Why is the burning of wax an irreversible reaction?

Answer 1

When the candle is burned, chemical reactions take place between the wax and oxygen. These chemical reactions release energy in the form of heat and light. These reactions are not going to easily be run backwards. If they did, we might have candles popping up all over the place....

On the other hand, melting the wax is just a phase change. The physical properties change, but the chemical properties do not. Since the wax melts because the temperature of the wax is raised, once the heat source is removed, the wax will begin to lose heat, assuming that ambient temperature is low enough. Eventually, the wax will go through another phase change and become solid.

So, the short answer is: burning is irreversible because it involves a chemical reaction, while melting is reversible because it's just a phase change. The caveat is: the chemical reaction is reversible too. Ever wonder where your ear wax comes from? Candles and crayons aren't a part of a normal diet....
A physical change is when the same compound/chemical just changes form, for example, water experiences physical changes when it goes from ice to liquid to steam. The identity of the compound is completely unchanged however.

A chemical change is one where bonds have changed, therefore the molecule that existed the change is now a completely different molecule. For example, burning glucose in oxygen gives you carbon dioxide and water. Notice the glucose has combined with the oxygen to form completely new chemicals, i.e. CO2 and H2O. (It is worth remembering that burning is a chemical reaction with oxygen).

So in your example, the flame melts the wax and there is a physical change occurring. The wax goes from solid to liquid, but it is always wax, just in different forms If the melted was cools again, it will go back to being the same solid wax as before. When the wax burns, and the wax combines with the oxygen to form CO2, carbon monoxide, carbon particles (incomplete burning/oxidation) and water. After it burns, you can't get the wax back! The burning has broken the bonds in the wax and made new compounds.

Note that when a candle burns, the wax does two things. First it melts, and then it burns. What burns is actually melted wax! The heat of the candle melts the nearby wax, and that melted wax is absorbed by the candle's wick. The melted wax burns on the wick -- the wick itself isn't burning, just the wax on it. However, as the candle burns down, what happens is there isn't enough wax nearby to melt, and so the end of the wick does burn a little, which is why it slowly gets shorter (at the same speed as the candle gets shorter).

It is a chemical change because there is new matter created but flammability is more of a property than a chemical change. A physical change is still the same matter but is in a different form like cutting paper. Its still paper. But if you burn paper, that's a chemical change because its new matter now and its really hard to change back

Answer 2

The burning of wax is an irreversible reaction because it undergoes a chemical change during combustion, converting the wax molecules into new molecules such as carbon dioxide and water vapor. These new molecules have different chemical properties from the original wax molecules and cannot be easily converted back into wax.

Answer 3

you start with wax (a hydrocarbon)

and end with CO2 and H2O.

Hard to chance that back to wax.

Put simply, the burning process uses up the fuel (the wax) and emits it into the air as non-solids.

Answer 4

A candle "burns" around the wick. It doesn't physically change the wick or the wax. The wax is physically pulled up the wick and heated by the suspended flame, and changed into hot liquid wax.

Answer 5

If you identify a set of objects well insulated from all the other objects (at least under the points of view you are interested in) you can call them a thermodynamic isolated system.

A burning candle that does not receive other energy from other objects is a good example of isolated system (at least in a good approximation).

in thermodynamics a thermodynamic potential exists, called entropy, so that in nature an isolated system spontaneously perform all the processes increasing entropy, while it is not able to perform processes decreasing entropy.

Processes where a system decreases the entropy can occur, but the system has not to be insulated and the entropy has to increase in other places outside the system to balance the decrease inside the system.

Entropy is related to the "order" of the microscopic state of the system. More a system is ordered, in the sense that its microscopic elements like atoms or molecules have a unique well defined position and state, less the entropy,. More the system is disordered, in the sense that the state and position of molecules and atoms is casual, greater the entropy.

A burning candle transform the well ordered candle form, were wax is well formed, in a much less ordered shape freeing energy, thus entropy increases and the process is spontaneous. The inverse process is possible, but it is not spontaneous. It happens when the candle is fabricated from natural wax and requires work and use of energy, thus interaction with other systems.

In general it is possible to demonstrate that fire, and almost all other forms of heath production, are all irreversible processes, related to passage from order to disorder.

Answer 6

Burning of a candle is a chemical change because:

1.When candle burns, Carbon dioxide and water vapor are produced, so new substances are formed.

2. The change is permanent and cannot be reversed.

3.Heat and light energy are evolved and released.

4.There is an increase in mass of the substance.

Chemical change- A change which is permanent, not easily reversible, in which the identity of the substance is altered which may result in formation of one or more new substances. The composition of the substance is altered in a chemical change. The mass of the substance changes,energy is also evolved or absorbed.

Answer 7

Put simply, the burning process uses up the fuel (the wax) and emits it into the air as non-solids.

Answer 8

becuase you can't "unburn" a piece of paper

Answer 9

Because burning occur, gases are released.

Answer 10

yes!