What is a solution?



SolutionsHonors ChemistryWhat is a solution?Back at the beginning of the school year we learned about types of matter. Solutions were one of the types we studied. Let’s see if we can remember. If you have sugar water, Italian dressing and distilled water, which is the solution? If you chose sugar water you were correct! But what makes sugar water the right choice? Why isn’t Italian dressing a solution? Or distilled water? If you don’t remember, take a minute to go back in your notes (or book) and find out.A solution contains two parts, the solvent and the solute. The solute is the thing being dissolved, while the solvent is the substance doing the dissolving. An example would be salt water. Salt is the solute and the water is the solvent. Typically, we think of solutions as being in a liquid state (and that is the majority of what we will study in this chapter), but that is not always the case. To be a solution, a substance must fit three criteria: It must be a combination of two or more pure substances, it must be able to vary in concentration, and its parts must be indistinguishable. Although it is not a liquid, a typical gold ring is a solution; it is a combination of gold and other metals. (This gives it extra strength.) As you may know, there are various types of gold jewelry, 10K, 14K or 24 K, based on the amount of gold in the mix. When you look at a ring, you cannot distinguish the gold atoms from the other atoms. The following are also solutions: carbonated beverages (before they are opened), air, and steel. Think about each of these in terms of the criteria to be sure you understand why they are solutions.Preparing SolutionsThere are several terms related to solutions you must be aware of. These are saturated, unsaturated and supersaturated.Let’s look at these terms in terms of cold beverages. If we are making iced tea, we put about 1-2 tablespoons of sugar in the glass. This will completely dissolve. We might put in another tablespoon or two, which will also dissolve. This is because the tea we prepared was unsaturated. It could hold more solute than I had placed in it. What if we keep adding sugar? Say we place sugar into the glass until no more can dissolve, and some is sitting on the bottom, undissolved, like sludge. Now we have saturated the solution. Saturated means that no more solute can be dissolved. But what if we really want to have more sugar in it? What if that just isn’t sweet enough? Then we want to make a supersaturated solution, one that holds more solute than it should be able to. How do we do that? It turns out that solubility is temperature dependent. This means that in most cases, increasing the temperature will increase how much we can dissolve. Let’s use some numbers to make this clear. At room temperature, we can dissolve about 4 tablespoons of sugar.At 80 degrees, we can dissolve about 8 tablespoons of sugar. Does that mean that if we heat up water to 80 degrees and dissolve 8 tablespoons of water we have a supersaturated solution? No, at 80 degrees, 8 tablespoons is what we should be dissolving. But what if we heat it up, add the 8 tablespoons and then let it cool down to room temperature. Now it is supersaturated. Why? Because it is holding (dissolving) 8 tablespoons at a temperature where that shouldn’t be possible. Both heating and cooling must occur before a solution can be considered supersaturated.Expressing Amounts with SolutionsThere are several ways to express amount for solutions based on how we intend to use the information and what we are trying to express.Molarity – Moles solute per Liter solution – abbreviation used: MMolarity is most commonly used in chemistry to express the concentration of prepared solutions. We have a beaker with 1 liter of water in it, and pour in 58 grams (one mole) of sodium chloride. As you should know, this sodium chloride will dissolve and spread evenly throughout the container. We label the container “one mole” and place it on the storage shelf. Now suppose a person comes and takes just a portion of this salt/water solution. Will they have one mole (58 grams) of salt? Of course not. What would we need to know to figure out how much salt they have? We would have to know how much solvent (liters) they took. This leads us to molarity. Molarity gives us a way to express the amounts in solutions that takes into consideration not just how much is dissolved, but also the amount of solvent it is dissolved in. Let’s look at some examples with numbers.Example 1: Salt water from above - 1 mole (58 grams) is dissolved in 1 L. Because the definition of molarity is moles/liter, this solution is called a 1M solution.Example 2: A salt water solution with 1 mole (58 grams) dissolved