Slide 1 / 39 New Jersey Center for Teaching and Learning Progressive Science Initiative This material is made freely available at www.njctl.org and is intended for the non-commercial use of students and teachers. These materials may not be used for any commercial purpose without the written permission of the owners. NJCTL maintains its website for the convenience of teachers who wish to make their work available to other teachers, participate in a virtual professional learning community, and/or provide access to course materials to parents, students and others. Click to go to website: www.njctl.org Slide 2 / 39 Solutions: Mixtures, Solubility and Concentration www.njctl.org Slide 3 / 39 Solutions The infamous saltwater crocodile cannot survive in freshwater. It needs a mixture of water and many solutes.
Slide 4 / 39 Mixtures vs. Pure Substances Mixtures contain two or more pure substances. Mixtures do not obey the law of definite composition therefore the relative amounts of each substance can vary depending on the sample. Pure water Salt water H 2 O contains H 2 O, Ca 2+ , Cl - , Na + ..... 89% O, 11% H composition by mass varies chemical separation method physical All mixtures can be separated using physical means while most pure substances cannot with the exception of thermal decomposition of certain pure substances such as metal carbonates and metal chlorates. What are some methods for physically separating mixtures? Slide 5 / 39 Mixtures Mixtures are classified as suspensions, colloids, or solutions based on particle size. Suspension Colloid Solution > 1000 nm 1000 nm <-->1 nm Particle Size < 1 nm Settling Yes No No Yes Homogenous No Yes Tyndall Effect (particles Yes Yes No scatter light) Given the similarity between colloids and solutions, the Tyndall effect is often key to distinguishing them apart. Click here to see a short animation of the Tyndall Effect Slide 6 / 39 Suspensions Due to their large particle size, suspensions can often be separated by filtration. When precipitates form a mixture of aqueous solutions, a suspension is created with the solid precipitate settling out in an aqueous medium. The solid precipitate can be easily separated by proper filtration. filtration precipitate + Other examples of suspensions include sand in water and snow in air.
Slide 7 / 39 Colloids Due to their smaller particle size, colloids cannot be separated by filtration. In addition, the particles neither settle nor dissolve in the greater medium. Fog is a classic example of a colloid, as the water droplets neither dissolve in the surrounding gaseous medium nor do they settle out. The Tyndall Effect is easy to see when driving through fog as the light from the headlights gets scattered by the particles as visualized below by sunshine on a foggy morning. Slide 8 / 39 Solutions Solutions contain the smallest solute particles that dissolve in a medium called the solvent. Solutions are homogeneous mixtures because regardless of sample size, the ratio of solute particles to solvent remains the same. Uniform mixture of solute (purple) and solvent (green) If you took a 5mL sample or a 200mL sample of a solution you would find the exact same ratio of solute to solvent. Slide 9 / 39 Solutions Solutions contain the smallest solute particles that dissolve in a medium called the solvent. Solutions are homologous mixtures because regardless of sample size, the ratio of solute particles to solven remains the same. Salt water is a classic example of a solution. The Na + and Cl - ions are dissolved in the solvent, creating a uniform material. - - Na+ - Na+ - - + + + + + + - + + + - Cl- + + - Na+ + - + + - + Cl- + + + + Cl- + Due to their small size and interactions with the solvent, the solute particles cannot be filtered out. How could you physically separate solutes from solvents in a solution?
