by No, it isn t! What is Cold? According to the dictionary, a body at a relatively lower temperature, especially when it is compared to the temperature of a human body is describedPasPa colder one. So, any object below the normal human body temperature aboutP37 degrees Celsius is aPcold thing. But wait a minute! When you touch an object, what does it tell you about the temperature of the object? Can you really judge if it is a cold one or a hot one? Unfortunately, our bodies aren t thermometers, we are not so smart when it comes to judging the temperature. Consider the following case. A book and a steel plate kept in the same environment for a long time attain the same temperature eventually (it is called thermal equilibrium). This can be checked by using a thermometer on both the objects. But, when people are asked to touch a metal plate and a book, they find the former to be much cooler. You can try this out yourself by touching differentPmaterials around you. You ll see how some things feel colder while the others feel warmer. A YouTube channel Vertasium conducted a social experiment to record this on camera. See the video below:
So, in the video, ice melts faster, if kept on steel plate than on a plastic plate, even when the steel plate feels colder. Common sense dictates that the colder thing is supposed to sustain the ice block for a longer time, just like your refrigerator does. So why does the opposite happen? According to thermodynamics, simply put, everything has heat in it. So, even a cold ice block has some amount of heat stored in it (say, around 273. 15 Kelvin or 0 degree Celsius). When one object comes in contact with other object, it loses or gains heat till their temperatures get equal or till they attain thermal equilibrium.
Which object loses heat and which one gains it, is decided by their relative temperatures. In case of ice and steel, ice has a lower temperature than steel (assuming it isn t already freezing out there). Therefore, here, ice gains heat from steel till they attain the same temperature and ice melts. Side note: The ice is also in contact with a relatively hotter atmosphere. Hence, it gains heat from there also. In this case, we are only concerned about the steel and ice interaction. Why does it melt faster on steel? There is a particular property which depends on the kind of material and is called thermal conductivity. This is the parameter which decides which objects lose heat quicker and which ones do it slower. Here, for instance, steel has a higher thermal conductivity than plastic. Hence, the steel plate gives away heat to the ice block faster than a plastic block does. As a result, ice melts faster on a steel plate than on a plastic one. Incidentally, this effect can also be used to explain why one plate feels colder than the other, in our hands. Think of it like this, the ice is replaced by our hand. So, a steel plate, due to its better thermal conductivity, draws heat faster from our hand than a plastic plate. This makes us feel that the steel plate is colder than the plastic one. As checked by a thermometer, both the plates have the same temperature, our bodies are only fooled into believing that the thing we feel is temperature; it isn t. None of the plates is actually colder than the other (according to the dictionary see first paragraph). We don t feel the temperature. What we feel is actually the rate of heat being drawn away from our hand. Faster an object draws heat, the colder it feels.
This demonstration shows, against intuition, that an ice cube melts more quickly when in contact with a metal block than a plastic block. There are no safety problems with this demonstration. A suitable ice melting kit is available from a Pass the metal and plastic blocks around the class; ask your students what differences they observe. They are likely to comment that the metal block feels colder to the touch than the plastic one. b Explain that you are going to place identical ice cubes on each block. Ask for predictions as to what will happen. c Place one ice cubes on each block. Observe the ice melting over a few minutes. The film below shows how to carry out this demonstration, together with typical results. d You could use a timer to determine the time for each cube to melt completely. Alternatively, attach a temperature probe to each block and observe how their temperatures change. 1 This demonstration can form the introduction to a structured development of ideas about energy transfers between objects at different temperatures. Ice cubes are placed on metal and plastic blocks; the cube placed on metal melts much more quickly than the cube placed on plastic. This is counterintuitive (for many students) because metals feel cold while plastics feel warm. 2 Energy is transferred to the ice cubes by conduction from the blocks on which they have been placed. A metal block is a better conductor and so energy is transferred more quickly to that ice cube. Why isn t this obvious? Metals feel cold to the touch. This is because, when you touch a piece of metal, energy conducts away from your fingers into the metal, lowering the temperature of your fingers. Plastics are good insulators so, even though the plastic is at a lower temperature than your fingers, little energy is lost to the plastic and it feels warm.
Hence it is best to start the demonstration by asking your students to feel the two blocks so that they may be misled by this experience. Then show that the ice on the metal block melts more quickly, and discuss the reasons. 3 You may then wish to take the discussion to a deeper level. Students may think that some materials (metals, water) are intrinsically cold, while others (plastic, wood) are intrinsically warm. (We talk about warm clothing ). So you could use thermometers to test the temperatures of different objects and materials in the room. Then repeat the demonstration with electronic thermometers monitoring the temperatures of the blocks as the ice cubes melt. Show that the two blocks are both at room temperature at the start, and observe the rapid drop in temperature of the metal block. 4 You could ask your students to explain why the temperature-time graphs for the two blocks are curved (they are roughly exponential). The reason for this is that the rate of transfer of energy from the block to the ice decreases as the temperature difference between them decreases. 5 Note that there is a complication to this analysis which we have avoided mentioning so far. The rate at which energy is transferred to the ice depends on both the conductivity of the block and its heat capacity. It might be that the ice on the plastic block melts very slowly because the temperature of the plastic block drops very rapidly to that of the ice. This would happen if plastic had a low specific heat capacity. This is shown not to be the case by when a temperature probe is used.