What is the opposite of FIRE ?

On http://www.straightdope.com/columns/read/2425/what-exactly-is-fire
I found your answer on what is fire.
But I would like to know:
—>What is the opposite of FIRE ?
(Thinking of a car’s engine).

Ice?

Why would there be an opposite? It’s like asking what is the opposite of elephants.

No fire.

In a ‘reverse engine’ put in burned fuel, smog and vaporised water.
Then make it work (turn) (push the car from a mountain)
ad the “opposite of fire”

and clean air and fuel comes out.

Is this do-able?
What do we need more?

Sorry for my English. I am from ze nezzerlenz

No, it is not do-able.

It is technically possible to resynthesise fuel from the products of combustion (not really with anything similar to the ‘reverse engine’ concept you describe) , but it will always require the input of a greater quantity of energy than could be obtained by burning the fuel.

You can’t get something for nothing - that’s the law (second, of thermodynamics)

That’s the first law. The second law is that you can’t round-trip something to something else and back again without paying.

Probably the closest to what you’re asking for is cold fusion, but it’s currently not possible.

If by “closest” you mean not related in any conceivable way, then yes.

Hire?

Actually, the first law is:

Heat is work and work is heat

And of course, the second law is:

Heat cannot of itself pass from one body to a hotter body
Heat won’t pass from a cooler to a hotter
You can try it if you like but you’d far better not-a
'Cuz the cold in the cooler will get hotter as a rule-a
'Cuz the hotter body’s heat will pass to the cooler

We can get energy from the sun and the wind.
Look at this machine more like an air-cleaning-machine.
And fridge-like and pressure-depressure-engine.
Clean air and oil (not fuel) should be possible.

Maybe it can bring down the temperature of the air as well.

I don’t really believe in the CO2 problem, the temp-rize because of that. (Less than 1 degree in so many years).
No.

We’ve been burning fuels and things for so many years now.
That’s where the heat comes from.

This must be reversable.
So I say: it’s do-able.

Opposite of “fire” is “no, don’t shoot.”

To paraphrase the above answers - the answer is that you can do it, but you will always have losses in the system, so it can never be 100% reversible. Your question is almost this: Can you drive up a mountain, and collect the exhaust gasses from the car whilst you do so, then using the energy available by running down the mountian again, reverse the chemical reactions involved in the combustion of the fuel? Which is a variant of a perpetual motion machine.

The answer is that you could, in principle make a device that was powered by the energy available, and that did indeed reverse the reactions. But, and a huge but, you would only find that you were able to reverse a fraction of the reactions that occured on the way up. As has been mentioned, the second law of thermodynamics is the one that kills you. In particular, a gentleman by the name of Carnot placed explicit limits on part of what you can do. Every part of the system is less than 100% efficient, and Carnot described the absolute limits to possible efficiency of any heat engine, which a car engine is a rather imperfect version of. You can’t get all the energy out of burning the fuel into moving the car, and due to other losses you won’t be able to get all the energy available in moving the car back into the fuel air synthesis.

The laws of thermodynamics can be paraphrased as:
You must play.
You can’t win.
You can’t break even.
You can’t leave the game.
Entropy requires no maintenance.

Well, fire is rapid exothermic oxidation of something, emitting light and heat.

The opposite of that would be: gradual endothermic reduction of something absorbing light and heat.

As an example from Encyclopedia Britannica:
“One endothermic reaction is the smelting reduction of zinc oxide (ZnO) by carbon monoxide (CO) to yield zinc (Zn) metal and carbon dioxide (CO2)”

Which is probably not something you had in mind. Problem is, what do you mean by ‘opposite’? Fire is a process, and we usually don’t have a clear opposite for processes. e.g. what’s the opposite of raining.

We’re getting close…

Imagine a car,
in second gear, on a mountain, and rolling backwards.
The engine turns backwards…
makes vacuums…
pulls in dirty air…

You’re not really getting the point that in order to reverse the process you need to add more energy into the system. You can’t win.

No need to win.
There’s solar and wind-energy we can use. Or even manpower.
That’s not the issue.

You write

…so you know how to do that?
Tell me.
Because that is the question mark, how can we really reverse that proces.?!?
How?

Let me rephrase your question. What you’re really asking is:

Is it possible to construct a chemical process where carbon dioxide and water (the two main products of combustion) are re-combined to make oxygen and some sort of fuel, realizing that this process will require a net input of energy?

I’m not a chemical engineer (hopefully one will be along shortly to confirm), but I’d guess that the answer is yes, it’s possible to construct such a chemical process.

However, the real question is why would you want to do such a thing? If the intention is to scrub carbon dioxide from the atmosphere, I imagine that there are cheaper and more efficient ways to sequester carbon dioxide than to turn it into methanol. If the intention is to create fuel, again I imagine that there are cheaper and easier ways to manufacture fuel.

Fossil fuels were made through the use of solar power. Plants used energy from the sun to grow, to take carbon, hydrogen, and oxygen out of the air in the form of carbon dioxide and water, and transform them into what we now call organic materials. Coal, oil, natural gas, peat, tar sands and all the rest can be burned to provide energy for our vehicles - although each requires in reality a long refining process that consumes large amounts of additional energy.

The solar power that grew the plants was essentially free for the taking. The sun shines everywhere. No individual plant was a major source of energy but the billions and trillions of plants over millions of years added up to a tremendous source of stored energy.

How to reverse this? There is no natural process by which stored organic matter energy can use the sun to reverse into carbon dioxide and water. And we don’t want to wait millions or hundreds of millions of years for it to happen. You would need to concentrate that energy and apply it not to billions of individual plants but small amounts of reclaimed oil. That’s a totally different issue. Concentrating energy is one of the hardest things for us to do. It takes vast amounts of energy to build solar cells, for example, and they are low in efficiency.

Every calculation we can make tells us that the process of reversing millions of years of energy concentration in organic substances would require millions of years and more energy than it took to make those substances in the first place. It’s not feasible, even theoretically.

Does that help?