Activity 2. Freezing Point of Liquids

1. Activity 2. Freezing Point of Liquids Which of the following liquids will freeze? Water, Oil, Alcohol, Diet Soda, Regular Soda, Coolant? Hypothesis: ____________________ Design a simple test. Set-up your experiment. Water Soda Oil Alcohol Coolant

2. FREEZING POINT Change in phase from liquid to solid Why? 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: http://www.factfrenzy.com/what-is-the-freezing-point-of-water/ Particles slow down Particles of a liquid Particles fuse

4. Why is salt added to a snow-covered driveway?

5. Activity 2. Freezing Point of Liquids Why add salt to a snow-covered driveway? Hypothesis: ____________________ Design a simple test. Set-up your experiment.

6. 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). http://www.stevespanglerscience.com/experiment/how-does-salt-melt-ice

7. BOILING POINT 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 liquids Boiling takes place beneath surface of liquid This 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: http://www.elmhurst.edu/~chm/vchembook/163boilingpt.html Animated GIF "Boiling" - Courtesy of General Chemistry Help - Purdue University

9. Physics in the Kitchen • It is the high temperature that • cooks the food. • Pressure affects boiling. • Increased pressure raises the BP. • Pressure cookers increase the • pressure inside and prevent • boiling. • The increased temperature cooks • food faster.

10. Never boil water in a Microwave Source: http://www.stevespangler.com/teaching-moments/warning-water-can-explode-in-the-microwave/ 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.

11. Never boil water in a Microwave Source: http://www.stevespangler.com/teaching-moments/warning-water-can-explode-in-the-microwave/ 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 diffuse the energy as it is heating in the microwave.

12. Energy is added: Solid  Liquid  Gas Energy is removed: Gas  Liquid  Solid