Conceptual Physics 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 surroundings.

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

Freezing point depression lab Crush ml of ice and create a chunky slurry of ice water Measure the temp (should be close to 32 F) Add a small hand full of rock salt and stir and watch what happens to the temperature Was thermal energy added into the system? The process of melting is what kind of process? (it warms or cools what) What has happened to the freezing point of this water?

Freezing Point Depression: Follow-up Is thermal energy added to the system? – NO! So what happened? – The salt lowered freezing point by destroying the ionic bonds between the ice crystals.

Question: Why is it never very cold when it snows? – Water goes from liquid to solid releasing thermal energy to the 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

Question: 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

Demo #6 – 2 Ice Cube Revisited Clearly the ice is melting on one block at a greater rate then on the other. Question: What is happening, temperature-wise, to the block on which the ice is melting quickly? – It is becoming cooler! pirt.asu.edu

Question: 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

Question: 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 or rate of transfer of thermal energy and thus is damaged 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 Heat of Fusion: 80 cal/gm – Amount of energy added during melting or given off during freezing. Heat of Vaporization: 540 cal/gm – Amount of energy added during vaporization or given off during condensation.

Phase Changes of Water

Demo #7 – Heat of Fusion Approximately 400 ml of ice on a hot plate. Temperature probe will record the temperature during heating process. Observations: – Initial temp 0⁰C – Temp remains constant until all ice is melted.

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

Refrigeration Process

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? Why do firefighters use different ways of spraying? joanking.hubpages.com HipVo

Why are air conditioners mounted like this?

Mythbusters – Hot water heaterHot water heater water to steam expansion ratio 1:1600 What happens to steam when it cools? – 1600:1 Steam to water pop can demodemo

Practice Calculate the energy absorbed by 20 g of water that warms from 30 ⁰C to 90⁰C – 1200 cal Calculate the energy needed to melt 50 g of 0 ⁰ C ice – 4000 cal Calculate the energy released by 20 grams of 100 ⁰ C steam that condenses and then cools to 0 ⁰ C water – 12,800 cal How much energy does it take to warm up 1000 grams of 0 ⁰ C water to 37 ⁰ C? – How much water would you have to drink to “burn” 2,000 food Calories? Remember 1 gram of water = 1 ml