Conceptual Physics/AOHS Change of Phase 03b Heat Flow, Phase Changes of Water

Heat Flow Which way does heat flow when water freezes? – Heat flows out of the water into the air. Said another way… thermal energy flows out of the water into the surrounding environment.

How can you melt ice? – Add thermal energy! OR – Destroy structure through salt. The ions get in the way of the bonds. Heat Flow

At-Home Demonstration Freezing Point Depression Lightly crush (chip) about 15 ice cubes creating a chunky slurry of ice water. Measure the temp (should be close to 32 °F (0 °C ) Add a small hand full of table salt, stir and watch what happens to the temperature. imaginationexpresspreschool.blogspot.com

At-Home Demonstration Freezing Point Depression How does salt melt ice? The process of melting is what kind of process? (warming or cooling) – Melting is a cooling process. In order to melt, change phase from solid to liquid the ice must gain thermal energy. This thermal energy is collected from the surrounding environment. What has happened to the freezing point of this water? – The freezing point of the water was decreased by adding the salt.

Why is it not always very cold when it snows? – As the water molecule in the atmosphere changes phase from liquid to solid it must give up thermal energy to its surroundings. – Thermal energy moves from “high to low.” Freezing is when energy is taken out of the liquid and released to its environment. Heat Flow

What is the opposite process of freezing? – Melting! Melting is when energy is taken in by a solid to change into a liquid and cools its environment. Heat Flow

Melting Blocks Revisited Revisit the Melting Block DemonstrationMelting Block Demonstration Clearly the ice is melting on one block at a greater rate then on the other. What is happening, temperature- wise, to the block on which the ice is melting quickly? – It is becoming cooler! – Thermal energy is being transferred by the metal block, because it conducts thermal energy better than the wood block, but is loosing the energy quicker than it can pull it from the air; thus getting colder. pirt.asu.edu

What is the opposite process of evaporation? – Condensation! Condensation – the changing of a gas to a liquid. What happens, temperature-wise, to the area where the liquid forms? – It warms! – Gas-to-liquid is a cooling process but warms the surface it lands on. Heat Flow

Why are steam burns so bad? – Condensation occurs; massive and quick transfer of thermal energy to the object on which condensation occurs. – In the case of a hand the flesh is unable to withstand the amount and/or rate of transfer of thermal energy and thus is ‘structure is damaged’ (skin destroyed) in the process. Heat Flow

Phase Changes of Water Depending on the direction of phase change energy is either released or absorbed. santasusana.org

Phase Changes of Water

Latent Heat of Fusion Observations: – Initial temp 0⁰C – Temp remains constant until all ice is melted. – Although thermal energy is being transferred from the environment to the ice all energy is going to change the phase from solid ice to liquid water at the same temperature!!

Energy must be ADDED to go from solid  liquid  gas. Phase Changes of Water

Energy must be REMOVED to go from gas  liquid  solid. Phase Changes of Water

Challenge Questions Why can you touch a hot iron with a moisten finger? – Hot iron transfers thermal energy to vaporize moist finger faster then finger transfers thermal energy. Why can you walk on red hot coals? – The coals, although super hot, are poor conductors. So if your feet don’t spend too much time on the coals very little thermal energy is transferred. – DO NOT TRY THIS AT HOME!! joanking.hubpages.com

Challenge Questions Why do firefighters use different ways of spraying? – Shooting of water into the fire, which is at different temperatures in different places, allows the water to absorb thermal energy to change phase from liquid to vapor removing energy from the fire. – For example ‘heavy drops’ into a exceptionally hot spot will take a lot of thermal energy away via the vaporization of the water.