Show The major component of a solution is called the solvent. The minor component of a solution is called the solute. By major and minor we mean whichever component has the greater presence by mass or by moles. Sometimes this becomes confusing, especially with substances with very different molar masses. However, here we will confine the discussion to solutions for which the major component and the minor component are obvious. Solutions exist for every possible phase of the solute and the solvent. Salt water, for example, is a solution of solid NaCl in liquid water; soda water is a solution of gaseous CO2 in liquid water, while air is a solution of a gaseous solute (O2) in a gaseous solvent (N2). In all cases, however, the overall phase of the solution is the same phase as the solvent.
A solution is made by dissolving 1.00 g of sucrose (C12H22O11) in 100.0 g of liquid water. Identify the solvent and solute in the resulting solution. Solution Either by mass or by moles, the obvious minor component is sucrose, so it is the solute. Water—the majority component—is the solvent. The fact that the resulting solution is the same phase as water also suggests that water is the solvent. Test Yourself A solution is made by dissolving 3.33 g of HCl(g) in 40.0 g of liquid methyl alcohol (CH3OH). Identify the solvent and solute in the resulting solution. Answer solute: HCl(g); solvent: CH3OH One important concept of solutions is in defining how much solute is dissolved in a given amount of solvent. This concept is called concentration. Various words are used to describe the relative amounts of solute. Dilute describes a solution that has very little solute, while concentrated. describes a solution that has a lot of solute. One problem is that these terms are qualitative; they describe more or less but not exactly how much. In most cases, only a certain maximum amount of solute can be dissolved in a given amount of solvent. This maximum amount is called the solubility of the solute. It is usually expressed in terms of the amount of solute that can dissolve in 100 g of the solvent at a given temperature. Table 11.2 “Solubilities of Some Ionic Compounds” lists the solubilities of some simple ionic compounds. These solubilities vary widely: NaCl can dissolve up to 31.6 g per 100 g of H2O, while AgCl can dissolve only 0.00019 g per 100 g of H2O.
When the maximum amount of solute has been dissolved in a given amount of solvent, we say that the solution is saturated with solute. When less than the maximum amount of solute is dissolved in a given amount of solute, the solution is unsaturated. These terms are also qualitative terms because each solute has its own solubility. A solution of 0.00019 g of AgCl per 100 g of H2O may be saturated, but with so little solute dissolved, it is also rather dilute. A solution of 36.1 g of NaCl in 100 g of H2O is also saturated but rather concentrated. Ideally, we need more precise ways of specifying the amount of solute in a solution. We will introduce such ways in Section 11.2 “Quantitative Units of Concentration”. In some circumstances, it is possible to dissolve more than the maximum amount of a solute in a solution. Usually, this happens by heating the solvent, dissolving more solute than would normally dissolve at regular temperatures, and letting the solution cool down slowly and carefully. Such solutions are called supersaturated solutions and are not stable; given an opportunity (such as dropping a crystal of solute in the solution), the excess solute will precipitate from the solution. It should be obvious that some solutes dissolve in certain solvents but not others. NaCl, for example, dissolves in water but not in vegetable oil. Beeswax dissolves in liquid hexane but not water. What is it that makes a solute soluble in some solvents but not others? The answer is intermolecular interactions. The intermolecular interactions include London dispersion forces, dipole-dipole interactions, and hydrogen bonding (as described in Chapter 10 “Solids and Liquids”). From experimental studies, it has been determined that if molecules of a solute experience the same intermolecular forces that the solvent does, the solute will likely dissolve in that solvent. So, NaCl—a very polar substance because it is composed of ions—dissolves in water, which is very polar, but not in oil, which is generally nonpolar. Nonpolar wax dissolves in nonpolar hexane but not in polar water. This concept leads to the general rule that “like dissolves like” for predicting whether a solute is soluble in a given solvent. However, this is a general rule, not an absolute statement, so it must be applied with care.
Would I2 be more soluble in CCl4 or H2O? Explain your answer. Solution I2 is nonpolar. Of the two solvents, CCl4 is nonpolar and H2O is polar, so I2 would be expected to be more soluble in CCl4. Test Yourself Would C3H7OH be more soluble in CCl4 or H2O? Explain your answer. Answer H2O because both experience hydrogen bonding
More Tutorials Join AUS-e-TUTE Contact Us Want chemistry games, drills, tests and more? You need to Join AUS-e-TUTE! Key Concepts⚛ A homogenous mixture is usually called a solution. ⚛ A substance is said to be soluble if it dissolves in a solvent to form a solution. ⚛ A substance is said to be insoluble if it does not dissolve in a solvent to form a solution. ⚛ The substance that dissolves in a solvent to form a solution is referred to as the solute. ⚛ A solution is formed when solvent particles completely surround solute particles which are then uniformly dispersed. · When a solute dissolves in water, the solution is referred to as an aqueous solution. ⚛ The amount of solute present in the solution is referred to as the concentration of the solution. ⚛ The terms dilute and concentrated refer to the relative concentrations of two or more solutions: · A dilute solution has less solute present in the solution than a concentrated solution. ⚛ A solution is said to be unsaturated if more solute could be dissolved in a given amount of the solvent at a specified temperature. ⚛ A solution is said to be saturated when no more of the solute can be dissolved in a given amount of the solvent at a specified temperature. ⚛ The solubility of a solute in a given solvent refers to the maximum amount of solute that can be dissolved in that solvent at a given temperature. Please do not block ads on this website. Types of SolutionsIf we consider solutions made up of only two components, known as binary solutions, we can see that there are 9 possible combinations:
The table below gives an example of each type of solution.
