Melting Ice
Some children were experimenting with warm water and ice cubes. The time it took for the ice to melt depended on the ways they tried to melt the ice with the water. The first child poured warm water over their ice, and the second child put their ice in a container with warm water. The children soon realised that putting the ice in the container meant it melted quicker. Below are some of the comments the children made while carrying out this activity.
“We need warm water”
“Bring water over, fill it right up!”
“Oh, it’s melted!”
“It’s in the water now”
The ice in the container melted quicker as all of the surface area of the ice was in contact with the warm water. This meant that the melting could occur all over the ice and not just in one area. The more surface area in contact with the warm water, the faster the ice can melt. We can use this to find ways to make the ice melt even faster.
If we were to cut an ice cube in half, we would now have two small ice cubes with the same combined volume as the original large ice cube. The two smaller ice cubes (these do not actually have cube dimensions anymore) would have a larger combined surface area than the large ice cube. This means that the two smaller ice cubes would melt quicker than the large ice cube. In the image below, we can see a cube with side lengths of \(4\text{cm}\) and two rectangular prisms with side lengths of \(4\text{cm}\), \(4\text{cm}\) and \(2\text{cm}\). The volume of the cube is \(64\text{cm}^{3}\) and the surface area is \(96\text{cm}^{2}\). The volume of the two rectangular prisms combined is still \(64\text{cm}^{3}\) but the combined surface area is now \(128\text{cm}^{2}\).
To alter the surface area of the ice, we could freeze the same volume of water into different shapes. Changing the shape of the ice changes the surface area to volume ratio. The greater the surface area to volume ratio, the quicker the ice will melt.
A sphere has the lowest surface area to volume ratio, meaning that if we froze a volume of water into a sphere, it would melt the slower than any other shape. When we freeze water into a cube shape, the corners of the ice cube melt quicker than the rest of the ice, as the surface area to volume ratio is higher there. This means that as the ice melts, it becomes a more rounded shape. Children could observe this for themselves in the classroom and carry out some experiments to deepen their understanding.
