L4b Dissolving PDF - Investigating Dissolving

Investigating dissolving  What do the words dilute and concentrated mean?  There are two different ways of making dilute and concentrated solutions, depending on whether you are considering the volume of solvent or the mass of solute. How might you explain this idea?  I can keep th...

Investigating dissolving  What do the words dilute and concentrated mean?  There are two different ways of making dilute and concentrated solutions, depending on whether you are considering the volume of solvent or the mass of solute. How might you explain this idea?  I can keep the mass of solute the same but change the volume of the solvent eg Dilute = 10g sugar in 1000ml of water Concentrated = 10g sugar in 100ml water  I can keep the volume of the solvent the same but change the mass of solute eg Dilute = 1g sugar in 100ml of water Concentrated = 10g sugar in 100ml water  A solution that has a high proportion of dissolved solute particles is said to be a concentrated solution. A solution that has a low proportion of dissolved solute particles is said to be a dilute solution. For coloured solutions like copper sulfate solution, the more concentrated the solution, the darker it appears. For colourless solutions like salt water, concentrated and dilute solutions appear the same. (However, the more concentrated the salt water solution, the saltier would taste.)  What does saturated mean? Saturation Even if a substance is soluble, there is a limit to how much of it can be dissolved, for a given set of conditions. For example, if salt was continually added to a beaker of salt water, eventually there would be a point where no additional salt will dissolve. This point is known as the saturation point, and the solution is said to be saturated.  On the right we see a sediment layer of salt because no more salt can dissolve- the solution is saturated.  The mass of a solute that can dissolve in a particular solvent, before a solution becomes saturated, is referred to as its solubility. Solubility depends on both the solute and the solvent. Therefore, they both need to be mentioned when giving a measure of solubility. For example, a maximum of 360 g of salt can dissolve in 1L of water, therefore we say that the solubility of salt in water is 360 g/L.  Solutions Notes 1. Copy out these examples of how to make a solution, label which one is dilute and which is concentrated relative to each other 5g salt + 100ml water 25g salt + 100ml water 2. Sketch diagrams of dilute and concentrated salt solutions in a beaker to show the relative number of salt particles in each 3. The example above varied the mass to change the concentration of the solution. Make up two other examples of dilute and concentrated solutions by keeping the mass of the solute the same at 10g salt but changing the volume of the solvent. 4. Explain what a saturated solution is in your own words Extension 1. Can you think of some mixtures where gases have dissolved in liquids? 2. What effect does pressure have on solubility? 3. What effect does temperature have on solubility? 4. Find out about supersaturation The effect of temperature on solubility Generally, the solubility of solids increases with temperature. For example, the solubility of copper sulfate in water is 320 g/L at 20 °C but increases to 620 g/L at 60 °C  Sugar will dissolve more readily in hot water than cold water. Why? (Image: rawpixel, Pixabay)  The molecules in a sample of hot water move more rapidly than those in a sample of cold water.  Dissolving a solid requires energy input to disrupt the forces holding the molecules together, both in the solid and the water. In hot water, molecules are moving around more, so there are more collisions between the water molecules and a solid.  The blue circles represent molecules of water. An increase in temperature caused the water molecules to gain energy and move more rapidly, This results in water molecules that are farther apart which creates more space for a solute to dissolve, and is why you can dissolve more of a solute at higher temperatures Dissolving gases Solutions can also be formed by dissolving gases in liquids. For example, oceans contain dissolved oxygen that allows fish and other organisms to survive under water. The amount of a gas solute that can dissolve is also affected by temperature, but in the opposite way to solids. Generally, the solubility of gases decreases with temperature. For example, the solubility of carbon dioxide in water is 1.7 g/L at 20 °C, but decreases to 0.6 g/L at 60 °C. Warmer ocean water contains less dissolved gases, such as oxygen and carbon dioxide. (Image: 12019, Pixabay) The effect of pressure on solubility The solubility of gases (but not solids) is also affected by pressure. Generally, the solubility of gases increases with pressure. For example, at 20 °C, the solubility of carbon dioxide in water increases from 1.7 g/L at 1 atmosphere of pressure to 15 g/L at 2 atmospheres of pressure.  Carbonated drinks, such as soda water, contain carbon dioxide gas that is forced into them under high pressure. Supersaturation More solute can be dissolved in a saturated solution by increasing temperature (for solid solutes) or pressure (for gas solutes). When a solution contains more solute than it can normally dissolve, it is said to be supersaturated. However, as the temperature or pressure decreases, the extra solute will come out of solution. For a solution containing a solid solute, such as a salt solution, this will result in the formation of crystals.  In the cold solution the solute is not dissolving In the hot solution it has dissolved, this makes a super saturated solution, as there is solute dissolved that wouldn’t normally dissolve, it has only dissolved due to the increase in temperature If the temperature is cooled, the solute comes out of solution and makes crystals  Start with 100g of salt At 25c only 91g will dissolve before the solution becomes saturated- the remaining 9g will not dissolve BUT if you heat the solution to 50c then the extra 9g will dissolve because an increase in temperature increases the solubility. The solution is now supersaturated However, as the solution cools, the extra 9g will come back out of solution and form salt crystals.  For a solution containing a gas solute, this will result in the gas escaping. Eg fizzy drinks are supersaturated with carbon dioxide gas and will eventually go flat (not fizzy any more) if the high pressure forcing / keeping the gas dissolved decreases eg once you’ve opened the bottle Three interesting articles for you to read at home to learn more about the science of fizzy drinks https://letstalkscience.ca/educational-resources/stem-in-context/chemistry-pop https://www.cnet.com/home/kitchen-and-household/appliance-science-the-compressed- chemistry-of-carbonation/ https://www.livescience.com/32461-why-do-soft-drinks-go-flat.html  Supersaturated sugar solutions are often used in making confectionary, such as toffee and caramel. The mixtures are heated to dissolve excess sugar, then cooled, resulting in crystallised sugar.  https://www.youtube.com/watch?v=DnuKHNpS2-M Sodium acetate crystals forming, 4 min https://www.youtube.com/watch?v=Ta3qM2swmvM Similar, 6 min If you want to try something similar at home look at the 2 videos below! Ask your parents first and check you have safety equipment! https://www.youtube.com/watch?v=AedL_NCv1Pw or https://www.youtube.com/watch?v=znsPa1BSaIM  Solubility Questions 1. How does an increase in temperature effect solubility of solids? (Clue- think about putting sugar in hot tea v cold tea) Explain this (think about how much energy is in solvents at different temperatures and how spread about the particles are). 2. How does an increase in pressure effect solubility of gases? 3. Give an example of a gas dissolving, state the name of the solvent and the name of the gas. 4. What is a supersaturated solution? 5. How can you create a supersaturated solution of salt water? 6. What happens when a supersaturated solution of salt water cools?