Some call it a solution; others call it a mixture. This can depend on which industry you are in. So you can call it either. Now let's think about what happens when we mix butane and propane liquids. They will mix quite happily as both are non-polar. With all that jostling around, many of the bonds between butane molecules will be broken and replaced by bonds between butane and propane molecules.
The bonding energies are so similar the molecules don't really care. The resulting liquid will look similar to pure liquid butane for instance. It fits the definition of an 'ideal solution' just as nicely as it conforms to 'mixture'. Paraphrasing one of my corrspondants Jim now: propane and butane interact via van der Waals forces, but they interact nearly equally with one another as with themselves.
For that reason the mixture is called an 'ideal solution', and why it conforms well to Raoult's Law. This is because the interaction energies between water molecules, between ethanol molecules, and between water and ethanol molecules, all differ.
Boiling happens when the total vapour pressure VP of the liquid exceeds the surrounding air pressure. The VP of water reaches 'one atmosphere' at C, and that's where it boils. Well, of course, if you are way up a mountain where the air pressure is lower than 'one atmosphere', the water will reach that pressure at a lower temperature and boil below C.
This happens. Exactly the same applies to the fuels we use. They 'boil' when their VP reachs the surrounding air pressure. But evaporation will still happen below the 'boiling point': think of hot water steaming away.
In fact a liquid is losing vapour at all temperatures - but the rate of loss will vary. Note that as a volume of liquid loses vapour it is losing the most energetic molecules, so the temperature of the remaining liquid will fall. Any user of gas stoves will know how cold the bottom of the canister gets when the stove has been running for a while.
Butane boils at The difference in boiling point BP is due to the difference in masses of the molecules and the different intermolecular forces between the molecules. This is all determined by the size and shape of the molecules. The detailed chemical construction of the molecule plays a significant part in this, but we will ignore that for the present.
With these as the extremes, the 'boiling point' of a mix of these two liquids will of course be somewhere in between. What is the lowest temperature you can get vapour from a canister? When the temperature of the liquids falls below its current boiling point the pressure inside the canister will no longer drive vapour out.
That will depend on the mixture of fuels in the canister, and is discussed further on. So far we have mostly considered ordinary butane, which boils at Some manufacturers, including MSR and Kovea, offer canisters which contain isobutane instead of butane. Isobutane boils at C. What is the difference, and what difference does this make?
The difference between butane and isobutane lies in the arrangement of the carbon atoms. There is only one way to string three carbon atoms together: in a simple chain of three. But butane has a fourth atom: where does it go? If it connects to the end of the chain you have butane or n-butane. If it connects to the atom in the middle the chain is no longer a simple straight one, and you have isobutane. For those with pencil and paper and an enquiring mind, you can now try to work out how many variations there might be for 5 or 6 or more carbon atoms.
Yeah - messy! The technical answer to what difference this makes is really 'not much': it's all a matter of degree. Sorry about the pun! The energy of the bond between the molecules changes a little because the shape of the molecule has changed, and this is responsible for the shift in the nominal BP.
But the mixture still behaves like an ideal solution and obeys Raoults Law. The big difference for us comes when we consider the lower limits of boiling. Does all the propane boil off leaving all the butane? You are quite close to the boiling point of butane and it will still be giving off lots of vapour. Think of a pot of very hot water - still giving off steam.
The liquid butane would still be giving off butane vapour at -5 C, just at a lower rate. What happens here with a mixture is that the vapour pressure VP in the canister will be a combination of the vapour pressures of pure butane and propane. Is it the cold-weather performance of propane, the light, energy-packed properties of isobutane, or a mixture of both?
Whatever your next adventure, be sure to fuel up at your local outdoor gear store. Jetboil Eureka! Johnson Outdoors Family Eureka! Find A Store. Johnson Outdoors Family. July 25, Propane Propane is one of the most widely used fuels, and for good reason.
Isobutane Isobutane has two distinct advantages over propane. Are there any similarities? Please Follow Us Online. Subscribe to Budget Propane's Blog Today! Get a Quote. Request a Delivery. Make a Payment. Customer Care. Call Now Contact Details. In reply to Tall Clare: Interesting question! It seems from the article that they are practically interchangeable, but I wonder if they need different jets?
The torch flares like a bastard and it's impossible to get a steady blue flame out of it. I haven't yet bought a pure propane one to compare, but it worked perfectly before and nothing else has changed, so I suspect that the butane may be causing the problem. In reply to Tall Clare: All gas cylinders for portable "camping" stoves used to be Butane.
Of course some of these cylinders made their way back from the expeditions and were highly sought after for alpine climbing.
Slowly the production of the mixed cylinders increased and they became available more generally, usually only in the specialist outdoor shops. Eventually the production of pure Butane cylinders all but ceased - I'm not even sure if those sold to the building trade for blow-torches and the like are still Butane only, though they do use the same fitting as camping stoves.
In reply to a lakeland climber: You'd expect Propane cylinders to be stronger than Butane cylinders.
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