The most common binary solutions you will use in your chemistry course will be aqueous solutions. An aqueous solution is one in which water is the solvent. The solute, the substance that dissolves in the liquid water solvent may be a solid, liquid or gas. ⚛ solid solute, place (s) after the formula for the solute: for example NaCl(s) ⚛ liquid solute, place (l) after the formula for the solute: for example C2H5OH(l) ⚛ gaseous solute, place (g) after the formula for the solute: for example NH3(g) In a chemical equation, an aqueous solution is indicated by placing aq in round brackets, (aq), after the formula of the solute. Examples of aqueous solutions are shown in the table below:
↪ Back to top Solvation: Dissolving a Solute in a SolventDissolving a solute in a solvent can be thought of a two step process:
The process in which solute particles are completely surrounded by solvent particles is known as solvation. If we pour a small amount of liquid ethanol, C2H5OH(l), into a large amount of liquid water, H2O(l), the attractive forces between the ethanol molecules are broken and each ethanol molecule is attracted to water molecules so that water molecules completely surround each ethanol molecule. Because the molecules making up the solution are in constant motion, the ethanol molecules become evenly dispersed through the solution resulting in an homogenous aqueous solution. We could represent a small amount of ethanol dissolving in excess water as:
When chemists write a chemical equation to show the process of dissolving a small amount of solute in excess water, the water molecules are often omitted from the equation because the presence of water molecules as the solvent is implied when an aqueous solution is formed.
However, the states of matter for the solute and the solution MUST be shown. ↪ Back to top Concentration of SolutionsThere are two main ways to express the concentration of a solution:
Note that in either case the quantity of solute is always expressed first. 1. Concentration of a solution is often expressed in terms of the quantity of solute present in the total quantity of solution. 2. Concentration of a solution can also be expressed as the quantity of solute dissolved per unit of solvent.
↪ Back to top Dilute and Concentrated SolutionsThe terms dilute and concentrated refer to the relative concentrations of two or more solutions. When comparing two or more binary solutions made up of the same solute and solvent:
If the solute is coloured, when it dissolves in the solvent the resulting solution will be coloured. For example, copper sulfate is a blue colour. When copper sulfate dissolves in water, the blue copper(II) ions are evenly distributed throughout the solution so that the solution appears to be a uniform blue colour. The more blue copper(II) ions there are present in solution, the more concentrated the solution is, and the deeper the blue colour. The fewer blue copper(II) ions there are present in the solution, the less concentrated the solution is, and the lighter the blue colour. ↪ Back to top Saturated and Unsaturated SolutionsA solution is said to be unsaturated if more solute could be dissolved in a given quantity of the solvent at a specified temperature. A solution is said to be saturated when no more of the solute can be dissolved in a given quantity of the solvent at a specified temperature. The solubility of copper sulfate in water at 25°C is known to be 14 grams per 100 grams of water (14 g/100 g of water). This means that a maximum of 14 grams of copper sulfate can be dissolved in 100 grams of water at 25°C. Imagine adding 7 grams of solid copper sulfate to 100 g of water in a beaker at 25°C. All the copper sulfate would dissolve and the solution would appear uniformly blue. We can add another 7 grams of solid copper sulfate to this solution and all of this copper sulfate will also dissolve resulting in a solution with an even deeper blue colour. But, if we add another 7 grams of copper sulfate to this solution, the copper sulfate will not dissolve because the solution has become saturated in copper sulfate. The colour of the solution will not get any darker and the excess solid copper sulfate will sink to the bottom of the beaker. Unsaturated Solution 7 grams of copper sulfate ≈ 2.7 x 1022 copper sulfate particles Number of copper sulfate particles dissolved in 100 g of water Solution is unsaturated because, if more copper sulfate is added to the solution some (if not all) of it will dissolve. Saturated Solution (a) 14 grams of copper sulfate ≈ 5.3 × 1022 copper sulfate particles Number of copper sulfate particles dissolved in 100 g of water Solution is saturated because if any more copper sulfate is added to the solution it will NOT dissolve. Saturated Solution (b) O = 4.4 × 1021 copper sulfate particles Black box represents the volume of 100 grams of water at 25°C
Number of copper sulfate particles dissolved in 100 g of water Solution is saturated because some copper sulfate has not dissolved in the solution. ↪ Back to top Sample Question: Introduction to SolutionsWhich solution is more dilute? Back to top |
What is formed when the maximum amount of solute is dissolved in a given solvent of a mixture?
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