While you’re playing with ice and cream… What state is the cream in when it comes out of the fridge? What observations support your conclusion? What state was it in after being in the salt ice mixture for 15 min? What was the purpose of adding the salt to the ice? What was the purpose of the shaking? How did the temperature of the ice and salt mixture change? Did the frozen cream taste like ice cream? What was missing to make it ice cream?

Ice & cream An explanation: When you see the * tap or click during the presentation. I have included below some questions and answers that could be used to help you or your students understand what is going on. I see this activity as raising questions in terms of practicing inquiry skills. Hopefully the students will ask questions related to some of the following “Can you eat the ice cream? What state is the cream in when it comes out of the fridge? What observations support your conclusion? What state was it in after being in the salt ice mixture for 15 min? What observations support your conclusion? How did the temperature in the container after the 15 min shaking compare with the ice before? Did the frozen cream taste like ice cream? What was missing to make it ice cream? What was the purpose of adding the salt to the ice? What was the purpose of the shaking? Since this activity fits in changes of matter we will focus our attention on the changes in matter. To develop an explanation we have to consider what we are going to look at. In this case the system consists of a plastic container, ice, water, cream, salt. Our focus will be on the ice and the cream.

X ICE Water(s) + energy Water(l) CREAM Water(l) Water(s) + energy “So let’s look at what is happening to the ice. Ice is a term we use to describe water solid*. What evidence do we have that ice, water solid, changes during the shaking? When we took the lid off we had a mixture of liquid and ice. What did we assume the liquid was? Water* what evidence have we that it is water? Clear, colourless, flows looks like water, not much really. What name do we give to this process? It is called melting. Normally what happens for ice to melt? Temperature has to rise or we have to heat it. So in some way energy has to be added.* So looking at the word equation we can see that the energy added to the solid is now stored in the liquid water. This energy is no longer available, we notice this by the drop in temperature to below zero which is the normal melting point of water. The salt and shaking help melt the ice. The cream on the other hand is mainly water. In what state was the cream in at the beginning? Liquid. What evidence do you have that it is liquid? It sloshed around. What state was it in after 15 min of shaking? Solid. What evidence do you have that it is solid? Doesn’t appear to slosh or flow any more. Could use a spoon to scoop it out and the shape of the chunks did not flow into a puddle. So in this case the water in the cream has solidified.* It is hard to imagine that what is in the little container after shaking for 15 min is solid water because what makes water cream is suspended in the water and it is now suspended in the ice. Cream is not a pure substance it is a mixture called a suspension. So if that is acceptable we now have water liquid turning into water solid which we call ice.* Now I’d like you to compare these two word equations. How are they the same and how are they different? Both involve water liquid and solid but they are on opposite sides of the arrow for example the solid is a reactant in the first and a product in the second. They differ in the the first has energy as one of the reactants. If we assume that these two word equations are the reverse of the other then how do we change the second equation? We add energy to the product side.* Is this reasonable? I’d like you to think about that while we explore some other questions. When we freeze water to make ice cubes how is this done? Water liquid is put in ice cube trays and the trays are put in the freezer. So we could say that freezing involves adding cold, the freezer, or Water(l) + cold to form Water(s) however scientists don’t talk about adding cold they refer to cold as the absence of heat. You may at this point not believe that when water freezes energy is produced so I’d like you to imagine you are at the top of a ski run when there is little or no wind and it begins to snow. You can feel it on your face as the temperature rises. If you haven’t experienced this remember what I said the next time you are out and it begins to snow.” It is also interesting to note that the energy produced by the freezing cream is used to melt the ice as well. CREAM Water(l) Water(s) + energy X Water(l) + cold Water(s) Water(l) - heat Water(s)