3Activity 1. Evaporation of Liquids Evaporation happens when atoms or molecules escape from the liquid and turn into a vapor.Describe what happens to the particles during this process. Explain why this happens.
4Activity 1. Evaporation of Liquids Which liquid will evaporate fastest? Water, Oil, Alcohol?Hypothesis: ____________________Design a simple test. Set-up your experiment.
6What factors can affect the rate of evaporation? The energy you can measure with a thermometer is an average of all the molecules in the system.There are always a few molecules with a lot of energy and some with barely any energy at all.
7NATURE OF THE SUBSTANCE AlcoholWaterOilSome liquids have __________ forces that hold particles together allowing some particles to escape.
8What factors can affect the rate of evaporation? The molecules with a lot of energy are able to build up enough power to become a gas. Once they reach that energy level, they can leave the liquid and thus, it has evaporated.Force of air pressing down
9Image source: http://www.vtaide.com/png/evaporation.htm Factors affecting the rate of evaporationImage source:
11Applying Understanding: Real-life situations Why do you spread out your wet clothes to dry?Why do you feel cool when you stand next to a fan?Why do you feel cold when you immediately step out of the shower?Why is it important to “ rehydrate? Relate “sweating” to evaporation and our body’s ability to regulate temperature.
14Activity 2. Freezing Point of Liquids Which liquid will freeze the fastest? Water, Alcohol, Diet Soda, Regular Soda, Coolant?Hypothesis: ____________________Design a simple test. Set-up your experiment.extra
16Change in phase from liquid to solid FREEZING POINTChange in phase from liquid to solidWhy?When energy is taken out from a liquid, particles slow down until forces of attraction cause them to fuse and form a solid.Image source:Particles slow downParticles of a liquidParticles fuse
17Water Diet Soda Regular Soda Coolant Alcohol (Isopropyl) Mineral Oil SubstanceFreezing Point (degrees Celsius)WaterDiet SodaSlightly lower than 0Regular SodaLower than 0 (b/n -15 to -20)Coolant-37Alcohol (Isopropyl)- 88Mineral Oil-30
19Activity 2. Freezing Point of Liquids Why add salt to a snow-covered driveway?Hypothesis: ____________________Design a simple test. Set-up your experiment.
20Salt dissolves in water pretty fast Salt dissolves in water pretty fast. When you sprinkle salt on ice, the salt lowers the freezing point of the water, keeping it from re-freezing as easily and helping to melt the rest of the ice. This is called lowering the freezing point.When you add salt to water, the salt (called sodium chloride, NaCl) dissolves into particles (actually called ions of sodium and chloride). The freezing point of water becomes lower as more particles are added until the point where the salt stops dissolving. On a driveway or street, sodium chloride can melt ice only down to about (-9°C).
23BOILING POINT Evaporation takes place at surface of liquids In a liquid, molecules are packed closely together. As a liquid is heated, the temperature is increased. As the temperature increases, the kinetic energy increases which causes increasing motion. Eventually molecules break free of liquid and become a gas. At the temperature of the boiling point, the liquid turns into a gas.Evaporation takes place at surface of liquidsBoiling takes place beneath surface of liquidThis animation shows how water molecules are able to break the forces of attraction .This is what is happening inside the gas bubble as it is rising to the surface to break and release the water gas molecules.Image source:Animated GIF "Boiling" - Courtesy of General Chemistry Help - Purdue University
24Activity 2. Boiling Point of Liquids How does a coolant helpprevent overheating?Car Engine
25Activity 2. Boiling Point of Liquids Aim: How does a coolant helpprevent overheating?Lab Demo:Observations:Explanation:WaterCoolant
26How does pressure affect the boiling point? Vacuum Chamber
27How does a pressure cooker work? Physics in the KitchenHow does a pressure cooker work?
28Physics in the Kitchen It is the high temperature that cooks the food. Pressure affects boiling.Increased pressure raises the BP.Pressure cookers increase thepressure inside and preventboiling.The increased temperature cooksfood faster.
29Never boil water in a Microwave Source:A man decided to have a cup of instant coffee, so he heated a cup of water in the microwave. When the timer went off, he removed the cup from the microwave and noticed that the water had not boiled.Just then, the water literally “blew up” in his face. His whole face was blistered with first and second degree burns, which left some permanent scaring and damage to his left eye.While at the hospital, the doctor attending him stated that his is a fairly common occurrence. Water (alone) should never be heated in a microwave oven.
30Never boil water in a Microwave Source:Why? This phenomenon is known as superheating. It can occur anytime water is heated - especially if the cup or bowl is new. What happens is that the water heats faster than the vapor bubbles can form. If the cup is very new, then it is unlikely to have small surface scratches in it that provide a place for the bubbles to form (called nucleation sites).Without bubbles, the water cannot release the heat that has built up, the liquid does not boil, and it continues to heat up past its boiling point. If the water is bumped or jarred, it's enough of a shock to cause the bubbles to rapidly form and the result is an exploding liquid that is scalding hot. One solution is to place a wooden stir stick or something non-metallic in the water to help spread the energy as it is heating in the microwave.