| 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: (Initial Observation)
Most solid chemicals need to be dissolved in water or solvents before they can be used. Knowing the best temperature for dissolving each specific chemical can help us save time and energy in the dissolving process. Scientists often perform their own tests in order to learn about the effect of temperature on the rate of dissolving and create a graph that can help them for future reference.
In this project, you will study the effect of temperature on the rate of dissolving any specific chemical in water. The chemical that you select can be among the chemicals that are low hazard and can be found at home. Salt, baking soda, citric acid, and sugar are some good examples. You may obtain other chemicals such as copper sulfate, Calcium chloride and lime locally. After performing necessary experiments you must create a graph to show the rate of dissolving in different temperatures.
Find out about dissolving as a molecular level interaction between solids and liquids. You may also learn about some related subjects such as the effect of polarity in dissolving and learn that ionic substances can only be dissolved in a polar liquid such as water and non-ionic substances can only be dissolved in non-polar liquids such as acetone. Read books, magazines or ask professionals who might know in order to learn about the methods that you may test the rate of dissolving in certain temperatures. Keep track of where you got your information from. The following are samples of information that you may find.
Ionic solids (or salts) contain positive and negative ions, which are held together by the strong force of attraction between particles with opposite charges. When one of these solids dissolves in water, the ions that form the solid are released into solution, where they become associated with the polar solvent molecules. Source…
Why is it important to know the temperatures that maximize solubility for each substance? Solid chemicals do not react on each other. With exception of explosives, all other chemical reactions are done with liquid chemicals. You first make a solution of each ingredient, react the solutions and then you separate the products by methods such as crystallization, filtration or precipitation. In all such reactions, we attempt to use the minimum amount of water because later we need to remove the excess water. Removing the excess water requires time, fuel and other costs associated with separation process. By knowing the temperature in which a substance has the highest solubility we can avoid excess water and related expenses in chemical reactions and minimize the production cost.
What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement. The purpose of this project is to learn how temperature affects the rate of dissolving of a certain solid in a certain liquid. More specifically I will examine the effect of temperature on the rate in which table salt or rock salt dissolves in water. Similar procedures can be used to determine the effect of temperature on the rate of dissolving for other solids and other liquids.
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other. The independent variable (manipulated variable) is the temperature. (0ºC to 100ºC) The dependent variable (responding variable) is the rate in which salt (a solute) dissolves in water (a solvent). Controlled variables are air pressure and other environmental factors. Constants are:
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis. Following is a sample hypothesis: The rate of dissolving salt in water will increase by temperature. My hypothesis is based on my personal observations that heat accelerates dissolving.
Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.” Experiment:Dissolving rate of salt in water at different temperatures. Introduction: The rate at which Rock salt dissolves in water at 11 different temperatures is observed. Procedure:
Your results table will look like this: Amount of salt dissolved in water at different temperatures
Use the temperature column and dissolving rate column of your results table to draw a line graph. Notes/ Remarks:
Make a graph: When your results table is ready, use it to make a bar graph. You will only use the first and the last columns of your table to draw a graph. In this way your graph will show the relation between the temperature and the solubility. Make one vertical bar for each of the temperatures you test. For example if you are testing four different temperatures, you will have four bars. The height of each bar represents the solubility rate at each specific temperature. For example you may use a 20″ bar for 20% solubility. Make sure you write the temperature below each bar. Also write the solubility rate above or over each bar.
Chemicals: Salt crystals available from local hardware stores as rock. Please only use clean and large crystals if possible. Equipment: 500-mL beakers chemical scoop or spoon Thermometer Scale Modification: Clear plastic cups may be substituted for beakers. If you only have one beaker instead of 11, do your experiments one by one and wash the beaker after each test. You can either use a food thermometer or a laboratory-glass thermometer.
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results. Your results table and graph will appear here.
You will need to calculate the ratio of salt to water for each temperature.
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments. It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did. Following is a sample conclusion. Increasing the temperature increases the rate of dissolving because, at higher temperatures, the solvent molecules are moving more rapidly and therefore come into contact with and solvate the solute molecules more rapidly. In Sodium Chloride or rock salt however; increase of solubility in higher temperatures is not much. As seen in the results table and graph, the solubility of salt in water increases from …….. to …….. when temperature changes from 0ºC to 100ºC. Later I found the following graph on the Internet that confirms my results.
The light blue color is for table salt or rock salt. As you see in some chemicals like calcium chloride, solubility increases a lot by increase in temperature. On the other hand for Cerium Sulfate solubility decreases by temperature.
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically. If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious. I noticed that the temperature will change slightly during the time that I am stirring the solution. To prevent such change, I could wrap the beaker in insulating material and reduce the stirring time from 5 minutes to 2 minutes. A blanket or a Styrofoam box can be used as insulator. Alternatively I could heat up or cool off the beaker to maintain the temperature.
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