Slide 10 / 39 1 Which of the following would be TRUE regarding mixtures? A A sample of a mixture will never be uniform in composition B They can be typically separated using only chemical methods C They vary in composition from sample to sample. D Only solutions are considered true mixtures E None of these are true Slide 11 / 39 2 A mixture cannot be separated by filtration and does not demonstrate the Tyndall Effect. Which of the following could this mixture be? A NaCl(s) B NaCl(aq) C Sand dissolved in water D Fog in the air E Pure water Slide 12 / 39 3 Which of the following physical methods is often employed to separate a suspension? A Distillation B Filtration C Evaporation D Chromatography E Lithography
Slide 13 / 39 4 Which of the following would NOT be TRUE of a solution? A There are no interactions between the solute and the solvent B Solutions cannot be separated by filtration C A sample of a solution will be uniform in composition D Solutions have smaller particles than do colloids and suspensions E Solutions do not demonstrate the Tyndall Effect Slide 14 / 39 Solubility The solubility of a solute is defined as the amount of solute that can dissolve in a certain quantity of solvent. The solubility of a solute depends on its state and its affinity for the solvent. Solubility is commonly expressed as g solute/100 g of solvent. Substance Solubility in water @23 C CH 3 OH infinite CH 3 Cl 0.47 g/100 g water CCl 4 0.081 g/100 g water Note: The more polar the molecule, the more affinity for water as it is also polar. CCl 4 is non-polar and therefore has an extremely small solubility in water. The phrase "like dissolves like" is applicable here. Slide 15 / 39 Solubility The solubility of a solute in a solvent is highly temperature dependent. In general, solids and liquids dissolve better at higher temperatures while gases are more soluble at lower temperatures. Solubility of NaCl (g/100g water) at different temperatures. 0 C 10 C 50 C 80 C 100 C 35.65 35.72 36.69 37.93 38.99 Solubility of NH 3 gas (mL/100 mL water) at different temperatures. 0 C 10 C 50 C 80 C 100 C 11.7 9.0 3.33 1.38 0.88 Note: The decrease in gas solubility with temperature can be explained by remembering that if the gas molecules have a high kinetic energy, they are likely to weaken any solute -solvent attraction and escape the solution.
Slide 16 / 39 5 Which of the following would likely be the LEAST soluble in water? A CO 2 B HCl C PH 3 D CHCl 3 E CH 3 OH Slide 17 / 39 6 Which of the following would be most likely to dissolve in hexane (C 6 H 14 )? A CH 3 OH B H 2 O C Br 2 D CH 2 Cl 2 E NaCl Slide 18 / 39 7 Which of the following is TRUE regarding solubility? A O 2 gas would be more soluble at 10 C than 20 C B Polar substances are most miscible in non-polar solvents C In general, as the temperature increases, the solubility of most solids decrease D Solubility is not temperature dependent E Solubility is not dependent on the polarity of the solute or solvent
Slide 19 / 39 8 Which of the following would NOT be a miscible pair of solute and solvent? A KOH and H 2 O B CCl 4 and C 6 H 6 C CH 3 OH and H 2 O D CH 3 OH and CH 3 CH 2 OH E CH 3 OH and CCl 4 Slide 20 / 39 Solubility Saturated solutions contain the maximum dissolved solute at that temperature. Unsaturated solutions contain less and supersaturated solutions contain more. Unsaturated Saturated Supersaturated In a saturated solution, undissolved solute and dissolved solute are in equilibrium. In an unsaturated solution any new solute will dissolve whereas in a supersaturated solution, the amount of undissolved solute is growing. Slide 21 / 39 Solubility Curves A solubility curve shows how much solute can dissolve in a certain amount of solvent at a given temperature. Temperature in Celsius The line represents the amount of solute necessary for the dissolved and undissolved amounts to be in equilibrium - a saturated solution. Below the line the solution is unsaturated and above the line the solution is supersaturated.
Slide 22 / 39 Solubility Curves The solubility curve for a given salt is difficult to predict. 2 0 Solubility, grams per 100mL H Slide 23 / 39 Solubility Curves The solubility curves for gases clearly show the inverse relationship of gas solubility and temperature. Note: Cooler streams have higher dissolved oxygen (DO) levels than warmer streams therefore supporting a different variety of life. Slide 24 / 39 Solubility Curves The solubility of a gas also depends on the pressure. The higher the partial pressure of that gas above the liquid, the greater the solubility. This is known as Henry's Law. We can view the dissolved gas and undissolved gas above a solution as an equilibrium situation. Gas(dissolved) <--> Gas (undissolved) If the partial pressure of the undissolved gas is increased above a liquid, the equilibrium will shift left and more gas will dissolve. Gas(dissolved) <--> Gas (undissolved) Note: This is how soft drink manufacturers carbonate your soda. They simply crank up the partial pressure of CO 2 above the liquid and that causes more CO 2 to dissolve. They then smack a lid on top so the pressure is maintained.
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