in 500 ml of solution. Again, using the definition, we take moles and divide it by liters (1 mole/.500L). This gives us a molarity of 2M.Example 3: A salt water solution where 25 grams is dissolved in 250 ml of water. Molarity must be expressed in moles. We convert 25 grams to moles (25grams/58 grams), this gives 0.43 moles. Dividing 0.43 moles by 0.250L gives a 1.7M solution.Molality – moles solute per kilogram solvent – abbreviation used: mMolality is often used in biological reports and is closely related to molarity. The difference is the way we express the amount of solution. Now for water, these are very similar because 1 L of water has a mass of 1 kg. However, if the solvent is something other than water, molarity and molality can be different. Example 1: 2.2 moles of sugar is dissolved in .500 kg of solvent. To calculate molality, we simply divide moles sugar by kilograms of solvent. 2.2 moles/0.500 kg = 4.4 m.Example 2: 45 grams of sodium chloride is dissolved in 333 grams of solvent. First, we convert the mass into moles (45/58 = 0.78 moles), next we convert the grams to kilograms (333 grams = 0.333 kg). Then we divide (0.78 moles/0.333 kg = 2.33 m).Mole Fraction – moles solute/total moles solution – abbreviation used: XMole fraction is the same mole fraction we observed in our gases chapter. Example 1: 2.2 moles of sugar is dissolved in 250 ml of water. The first thing we need to do is convert 250 ml of water into moles of water. Remember that 1 ml of water = 1 grams of water, so we have 250 grams of water. 250 grams/18 grams (the molar mass for water) gives us 13.9 moles of solvent. Now remember, a mole fraction is moles solute divided by total moles, so we must add the sugar moles and water moles (2.2 + 13.9 = 16.1). Finally, 2.2 moles of sugar/16.1 moles of solution = a mole fraction of 13.7%.Percent Composition or Parts per hundred – Grams solute per total grams solution - abbreviation(s) used: % or pphThis method of expressing concentration is seen most often in environmental science and other places where small amounts of solute can make a big difference. An example of this would be lead or mercury in your drinking water. Example 1: 15 grams of salt is dissolved in 25 grams of water. According to the formula, we divide the 15 grams by the total grams (15+25). This gives us 0.375; multiplying this by 100 is 37.5 pph or 37.5%.Example 2: 250 grams of sugar is dissolved in 350 ml of water. One ml of water = 1 gram of water. So 350 ml = 350 grams. To solve: 250 grams/(250+350g) = 0.416 X 100 = 41.6 pph or 41.6%.Colligative PropertiesWhat are colligative properties? A colligative property is a property of solutions that is dependent upon the concentration of the solution. In other words, as the concentration of a solution changes, these properties will change along with it. One everyday example of this is antifreeze. We add antifreeze to our radiator to change the temperature at which water will boil. The more antifreeze we add (thereby increasing the concentration) the higher the boiling point will go. There are four colligative properties: boiling point elevation, freezing point depression, vapor pressure, and osmotic pressure.You should already know something about freezing, boiling and vapor pressure, so let’s just spend a little time describing osmosis. Osmosis is common in biological systems such as your body. It is defined as the movement of solvent (usually water) across a semi-permeable membrane. A semi-permeable membrane is like a wall with holes in it. The holes let some things through while others are held in place. Take a look at the picture below:148590093345In this picture, water is the solvent and the orange spots represent the solute. See the labeled membrane between the two sections. Notice that there are more solute particles on the left side. In order to achieve balance, water will move from the right side, to the left side so that concentration will be equal. What keeps the solute particles from moving over, too? The semi-permeable membrane. It is “designed” to let water molecules flow freely but to block the solute molecules. This movement by water across a membrane is called osmosis.How do colligative properties work? Colligative properties work by interrupting the normal intermolecular attractions. To understand this, let’s look at each colligative property separately starting with vapor pressure. You may remember that vapor pressure is related to the speed of evaporation. Also remember that evaporation occurs at the surface. If something increases in surface area you would expect a faster rate of evaporation, right? Well, dissolving solute to create a solution does exactly the opposite, it decreases surface area. Let’s look at pictures