What causes colour to change when temperature changes?

Answer 1

A good question that nags many chemistry students simply because it is keenly ignored/avoided.

I guess that's because a decent explanation requires quantum mechanics...which I won't go into because it'd take a while to do it justice. But actually it's really simply stuff.

Anyhow, we can still get a good idea without any real quantum mechanics.

When you shine light at a molecule, the photons hit the electrons and excite them (energise them).

Essentially, molecules can absorb light but ONLY light at the absolutely correct energy (as it takes a specific energy to excite that electron and no more or less will work)...so now we can see molecules will only absorb certain energies of light...and thus, certain wavelengths...or certain colours - if you want to look at it that way.

So what happens to the light that isn't absorbed? It is scattered (reflected) and that's the light that hits our eyes and we see. What's interesting is that we see what is called the "complementary colour".

This is the colour that appears when you use all of the visible light spectrum EXCEPT the one absorbed. So if we absorb yellow light, we get everything else...which turns out to be blue when you mix it all up.

We kinda see the colour 'opposite' to that which is absorbed.

Worth noting, visible light is just a tiny part of the electromagnetic spectrum. You'll learn that so many compounds absorb U.V light. If we could see U.V with our eyes then many compounds that are colourless in the visible spectrum (like methane, a colourless gas) may well be visible to us.

And finally, you do tend to get strong colours from transition metal complexes. This is due to d-electron transitions...but if you need to know that you will be taught it specifically.

Answer 2

Firstly it must be noted that not all substances are changed in nature by heat.

The substances that are changed are those that are available in multiple forms. i.e solid liquid or gas. For instance,ice, water and water vapour.

What differentiates ice from water or water from vapour is the force holding the molecules together which can be changed by the adsorption or release of heat from the substance.

The thing about heat that changes the nature of substances is it weakens the force holding the molecules together.

On providing heat to a substance, the kinetic energy increases. As the kinetic energy increases, the particles start vibrating with greater speed and overcome the forces of attraction between the particles. Due to this, the particles leave their fixed positions and start moving freely and thereby changing the state of the substance.

Answer 3

As temperature increases, the energy of the system increases and vibrations become more rapid - which corresponds to changing colors.

It's actually a shift in the range and intensity of the vibrational spectrum, but it takes a spectroscope or prism to split the colors apart and a photocell to measure the intensity of the light at the different frequencies.

Answer 4

The question is ambiguous, so let's break it up into parts.

How does heat affect different materials?

Different materials have different heat capacities. The heat capacity of a material can be thought of as a measure of how much heat energy it takes to raise the temperature of a fixed amount of that material by one temperature unit. For example, the heat capacity of liquid water is somewhere around 1 calorie (of heat energy) per gram per kelvin.

Of course, different materials melt or boil at different temperatures also, and the amount of heat energy required to melt/boil different substances is also different for each substance.

How does heat affect different colors?

There are three branches to this.

The simplest one is that darker (for a certain value of darker, which I'll explain shortly) colors absorb heat more readily. The trick here is that "darker" means "less reflective at the specific wavelengths associated with the heat energy in question". Something that appears to your eye as white, but is "dark" in the infrared spectrum, might absorb heat from a heat lamp better than something that appears dark red, but is very reflective in the the infrared spectrum.

The second simplest one is something called Black Body radiation, aka Cavity radiation, which basically says "hot things glow, and what color they glow depends on the temperature." There's an equation for this, but it's pretty easy to find on the internet if you just search for either of those terms, so I'll just leave it out of this answer.

The most complicated one is that some "colors" react to heating by changing either their chemical or physical structure, which causes them to appear a different color. One example of this is a "mood ring", where temperature affects the spacing of the liquid crystal layers, which in turn causes the ring to change colors. Also, some pigments denature or undergo a chemical change with heat, either reversibly (in which case they change back when the heat is removed) or irreversibly (in which case they don't).

Answer 5

The question is phrased a little vaguely.

Heating a substance may cause it to change color because of a chemical reaction (like bread turning brown when you bake it). Or it might change color because it induces a physical change in the substance (the stuff in "mood rings" works like this). If you heat it strongly enough, it changes color because it begins to emit light according to the cavity radiation laws ("red-hot").

Some other things might change color not because of a chemical reaction exactly, but because water is being driven off. "Blue vitriol" (copper (II) sulfate pentahydrate), which is ... well ... blue ... will, when heated strongly, turn white as the waters of crystallization are driven off. If it's allowed to cool again in the presence of moisture, it will resorb water and turn blue again.

Answer 6

Because it changes the chemical makeup of the substance

Answer 7

The heat expells the proteins leaving it to it's original colour

Answer 8

It changes the molecular structure when heated