Have you ever noticed that when it begins to snow outside the snow seems to melt on the sidewalk initially but does not melt as fast on the grass? Why does this happen? Let’s do a quick experiment to see if we can figure it out!

Materials

• Ice cubes that are of uniform or similar size
• Metal bowl
• Plastic bowl
• Optional: Other types of bowls (glass, wood, etc.)

Make a prediction

Touch both the metal bowl and the plastic bowl. What do you notice? Does one bowl feel warmer than the other? What do you think will happen if you put an ice cube in each bowl? Will ice melt faster in one bowl than another, or will the ice cubes melt at about the same time in each? Make your prediction!

Experiment

Time to do the experiment! Place a similar-sized ice cube in both the metal and the plastic bowl. Observe what happens over time. 

What’s happening?

You probably noticed that the metal bowl initially felt colder to your touch than the plastic bowl, even before you placed an ice cube in each. This is because metal can “pull” the heat out of your body better, or more, than plastic. Both bowls are initially at the same temperature, which is the temperature of the room. Placing an ice cube in the bowls lowers their temperature, which causes heat from the room to flow back into the bowls, warming them back up and melting the ice. How quickly this process occurs, and therefore how fast the ice melts, is due to the heat capacity of the material that the bowl is made from. Metal and plastic have different heat capacities. In fact, every substance has a unique or different heat capacity.

The heat capacity of a substance tells us how much heat is transferred to or from one gram of the substance when its temperature changes by one degree on the Celsius scale (°C). The metal bowl has a heat capacity of about 1.0 Joule for every gram and °C of temperature change. (Joule is the unit that scientists use to measure the amount of heat.)

Plastic has a higher heat capacity, about 1.6 Joules for every gram and °C of temperature change. If a substance has a low heat capacity, it takes less heat to change its temperature, that is, to warm it up or cool it down. Because metal has a lower heat capacity than plastic, the metal bowl warms up more quickly than the plastic bowl when heat flows back into the bowls from the surrounding air. This difference causes the ice to melt faster in the metal bowl than in the plastic bowl.

Extension

By testing how fast an ice cube melts in bowls made of different substances, you should be able to arrange those substances in order of their heat capacity, from lowest to highest. Gather different bowls, such as glass and wood, in addition to the metal and plastic bowls that you already tested. Place a similar-sized ice cube in each bowl. (It’s a good idea to test the metal and plastic bowls again, alongside the others, to keep the test conditions the same for each substance.) Observe how fast an ice cube melts in each bowl and, based on the results, arrange the substances in order from lowest to highest heat capacity. 

Tying it together

Let’s go back to our first question: why does snow melt faster on the sidewalk or driveway than on the grass? Would you have guessed that soil has a heat capacity of about 1.5 Joules for every gram and °C, while concrete or asphalt has a heat capacity of about 0.9 Joules for every gram and °C?

Reference

https://www.youtube.com/watch?v=ZoyVTEHhpxw

To view past “ChemShorts for Kids” activities, go to:
https://chicagoacs.org/ChemShorts

- PAUL BRANDT