Hi shaunm,
If you have a solution of a chemical dissolved in water (or any other solvent, like ethanol for example), you can make the solution more concentrated either by adding more of the chemical, or by removing some of the solvent. You could do this by heating the solution (carefully!) or just leaving the solution to sit at room temperature with the lid off for a while – some of the solvent will then evaporate and your solution will then be more concentrated. If you have a solid or liquid chemical that is not actually dissolved in anything (we would describe the chemical as being ‘neat’) then that is as ‘concentrated’ as it will get.
In the chemistry lab we use a machine called a rotary evaporator (rotavap for short) to remove solvent from solutions (ie. to concentrate our solutions). Have a look at this site if you want to see what one looks like: http://www.buchi.com/rotary-evaporator_rotavapor.4695.0.html
I’m going to try explaining how a rotavap works, although it is difficult trying to explain this without being able to actually show it to you! Here goes anyway… 🙂 (look at the picture as you read through this)
You attach your flask to the rotavap and lower it into the water bath (on the right in the picture), where you would then have the flask rotating. You also have the rotavap connected to a vacuum pump by a pipe/hose (you would connect this pipe to the knob next to the tap on the left in the picture). When you have the rotavap under vacuum, the pressure inside it is lower than the atmospheric pressure. When you have a pressure that is lower than atmospheric (normal) pressure, the boiling point of liquids is lower than what it would be at atmospheric pressure (if you haven’t learnt gas laws yet then this might not make much sense to you yet – you will learn about this eventually). The idea is that you have the rotavap system under a lower than atmospheric pressure, so then your solvent will boil at a lower temperature than its normal boiling point. Your solvent then evaporates from your flask, which you have spinning, and then goes and hits the coil that is full of cold water (top in the picture). The solvent vapour hits the outside of the cold coil, so then it become liquid again (condenses) and falls into the collecting flask (under the coil in the picture). How strong a vacuum you use (ie. how low your pressure gets) depends on what solvent you are trying to remove and what its normal boiling point is. If your solvent has a low BP (we would describe this solvent as being ‘volatile’) then you would not use as strong a vacuum (as low a pressure) as you would for one that has a higher BP (which is described as being less volatile). The reason you have your flask spinning is so that the solution in your flask gets plenty of surface area and so that the solvent will boil evenly. And you have it in a water bath so that you can heat the water a little bit if you want to (this comes in handy on really really cold days, but its usually a good idea not to heat your solution too much – the whole point of a rotavap is to remove your solvent at a low temperature!).
I hope my explanation of how a rotavap works makes some sense to you. If you are really interested, you might be able to find some videos on YouTube.
Hi shaunm,
If you have a solution of a chemical dissolved in water (or any other solvent, like ethanol for example), you can make the solution more concentrated either by adding more of the chemical, or by removing some of the solvent. You could do this by heating the solution (carefully!) or just leaving the solution to sit at room temperature with the lid off for a while – some of the solvent will then evaporate and your solution will then be more concentrated. If you have a solid or liquid chemical that is not actually dissolved in anything (we would describe the chemical as being ‘neat’) then that is as ‘concentrated’ as it will get.
In the chemistry lab we use a machine called a rotary evaporator (rotavap for short) to remove solvent from solutions (ie. to concentrate our solutions). Have a look at this site if you want to see what one looks like: http://www.buchi.com/rotary-evaporator_rotavapor.4695.0.html
I’m going to try explaining how a rotavap works, although it is difficult trying to explain this without being able to actually show it to you! Here goes anyway… 🙂 (look at the picture as you read through this)
You attach your flask to the rotavap and lower it into the water bath (on the right in the picture), where you would then have the flask rotating. You also have the rotavap connected to a vacuum pump by a pipe/hose (you would connect this pipe to the knob next to the tap on the left in the picture). When you have the rotavap under vacuum, the pressure inside it is lower than the atmospheric pressure. When you have a pressure that is lower than atmospheric (normal) pressure, the boiling point of liquids is lower than what it would be at atmospheric pressure (if you haven’t learnt gas laws yet then this might not make much sense to you yet – you will learn about this eventually). The idea is that you have the rotavap system under a lower than atmospheric pressure, so then your solvent will boil at a lower temperature than its normal boiling point. Your solvent then evaporates from your flask, which you have spinning, and then goes and hits the coil that is full of cold water (top in the picture). The solvent vapour hits the outside of the cold coil, so then it become liquid again (condenses) and falls into the collecting flask (under the coil in the picture). How strong a vacuum you use (ie. how low your pressure gets) depends on what solvent you are trying to remove and what its normal boiling point is. If your solvent has a low BP (we would describe this solvent as being ‘volatile’) then you would not use as strong a vacuum (as low a pressure) as you would for one that has a higher BP (which is described as being less volatile). The reason you have your flask spinning is so that the solution in your flask gets plenty of surface area and so that the solvent will boil evenly. And you have it in a water bath so that you can heat the water a little bit if you want to (this comes in handy on really really cold days, but its usually a good idea not to heat your solution too much – the whole point of a rotavap is to remove your solvent at a low temperature!).
I hope my explanation of how a rotavap works makes some sense to you. If you are really interested, you might be able to find some videos on YouTube.
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No need for me to answer I see. Top work @Arti 🙂
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Great explanation Arti,
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