Chapter 6.6 Measuring Concentration and Solubility of Solutions PDF

Measuring the Concentration and Solubility of Solutions 6.6 Concentration When a solid dissolves in a liquid, the liquid that does the dissolving is called the solvent. The solid th...

Measuring the Concentration and Solubility of Solutions 6.6 Concentration When a solid dissolves in a liquid, the liquid that does the dissolving is called the solvent. The solid that dissolves is called the solute. For example, in a solution of orange-drink crystals and water, the water is the solvent and the orange-drink crystals are the solute (Figure 1). Figure 1 The drink on the right has more solute than the drink on the left. How can you tell? Have you ever made a drink by dissolving drink crystals in water and found that it tasted watery? This happens when you do not have the right concentration of solute in the solvent. Concentration [kon-suhn-TRAY-shun] is the amount of solute that is dissolved in a given quantity of solvent or solution. Solutions that are made with the same substances may contain different amounts of each substance. A solution with a low concentration of solute is said to be dilute [die-LOOT]. A solution LEARNING TIP with a higher concentration of solute is said to be more concentrated. The key vocabulary words For example, lemonade with a small amount of dissolved sugar is a on this page are illustrated. If you are having trouble more dilute solution than lemonade with a lot of dissolved sugar. with these vocabulary The lemonade with more sugar is a more concentrated solution. words, look at Figure 1 for It tastes sweeter than the more dilute solution. clarification. NEL 6.6 Measuring the Concentration and Solubility of Solutions 153  LEARNING TIP Solubility Make a web to show what you already know about You can make orange drink because the orange-drink crystals substances that dissolve dissolve in water. Another way to say that a substance dissolves in in water. water is to say that it is soluble in water. Can you think of some other substances that are soluble in water? If the orange-drink crystals did not dissolve in water, you would not be able to make the drink. An insoluble substance is a substance that does not dissolve. Can you think of some substances that are insoluble in water? Can a substance that is insoluble in water be dissolved in another solvent? TRY THIS: OBSERVE DIFFERENT SOLVENTS Skills Focus: predicting, observing, classifying Predict whether salt, sugar, butter, and wax will dissolve in water and in ethanol. Now try to dissolve each of these solutes in the two different solvents (Figure 2). Record your results. Observe Different Solvents Dissolves in water? Dissolves in ethanol? salt sugar butter wax Figure 2 Adding sugar to ethanol to see if the sugar dissolves 154 Unit B Chemistry NEL Saturated and Unsaturated Solutions Even if a substance is soluble in a solvent, there is usually a limit to how concentrated the solution can become. For example, there is a limit to how many orange-drink crystals you can dissolve in a glass of water. Imagine that you add more and more drink crystals to a glass of water, stirring constantly. Eventually, the drink crystals will just stay at the bottom of your glass (Figure 3). The drink solution will not be able to dissolve any more drink crystals because it is saturated with them. A solution is saturated with a solute when no more of the solute can be dissolved in it. A solution is unsaturated with a solute when more of the solute can be dissolved in it. Figure 3 How much sugar do you think can be dissolved in this lemonade? The ability of a substance to dissolve in a solute is called solubility. You can measure the exact amount of solute that is required to form a saturated solution in a certain solvent at a certain temperature. Temperature is important because you can generally dissolve more solute in warm water than in cold water. NEL 6.6 Measuring the Concentration and Solubility of Solutions 155  Solubility is different for each combination of solute and solvent. The amounts of different solutes that are needed to saturate a certain volume of solvent varies enormously. For example, more sugar than salt is needed to saturate 100 mL of water at room temperature (20°C) (Table 1). Table 1 Solubilities of Common Substances in Water Solute Temperature (°C) 0 20 50 baking soda 6.9 g/100 mL 9.6 g/100 mL 14.5 g/100 mL table salt 35.7 g/100 mL 36.0 g/100 mL 36.7 g/100 mL sugar 179 g/100 mL 204 g/100 mL 260 g/100 mL TRY THIS: DISSOLVE SOLUTES Skill Focus: predicting, observing 1. Make a saturated solution of water and salt by stirring small amounts of salt into about 100 mL of water until no more salt will dissolve. 2. Now that the water is saturated with salt, do you think you will be able to dissolve anything else in the water? Make a prediction. 3. Test your prediction by trying to dissolve sugar in your saturated salt solution. Supersaturation A very few solid solutes can be used to create a solution that is more than saturated. A solution that contains more of the solute than can be found in a saturated solution is called a supersaturated solution. You can make a supersaturated solution by starting with a saturated solution at high temperature and then allowing the solution to cool slowly. Normally, as a solution cools, the solute particles lose energy. Some of the solute particles draw together and form the crystal pattern of the solid. In a supersaturated solution, the solute particles are not able to get into a crystal pattern. As a result, the solution remains liquid even when it is at a temperature at which it would normally be a solid. 156 Unit B Chemistry NEL  If the supersaturated solution is not disturbed, all the solute may LEARNING TIP remain dissolved. If you strike the container lightly with a stirring Pause and think. Ask rod or a spoon, however, the resulting vibrations may cause some of yourself, “What did I just read? What did it mean?” the solute particles to move into a crystal pattern. Immediately, the Try to reword the rest of the extra solute will fall out of solution and join the crystal. information on You can produce a similar effect by adding a seed crystal of the supersaturation in solute for the excess solute particles to build on (Figure 3). your own words. Figure 3 Adding a seed crystal causes the rapid formation of crystals in a supersaturated solution of sodium acetate. CHECK YOUR UNDERSTANDING LEARNING TIP Locate the information 1. Identify the solute and the solvent in needed to answer these the photo to the right. questions by scanning the 2. List two liquid solutions that do not text for key words. contain water. 3. Suppose that you add one teaspoon of sugar to your cup of tea. A friend adds four teaspoons of sugar to his cup of tea. Whose tea is a more concentrated sugar solution? 4. Is the solubility of all solutes the same? Explain. 5. Describe how you can tell the difference between an unsaturated solution and a saturated solution. NEL 6.6 Measuring the Concentration and Solubility of Solutions 157