Expert: George Maxwell - 8/27/2007

Question
QUESTION: why do different chemicals like calcium chloride, sodium chloride etc emit different colors of flames? what is the whole process they go through and what reaction occurs between them? does the same reaction occur in every chemical burning? I'm a student in grade 10... and i looked all over the internet and in my chemistry book but i couldn't find anything.. and even if i did.. i didnt understand. can u please explain it to me in an understandabe language?PLEASE, ITS IMPORTANT AND URGENT! THANKU :)

ANSWER: Hi there, and thanks for your question.

The first thing is that this is not a proper chemical reaction: all you're doing is heating up chemicals and cooling them down. You aren't burning the sodium chloride, calcium chloride etc, just making it a bit hotter.

For this explanation, we'll think of atoms as being tiny little blobs of material. Each chemical element is made up of different atoms (so a blob of hydrogen is different to a blob of sodium).

At room temperature, the heat from the surroundings makes the atoms in a chemical compound move around fairly slowly. When you put the compound in to a flame, you're suddenly giving it loads of extra heat energy, and it starts moving around really fast (in the textbook, this is more accurately explained as going from the "ground state" -little bit of energy- to the "excited state" -lots of energy).

The hot, energetic atoms move out of the flame, and immediately cool down. There isn't enough energy in the surroundings to keep the atom moving around really fast (the exctited state), so it slows back down to the normal (ground) state. When it does this, each atom has to lose the extra energy it got when it was heated. The easiest way for this to happen is for each atom to get rid of its extra energy in the form of electromagnetic radiation (including light). So what you're seeing is the atoms getting rid of the extra energy you gave them by heating them up, as they cool back down to room temperature.

Why do different compounds give off different colours?
Well, as we said earlier, each element is made of different atoms. Different atoms need different amounts of energy to get them to the excited state, and so they release different amounts of energy going back to the ground state: an atom that needs lots of energy put in to excite it will release lots of energy when it cools down. When the energy is emitted as light, a different energy corresponds to a different colour.

Hope this helps: it's not a complete explanation, and the ground state - excited state things is a little bit more complicated than the quick explanation here. However, this explanation is basically what's going on when you heat up different compunds and they emit light, so I hope it helps.

Please don't hesitate to contact me again if I've missed anything out or if you need more info.

Good luck and best wishes.

---------- FOLLOW-UP ----------

QUESTION: hi again.. thanku so much for the easy understandable explanation. and i wanted to ask to something else also. i found this information on the internet after a lot of hard work.. phew.. here it is:

"In addition to using a spectroscope, scientists can also use flame tests to identify elements in a substance. When elements are subjected to the intense heat of a flame, the atoms become energized, and the element gives off a distinctive color. For example, the sodium and chlorine atoms in table salt are split apart when heated by a flame. Each solitary sodium atom then has one unpaired electron in its outer shell (previously shared with a chlorine atom, which lacks an electron in its outer shell). This electron gains energy from the flame’s heat and then loses its excess energy as heat and light"
so is this the same this that happens to every compound thats heated...like they split up and loose and gain energy? and was it that the atoms of sodium caused the distinctive color of flame? and if calcium chloride is heated then would it be calcium that caused the colored flame?  

Answer
Hi, and thanks for the follow up.

Spectroscopy is the main use of this weird property of metal compounds.

It's important to note that the atoms themselevs don't split up: what's happening is that the compound (two or more atoms joined together, like Sodium and Chlorine in sodium chloride) is being broken in to individual atoms, which then undergo that excited / ground state thing, losing and gaining energy. The internet explaination you found describes this process in a bit more detail than I did.

Although most atoms can undergo excitation, only metal atoms take part in the processes we've described here: non-metal atoms just "sit back and watch". So yes, it's the metal that causes the colour. In the case of sodium chloride, bits of the compound break up in to sodium atoms and chlorine atoms, and the sodium atoms undergo the excitation - emission process.

Try sprinkling sodium chloride (salt) and sodium hydrogencarbonate (baking soda) on to a gas flame: they'll both give the intense yellow flame characteristic of sodium, even though the non-metal bits of the compounds are different.

Flame colour can't tell you exactly what compound you've got, but it can tell you what metals are present. Doesn't matter what the compound is, the flame colour is charecteristic of the metal involved:

Sodium compounds = Yellow
Strontium compounds = Red
Potassium compounds = Purple
Copper compounds = green-blue

This is how colourful fireworks are made: you just include a little bit of the appropriate compound in with your explosive mixture and you get the intense colours of a fireworks display.

Hope this helps, but again, don't hesitate if there's anything I've missed.

Thanks again.