law Constants for Aqueous Solutions at 25oC

The inverse of the Henry's law constant, multiplied by the partial pressure of the gas above the solution, is the molar solubility of the gas. Thus oxygen at one atmosphere would have a molar solubility of (1/756.7)mol/dm3 or 1.32 mmol/dm3. Values in this table are calculated from tables of molar thermodynamic properties of pure substances and aqueous solutes

Summary of Factors Affecting Solubility

Normally, solutes become more soluble in a given solvent at higher temperatures. One way to predict that trend is to use Le Chatelier's principle. Because DHsoln is positive for most solutions, the solution formation reaction is usually endothermic. Therefore, when the temperature is increased, the solubility of the solute should also increase. However, there are solutes that do not follow the normal trend of increasing solubility with increasing temperature. One class of solutes that becomes less soluble with increasing temperature is the gasses. Nearly every gas becomes less soluble with increasing temperature.

Another property of gaseous solutes in summarized by Henry's law which predicts that gasses become more soluble when their pressures above a liquid solution are increased. That property of gaseous solutes can be rationalized by using Le Chatelier's principle. Imagine that you have a glass of water inside of a sealed container filler with nitrogen gas. If the size of that container were suddenly halved, the pressure of nitrogen would suddenly double. To decrease the pressure of nitrogen above the solution (as is required by Le Chatelier's principle), more nitrogen gas becomes dissolved in the glass of water.

Introduction
Have you ever added a spoon of sugar to your tea and wondered why it disappeared? Where did it go? The sugar did not actually disappear—it changed from its solid form into a dissolved form in a process called chemical dissolution. The result is a tea–sugar solution in which individual sugar molecules become uniformly distributed in the tea. But what happens if you increase the amount of sugar that you add to your tea? Does it still dissolve? In this activity you will find out how much of a compound is too much to dissolve.

Background
Chemistry is the study of matter and how it behaves and interacts with other kinds of matter. Everything around us is made of matter, and you can explore its properties using common chemicals around your home. The way it behaves is called a property of matter. One important property is called solubility. We think about solubility when we dissolve something in water or another liquid. If a chemical is soluble in water, then the chemical will dissolve or appear to vanish when you add it to water. If it is not soluble, or insoluble, then it will not dissolve and you will still see it floating around in the liquid or at the bottom of the container.

When you dissolve a soluble chemical in water, you are making a solution. In a solution the chemical you add is called the solute and the liquid that it dissolves into is called the solvent. Whether a compound is soluble or not depends on its physical and chemical properties. To be able to dissolve, the chemical has to have the capability to interact with the solvent. During the process of chemical dissolution, the bonds that hold the solute together need to be broken and new bonds between the solute and solvent have to be formed. When adding sugar to water, for example, the water (solvent) molecules are attracted to the sugar (solute) molecules. Once the attraction becomes large enough the water is able to pull individual sugar molecules from the bulk sugar crystals into the solution. Usually the amount of energy it takes to break and form these bonds determines if a compound is soluble or not.

Generally, the amount of a chemical you can dissolve in a specific solvent is limited. At some point the solution becomes saturated. This means that if you add more of the compound, it will not dissolve anymore and will remain solid instead. This amount is dependent on molecular interactions between the solute and the solvent. In this activity you will investigate how much of various compounds you can dissolve in water. How do you think sugar and salt compare?

Materials

• Distilled water
• Measuring cup that measures milliliters
• Eight glasses or cups that each hold eight ounces
• Four spoons
• Measuring spoon
• Epsom salts (150 grams)
• Table salt (50 grams)
• Table sugar (cane sugar, 250 grams)
• Baking soda (20 grams)
• Scale that measures grams
• Marker
• Masking tape
• Paper
• Pen
• Thermometer (optional)

Preparation

• Using the marker and masking tape label two cups for each compound: “table salt,” “table sugar,” “baking soda” and “Epsom salts.”
• Into one table salt cup measure 50 grams of salt.
• Into one table sugar cup measure 250 grams of sugar.
• Into one baking soda cup measure 20 grams of baking soda.
• Into one Epsom salts cup measure 150 grams of Epsom salts.
• For each cup weigh it and write down the mass (weight).
• Add 100 milliliters of distilled water into each cup. Use the measuring cup to make sure each cup has the same amount of water. The water should be at room temperature and the same for all cups. You can use a thermometer to verify that.

Procedure

• Take both of the cups you labeled with table salt. With the measuring spoon carefully add one teaspoon of table salt to the 100 milliliters of distilled water.
• Stir with a clean spoon until all the salt has dissolved. What do you notice when you add the salt to the water?
• Keep adding one teaspoon of salt to the water and stirring each time, until the salt does not dissolve anymore. What happens when the salt does not dissolve anymore?
• Repeat these steps with both cups labeled Epsom salts. At what point does the Epsom salts solution become saturated?
• Repeat the steps with the baking soda. How many teaspoons of baking soda can you dissolve in the water?
• Repeat the steps with the sugar. Did you add more or less sugar compared with the other compounds?
• Put each of the cups containing the remaining solids onto the scale and write down the mass (weight) of each one. How much of each substance did you use?
• Subtract the measured mass from your initial mass (see Preparation) for each compound. What does the difference in mass tell you about the solubilities of each of the compounds? Which compound is the most or least soluble in distilled water?
• Extra: Does the solubility change if you use a different solvent? Repeat the test, but instead of using distilled water use rubbing alcohol, vegetable oil or nail polish remover as solvent. How does this change your results?
• Extra: Can you find other substances or chemicals that you can dissolve in distilled water? How do their solubilities compare with the compounds you have tested?
• Extra: Solubility of compounds is also highly dependent on the temperature of the solvent. Do you think you can dissolve more salt or sugar in hot or cold water? Test it to find out!

Observations and results
Did all of your tested compounds dissolve in distilled water? They should have—but to different extents. Water in general is a very good solvent and is able to dissolve lots of different compounds. This is because it can interact with a lot of different molecules. You should have noticed sugar had the highest solubility of all your tested compounds (about 200 grams per 100 milliliters of water) followed by Epsom salts (about 115 grams/100 milliliters) table salt (about 35 grams/100 milliliters) and baking soda (almost 10 grams/100 milliliters).

This is because each of these compounds has different chemical and physical properties based on their different molecular structures. They are all made of different chemical elements and have been formed by different types of bonds. Depending on this structure it is more or less difficult for the water molecules to break these bonds and form new ones with the solute molecules in order to dissolve them into a solution.

Cleanup
You can dispose of each of your solutions in the sink. Keep the water running for a while afterward to flush your sink properly. Dispose of all remaining solids in the regular trash. Wash your hands with water and soap.

More to explore
Saturated Solutions: Measuring Solubility, from Science Buddies
Salty Science: How to Separate Soluble Solutions, from Scientific American
Solubility Science: How to Grow the Best Crystals, from Scientific American
Science Activity for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

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