Conceptual Physics/AOHS Change of Phase 03a Evaporation, Boiling

Evaporation Question: Why do we sweat when we are warm? – To remove thermal energy from our bodies. Which would feel more cold on a cool day? Standing in front of a fan while dry or soaking wet? Explain. – Standing in front of a fan! I cool down faster. Evaporation is the process of a liquid changing to a gas and cools the surface left behind.

Open the PhET Simulation States of MatterSimulation States of Matter – Select Run Now – Select Water and Liquid – Take note of the motion of the water molecules in the liquid state – Add Heat by holding the slider up and compare the molecular motion after adding thermal energy. – Remove thermal energy by holding the slider down toward Cool and again compare the molecular motion. Simply put, the addition or subtraction of thermal energy either increases or reduces molecular kinetic energy of motion. This kinetic energy manifests itself as rapidly moving molecules having the ability to run into surfaces with great force affecting the pressure in the vessel. If there is no lid on the vessel then the molecules can simply escape…evaporate! Molecular Kinetic Energy

Fill in the blanks. Recall evaporation demo. – Evaporation occurs faster when the liquid is closer to boiling point. Evaporation

Fill in the blanks. Recall evaporation demo. – Evaporation occurs faster when the liquid is closer to boiling point. – When a liquid evaporates, it takes thermal energy with it. Evaporation

Fill in the blanks. Recall evaporation demo. – Evaporation occurs faster when the liquid is closer to boiling point – When a liquid evaporates, it takes thermal energy with it. – When a liquid evaporates, the liquid left behind has lower thermal energy. Evaporation

Fill in the blanks. Recall evaporation demo. – Evaporation occurs faster when the liquid is closer to boiling point – When a liquid evaporates, it takes thermal energy with it. – When a liquid evaporates, the liquid left behind has lower thermal energy. – A liquid in life will fully evaporate because thermal energy is added to the system Evaporation

Fill in the blanks. Recall evaporation demo. – Evaporation occurs faster when the liquid is closer to boiling point – When a liquid evaporates, it takes thermal energy with it. – When a liquid evaporates, the liquid left behind has lower thermal energy. – A liquid in life will fully evaporate because thermal energy is added to the system. Evaporation

At-Home Demo – Water vs. Alcohol Rub two different patches of your forearm, one with water and one with rubbing alcohol, and then blow on the patches. Explain the differences felt. – The patch with alcohol feels A LOT cooler. This is because the alcohol is evaporating more quickly, which is to say that the alcohol is removing thermal energy at a faster rate than is the water. Rubbing alcohol evaporates at a lower temperature; requires less energy to evaporate. How will your skin’s temperature actually measure? – Skin temperature will be lower on the patch of alcohol versus the patch with water.

Question: What are some ways to cool off a hot drink? – Blowing on it. Why? Increases KE of molecules thus increasing the rate of evaporation. – Increase surface area for evaporation. Why? More molecules able to escape at one time. – Put silverware in it to act like a radiating antenna. Why? Metal is a good conductor of heat; will carry thermal energy away from hot drink. – Put ice in or something cool. Why? Thermal energy moves from high to low regions. Evaporation

Boiling What is ‘boiling’? Boiling is the change in phase from a liquid to a gas. It takes thermal energy to break the bond between the molecules in the liquid.

Question: What are the air bubbles in a pot of boiling water? – Liquid water turning to gas. – Water vapor is greatly increasing the volume it occupies thus forming a bubble. What is adding the thermal energy? – Burner. How is thermal energy leaving this system? – Evaporation! In something that is boiling or evaporating, what part is loosing or gaining thermal energy? – Water is losing thermal energy, air is gaining thermal energy. Boiling

What will happen to the water vapor bubbles if the pressure is increased? – In order to have enough molecular kinetic energy to move around fast enough to ‘push everyone out of the way’ to create a bubble of space the water has to get to a higher temperature to withstand the added pressure of the environment. – Where boiling temperature at see level is 100 ⁰C the temperature thus gets OVER 100 ᵒC!! What is the benefit of a pressure cooker? – Higher temps mean that the food cooks faster. Boiling

So what will happen to the boiling point if we remove air pressure? – Boiling point will be lowered; due to a greater ease of ‘pushing others out of the way’ to create a bubble. – Bubbles are easier to form. Which has a higher boiling point, Colorado springs or San Diego? – San Diego – more air pressure – boiling point at 100 ᵒC. – Colorado Springs – around 94 ᵒC. Boiling

What about in a near vacuum? – Boiling water in a vacuum Boiling water in a vacuum – The water boils because the pressure in the environment is removed NOT because heat is added to the system. – The reduction in pressure simply makes the molecular kinetic energy of the water enough to ‘push other out of the way’ in order to create the bubble…thus boil! Yes, that’s cool! Boiling

When water is boiling at sea level, does it ever go above 100 ᵒC? – No! Never! – 100 ⁰C is the boiling point of water…period! Heat of fusion (L)– (next lesson) – Idea that water stays at 100 C when boiling. Boiling

What will happen to a paper cup if I put it over a flame? – The cup would ignite and burn! What will happen if I put water in the cup and then over a flame? – The cup doesn’t burn! The water absorbs the thermal energy versus the paper cup. Boiling Water in a Paper Cup Boiling in a Paper Cup

Space Craft Heat Shields Space Shuttle: – Thermal Protection System… tiles. tiles – Insulator – prevents heat transfer. Apollo 11 Capsule Re-entry:Re-entry – High melting point. – Surface melts and vaporizes away, referred to as “ablation,” carrying thermal energy away from spacecraft. – Entry angle: b/w 5⁰ and 10⁰ scibuff.com