Heat and Melting Point热、熔点.doc

Heat Anton Wintner18/04/07Materials:Batter (provided by Shawn):1/2 cup sugar1/4 teaspoon of salt + extra salt1 cups of whole milk3 beaten egg yolks1 tablespoons of Vanilla Extract2 cups of heavy whip creamDisposable spoonsZiploc BagIcePaper towelsTrash bags to separate ice for different classesThermometerDay Before Class (Shawn will take care of this):Do all of the necessary preparation; Over low heat combine sugar, salt, and milk. Stir in egg yolks. Continue cooking until small bubbles form at the edge of mixture. Then let cool to room temperature. Mix in vanilla and whipping cream and put in a medium sized Ziploc bag. Place this bag inside a second bag of the same size, just to be plete rest of procedure in class.In Class:- Review from last week (if you taught last week)- How would you melt something? try to guide kids to say that you would heat it up. - But we dont have a stove can you think of any other ways to melt things.like this ice cube for example (give every child a small ice cube)- Try to guide the discussion to where someone says that you should hold it in your hand (or something similar).- Try to figure out why because your hand is warmer than the ice cube. - What happens to your hand when you hold the ice cube in it? your hand gets colderUse the chalk board as an aide for drawing diagrams:- It does this because the ice cube and your hand want to be at the same temperature (warmth). But since your hand is warmer, it must give some of its heat to the ice cube. So when you hold the ice cube in your hand, your hand gets colder because it loses some of its heat. - So where does the Heat go? into the ice cube. Heat always goes from the warmer object to the colder.- What would happen if you left two things of different temperatures together for a long time? Their temperature would eventually be the same. We call this reaching an equilibrium, as the temperature of everything involved is equal.- So what happens to water when you put it into the fridge it will eventually become the same temperature as the air in the fridge. What about when you put it into the freezer? It also becomes the same temperature as the air in the freezer. But as you know something special happens it turns to ice. The temperature that this happens at is known as the freezing point.- If I told you that the ice cream batter is warmer than the ice, what would you expect to happen when you put them together? Which way would the heat flow? The ice would get warmer and the batter would get colder.- Lets do the experiment. Put the bag of batter inside a XL Ziploc bag of ice and see what happens. Nothing happens because the bag of batter cant get cold enough to form ice cream. Using the thermometer, show the students that the temperature of the water/ice is about 32F, about the freezing point of water.- In the winter, what do people put on the road and sidewalks to keep ice from forming? they put down salt. When salt combines with water, it makes water harder to freeze (the salt ions block the formation of ice crystals, you can try to explain this in simple terms if youd like). Therefore you can get water colder without freezing it if there is salt in the water.- In order to be able to make our milk turn into ice cream, we need to take enough heat away from the batter to make it freeze. To do this, we need our ice water to be colder than ice, have a temperature below 32F. How can we do this? Use salt water, just like people do in the streets in winter. By adding salt to our ice water, we allow the water to get colder without being able to freeze. But the ice cream doesnt have salt, so it still freezes at the normal point. This allows our ice cream to freeze, and keeps the water at a temperature lower than freezing.- Putting the bags of ice cream batter into the ice water causes heat to be transferred. Which way does it go? The heat from the warmer ice cream batter goes into the ice water, melting some of the ice, but freezing the batter. Lets see if what I just told you really works. - Add a handful of salt to your ice/water mixture. Show the students that the temperature of the water decreases to lower than 32F, making it possible for the batter to get colder than freezing. Take your batter and divide it into small ziplock bags, one for each student. Either taking turns or doing a few bags at a time, let them place their batter in the now colder water. The ice cream should harden. Shawn will give a more detailed report about how long this takes on Sunday after it has been experimented with it a bit, but online it says that this takes about 15 minutes, so you may have to do all of the ice cream at once. Be sure to stir things around and shake the ice cream bags so they harden