to help: 297180066675H2O with solute00H2O with solute114300066675H2O only00H2O onlyThe beaker on the left represents distilled water, or solvent only. Notice that every bit of surface area is available to the water. Now look at the beaker on the right. A solute has been dissolved (represented by the squares) which now takes up some of the surface area. There is less area for the solvent to evaporate. This will decrease the rate of evaporation. What effect will this have on vapor pressure? If you said it would decrease, you are right! Lower rate of evaporation means lower vapor pressure. Adding a solute to a solvent decreases vapor pressure. The more particles of solute added, the greater the decrease.Now, let’s look at boiling point. To understand the effects on boiling point, we must understand the relationship between vapor pressure and boiling point. Let’s go back to our pictures. In a substance below boiling point, there are very small pockets of gas within the liquid where the molecules are not moving fast enough to escape. The pressure within these pockets of gas are equal to the vapor pressure of the liquid. If vapor pressure is low enough, these particles are held in the liquid by atmospheric pressure. If you heat the liquid, more and more molecules begin moving faster. More and more molecules become gaseous and these bubbles become larger. The vapor pressure inside the bubbles becomes greater, and the bubbles start pushing more and more against the atmosphere. As you may recall, boiling occurs when the vapor pressure equals atmospheric pressure. See the pictures below:Atm. Pressure(equal)Vapor pressure now equal to atmospheric pressure. Boiling occurs.Atm.Pressure(greater)Vapor pressure increasing but still lowerAtm.Pressure(greater)Small vapor pressure(lower)Understand that in drawing these pictures, we have exaggerated the size of the gas pockets for illustration purposes. What you should understand is that boiling occurs when vapor pressure and atmospheric pressure are equal. Back to colligative properties. If adding a solute decreases the vapor pressure, then there is a greater difference between vapor pressure and atmospheric pressure. That means that we will have to add even more energy to get something to boil. Adding solute to a solvent increases the boiling point. This is called boiling point elevation.How about the freezing point? As freezing occurs, the molecules in the substance arrange themselves in an orderly fashion to form a solid. In other words, they like to line up nice and neat. In a pure substance, however, we have only one type of molecule, and organization is relatively easy. However, adding a solute (a substance with its own shape and size) will make it more difficult for the solvent molecules to line up neatly, and will want to remain a liquid. This change causes a lowering of the freezing point which we call freezing point depression.Lastly, let’s look at osmotic pressure. Earlier in this section we learned that osmotic pressure is what happens when a solvent (usually water) moves through a membrane in an attempt to balance concentration. The more solute particles present on the other side of the membrane, the more solvent we will observe moving. This movement of solvent creates osmotic pressure. More solute, more water moving, more osmotic pressure.Each of these properties is related to concentration. That’s what colligative means. In each property, having a solute present changes how the solvent behaves. It may change how it boils or how it freezes, or the pressure it exerts. But all of these changes are directly related to how much is dissolved. Solution StoichiometryBy now, you should feel like an old hand at stoichiometry. In the past, we have converted from moles of A to moles of B, from grams of A to grams of B and from liters of gas A to liters of gas B. Now we will be using equations involving solutions, specifically molarity of solutions.Sample Question: How many grams of zinc are necessary to react completely with 40 ml of a 2.5 M solution of HCl?Step one: We need a balanced equationZn + 2 HCl ZnCl2 + H2Step two: Stoichiometry requires a mole – mole ratio, so we must convert the given information to moles. Converting from liters and molarity, we can find moles like this:Given: 40 ml, 2.5 M HCl (remember M means moles/liter) X = 0.1 molesStep three: Set up the mole-mole ratio zincStep four: Solve for the appropriate unit (in this case grams of zinc)Most problems will be similar to this example. Watch, however, for the occasional problem that will force you to bring back you knowledge of gas stoichiometry. (Hint: what is the volume of 1 mole at STP?) ................
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