What melts ice faster: running cold water or still warm water?
Students will have a variety of predictions based on this question, making it a quality question to start off your lesson. Most will focus on the temperature difference between the two scenarios in their decision making. Some might want to go against the grain and select the cold water but aren’t really sure why. This accessible question offers students an opportunity to share what they know and utilize past experience to begin going through the scientific method.
After making predictions, students will observe a demonstration comparing the two scenarios. I like to have two students come up to the front to hold the ice cubes for me. Each student is given a sink strainer (or pair of tongs) to hold their ice cube. When we say “GO,” one student holds their ice cube still in a cup of warm water while the other holds their ice cube over an empty bin as I pour cold water on it. It is crucial that I make sure that I am pouring water directly on the ice cube. The students are instructed to shout out when their ice cube has melted. And… the student to shout first is always the one with the… cold water! Wait WHAT? How could that be? Students all around the room start questioning EVERYTHING.
Was the water was really warmer?
Were the ice cubes the same size?
Did the ice cube really melt completely?
The key is to not focus on who got it wrong but rather get students asking more questions or rethinking their explanation of what would happen. Discourage students from changing their hypothesis now that they know the result. A hypothesis is NEVER wrong. But they all got the chance to discover something new, and doesn't that feel good?
Does cold water melt ice faster than warm water? That doesn’t make sense with what we’ve experienced about ice before. We examine the experiment a little closer and realize there were two variables being tested. We changed two parts of the experiment at the same time, the temperature and the movement of the water. Now, we don’t really know whether one or both contributed to the melting time difference. This is why it’s important to identify one independent variable prior to the experiment. The water temperature or the water movement can be the independent variable while the other can be one of the control variables.
Students still getting confused on which variables are which? Utilize this FREE poster in your classroom for students to reference. We also have a worksheet that gives students an opportunity to run through a hypothetical experiment to practice these skills.
As a class, we discuss a plan to test these variables separately. This can be expanded to having students write their own procedure if time allows. Then, it’s the student’s turn to collect data. They measure the time it takes for the ice to melt in cold vs. hot water that’s not moving and still vs. moving water at the same temperature. From this change in the experimental design, students determine how each variable impacts the melting rate.
Interested in bringing this investigation to your classroom? Here’s a copy of the lab document that you can use.
Keep the investigation going by discussing what else the class could discover.
We tested warm ____ degree Celsius water, but what about even hotter water? You could create a graph of temperature vs. melting time by collecting data for multiple water temperatures. Then have students take their data to math class to analyze what type of mathematical model could be used to describe the relationship.
Is there a temperature where the still water melting time matches the moving water temperature melting time? This would involve a graph with two lines to see if the still and moving water ever would intersect. Students can learn how to make predictions from data in this expansion.
But wait, what’s the science behind melting ice?
Running water essentially rinses away the water molecules that are colder than the running water, allowing for constant contact with water that is warmer than the ice. Ice in still water melts and creates a layer of cold water around the ice cube that insulates it slightly from the surrounding warmer water.
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