Turn Soot into Silver

Introduction

You might have heard the saying “not everything is as it seems.” Something might look great from far away, but when you take a closer look, it might not turn out to be as beautiful as you thought! In this activity, you will create the opposite experience—you will turn a spoon blackened with soot into shiny silver. Even though it is only an illusion, you will not know it when you see it. Try it out for yourself!   

Background

How do we see what we see? Vision is based on light—without light, we are not able to see anything. Light travels through space in waves that eventually reach our eyes, where signals are sent to our brain. Our brain translates these signals into information such as the color, location, or appearance of the object we are looking at. When light waves strike an object, they can interact with the object in several ways: they can be reflected back from the object like those from a mirror; they can be absorbed, which means they are captured by the object; they can be transmitted and pass through the object; or they can be scattered and deflected in different directions. Additionally, light that passes through a transparent object such as air, water, or glass can also be refracted, which means that the light wave bends when it passes from one transparent substance into another. Without this bending of light due to refraction, we would not be able to use lenses or magnifying glasses.

How light interacts with an object depends mostly on the properties of its material. Objects that absorb most of the light appear dark or opaque. If an object has a very smooth surface, the light can be reflected and bounces off of the material’s surface. One example is a mirror’s smooth surface, which reflects light at an angle equal to the angle of the incoming light wave. This is the reason why we see ourselves in the mirror: the image reflected by the flat surface is reproduced. Rough surfaces reflect light as well, but in more than one direction. The incoming light wave is scattered from the surface in all directions because the surface is uneven. In this activity, you will see for yourself how a material’s surface can determine how light interacts with it. The result will surprise you, as the exact same object can appear in very different lights—literally!

Materials

Observations and Results

A metal spoon looks very shiny because its surface is very smooth, which is why it acts like a mirror and reflects light very well. If you paint it with black paint, it looks black. This doesn't change when you put the painted spoon in water. It still looks black from all angles. When you put the metal spoon into the flame, you should have observed that after a while, the spoon turned black. This is because the candle produces soot from its wax that is made up of carbon and hydrogen. The yellow region of the flame contains tiny carbon particles that are deposited on the metal surface. Under a microscope, you would see that the soot on the spoon looks very bumpy and rough, almost like a mountain landscape. The surface looks black because, due to its roughness, any light that reflects from one particle will hit another one and get absorbed.

However, once you put the black spoon into the water, you should have observed that the previously black surface suddenly looked shiny and silver. Why is that? The answer is that due to the surface roughness of the soot, tiny air bubbles get stuck on its surface when you submerge the spoon in water. That means that on top of the soot, there is a very thin layer of air bubbles. This air layer creates a very smooth surface on top of the rough soot surface. When light hits the boundary between the water and air layer, it is no longer absorbed, but reflected in the same way as a mirror; the surface looks shiny. This phenomenon is also called “total internal reflection,” which can only happen at the border between two different materials the light passes through; in this case, water and air. When you take the spoon out of the water, the spoon looks black again as the light once more gets absorbed by the rough, sooty surface. With the painted spoon, there is no water-air boundary under water because no air bubbles get stuck to the paint. That is why you do not see this phenomena with the painted spoon.

Additional Resources

• Light Absorption, Reflection, and Transmission, from the Physics classroom
• Refraction of light, from Science Learning Hub
• Total internal reflection, from Kids.Net.au
• Science Activity for All Ages!, from Science Buddies