Homework answers / question archive /  Lab: Solutions Solutions 1

 Lab: Solutions Solutions 1

Sociology

Share With

 Lab: Solutions Solutions 1.0 Introduction Solutions are homogeneous mixtures of two or more components. One component, which dissolves the other one, is called the solvent and the component that is dissolved is called the solute. This distinction is most clear when a solid solute dissolves in a liquid solvent, such as when salt dissolves in water. When solid solutes dissolve in liquid solvents we call them soluble. If a solid does not dissolve in a particular solvent, we call it insoluble. When ionic solids dissolve, cations and anions form in solution since the ionic crystal must dissociate in order to dissolve. Covalent compounds often dissolve without forming ions since they are molecular. A simple way to test for ionic or covalent solutes is to check the solution for conductivity. A conductive solution indicates the presence of ions and the dissolved compound is then called an electrolyte. A solution that has no conductivity indicates that the solute is covalent since no ions have formed. When liquid solutes mix with liquid solvents, they are said to be miscible. Examples of this would be vinegar and water or alcohol and water. Gases are also said to be miscible in each other when they form solutions. When two liquids do not mix, such as oil and water they are said to be immiscible. In this case, layers will form with the less dense layer on top. Gases are always miscible since there is so much space between the gas particles. In the cases of liquid-liquid and gas-gas mixtures it is not always easy to identify the solvent and solute. Often the solvent is considered to be the component that is most abundant, but other times this distinction is not possible. Consider as an example, a mixture of 10 mL of water with 10 mL of alcohol or 2 L of O2 mixed with 2 L of N 2. Two terms often confused when describing the properties of solutions are clear and colorless. A solution is clear when you can see through it, but it might be colored. Consider filtered apple juice. Often what we call “clear” is both clear and colorless such as a salt water solution. The opposite of clear is often cloud or turbid. One event that often happens with solutions is the formation of precipitates. A precipitate is the formation of one state from another. Thus, when weather people talk about precipitation they are referring to a liquid forming from a gaseous solution – rain from the atmosphere. When a chemist refers to precipitation, it is most often a reference to a solid forming from a liquid. In notebooks this can be abbreviated as “ppt”. The formation of a precipitate is often an indication of a chemical reaction. Another possibility is that a supersaturated solution is re-crystallizing, such happens in one type of hand warmer. Often a precipitate dissolving also indicates a chemical reaction might be taking place, but once again, it pays to carefully observe the conditions. Did the precipitate dissolve as the temperature was raised as when a hand warmer is boiled to recharge it? Or did the precipitate form as another chemical was added to the solution? When solids dissolve in liquid solvents, there is both the rate of solution and the amount of solute that dissolves to consider. The amount that dissolves is referred to as the solubility and is often given in grams per 100 mL. The solubility of solid solutes is affected by the nature of the solvent particularly its polarity or lack of it and the temperature of the solvent. The rate at which the solution forms is affected by mixing and the surface area of the solute particles. It is important to remember that these factors do not change the solubility but rather the rate of solution formation. However, temperature has an effect on both the solubility and the rate of Page: 1 of 5 192/410 Lab: Solutions solution formation. Higher temperatures usually favor a faster solution formation and a higher solubility. A solution that has reached maximum solubility is said to be saturated. Occasionally, a solution can contain more than the normal maximum amount of solute in which case it is said to be supersaturated. This latter condition is not stable and the solute is subject to begin precipitating at any moment. Consider sugar crystallizing from recipes as they cool as an example of this behavior. 2.0 Procedure I. Rate of Solution Remember: The rate or speed at which a substance dissolves is not necessarily an indication of the degree or amount of its solubility. A. Effect of temperature on the rate of an endothermic solution process. Fill a 250 mL beaker about half full with tap water. Get one spoonful of table salt, note the time and quickly add the spoonful of salt to the beaker. Mix the solution until the crystals are dissolved. Record the time (seconds) required for the solute to completely dissolve. Discard the salt water down the drain. Rinse the beaker well and refill with the same volume of tap water. Place the beaker with water in a microwave and heat for 1 minute. Again note the time and add one spoonful of salt to the now hot water. Mix the solution until the crystals are dissolved and record the time (seconds) required for the solute to completely dissolve. Discard the salt water down the drain. Seconds required for the salt to dissolve Cold water Hot water What can you conclude about the rate of solution and the temperature of the solvent? Will this be true for all solutions? B. Effect of surface area on the rate of solution. Place equal amount of table salt and rock salt in 2 separate beakers of the same size. Fill two 250 mL beakers half full of tap water. At the same instant add the water into the table salt and rock salt beakers. Mix the solutions simultaneously by swirling. Which dissolves faster? Why? The rate of dissolving is also affected by the rate of stirring. How and why is this true? Page: 2 of 5 192/410 Lab: Solutions II. Nature of Solute and Solvent A. Put about 3 mL deionized water into one test tube and 3 mL of oil into another. Drop a crystal of I2 into each tube and shake. Observed the degree to which iodine dissolves in each solvent as evidenced by the color of the liquid. What is the polarity of I2? ______________ Now use the principle of “like dissolves like” to indicate the relative polarity of each solvent. Water is _________________ and the oil is __________________ Now pour the contents of one test tube into the other test tube and mix. Let the mixture settle. What do you observe? Which is denser oil or water? B. What do you predict would happen if you used NaCl as the solute instead of I2 in oil and in water? Why? Try it. Was your prediction correct? C. What about peanut butter? Try dissolving a small spatula of peanut butter first with water and then with oil in a plastic cup. Results? D. Clean up: ? Place oily test tubes into appropriate waste container ? Dispose of the plastic cups of peanut butter/water/oil in the regular trash can Page: 3 of 5 192/410 Lab: Solutions III. Solution Conductivity A. Put about 3 mL of water in a test tube and add a spatula of unknown 1 and stir to dissolve. Now check the conductivity of the solution. Is the solute covalent or ionic? Why? B. Put about 3 mL of water in a test tube and add a spatula of unknown 2 and stir to dissolve. Now check the conductivity of the solution. Is the solute covalent or ionic? Why? IV. Saturated vs Unsaturated Do a test to see if unknown 3 is a saturated or unsaturated salt solution. What did you find and how did you determine this? V. Precipitation A. Place a very small amount of Ca(NO3)2, calcium nitrate, in a test tube. Add enough water to dissolve the solid. (Stir with a clean stirring rod to facilitate dissolving.) Would this solution conduct electricity? ___________ SAVE THIS SOLUTION. B. Repeat the procedure above with a small amount of Na2CO3, sodium carbonate. Would this solution conduct electricity? ___________ SAVE THIS SOLUTION C. What happens when you mix these two solutions together? Did a chemical reaction occur? Using the two reactant compounds write an equation for the reaction: What is the most likely formula of the precipitate? Page: 4 of 5 192/410 Lab: Solutions Solution Concentration Problems Show your work. A. How many grams of solute are required to make 100.0 g of a 4.5% (w/w) solution? B. How many mL of a liquid solute are necessary to make 300.00 mL of a 5.00% (v/v) solution? C. How many grams of solute are required to make 5.0 mL of a 20.00% (w/v) solution? D. How many grams of NaCl, sodium chloride, would be needed to make 100.0 mL of a saline solution that is 0.5 M?