Been doing some chemistry research and cannot find the answer to this - what does methylene chloride decompose into when heated at around 200-300 F?
It decomposes at much higher temperatures, over 500 C. Here is a paper on it:
https://royalsocietypublishing.org/doi/abs/10.1098/rspa.1959.0058
Great, thanks. So at low temps (ie 300 f) it just evaporates?
It sure does. I’ve evaporated thousands of liters of methylene chloride after using it to do liquid-liquid extractions. You do it in a kuderna danish flask topped with a three-ball Snyder column. It boils just over body temperature, around 103 degrees Fahrenheit.
urban archeologists might come across some glass balls filled with a clear liquid. These are old-school fire extinguishers, filled with carbon tetrachloride-a close cousin of dichloromethane. You throw the balls at the base of the fire. The glass breaks, releasing the carbon tet. Because of its very low boiling point, it instantly vaporizes, displacing oxygen. Also, the chlorine scavenges the free radical reactions which feed the fire.
Very efficient, very simple, and only somewhat carcinogenic. Eventually.
If methlyene chloride boils at 103 F, what is it at room temperature (60-80 F?) Vapor or liquid?
Not to mention those “bubbling” Christmas lights.
If I recall correctly, the instructions that came with those lamps told you to just let the liquid evaporate if the lamp broke.
Is this a trick question? Room temperature is lower than the boiling point (and above the melting point), so it will be a liquid. Assuming standard pressure, of course 
It’d be a mixture of vapor and liquid. Precisely what mixture would depend on the size of the room or other container, and the amount of the substance.
If you keep it sealed in a bottle, at 78 F and atmospheric pressure, the liquid at the bottom will have around 0.02% (by volume) air, while the air on top will have around 60% (by volume) methylene chloride.
Provided that the bottle was sufficiently small that there was any liquid left in it at all.
Yeah Chronos is right : there has to be liquid left in it. But the size of the bottle doesn’t matter as long as it is at 1 atm (highly volatile liquid containers have a small vent like your gasoline tank) and 68 F.
Follow-up question…is methylene chloride water-soluble?
Slightly. Maybe 20 grams of methylene chloride into a liter of water, depending on the temperature. More into cold water, less into warm. It’s why it’s commonly used for liquid-liquid extractions of non-polar analytes in separatory funnels.
Second that. Here’s a table (Table 1 ) and an equation for solubility at different temps
Is this something that was abandoned after rotovaps? Never heard of it.
No, the Kuderna-Danish flask and three ball Snyder is still in common use. For some procedures it’s the only approved method if you’re doing an EPA procedure for pesticides, or at least is was when I was a chemist. That’s been a couple of decades so this is from memory.
You’d take a liter of water from a test well or other source, put it in a separatory funnel, extract it with 80 milliliters of methylene chloride collecting the methylene chloride in a Kuderna-Danish flask, repeat the extraction twice more then evaporate your combined extract using a three-ball Snyder down to 10 milliliters or less. transfer to a volumetric flask and dilute to an appropriate concentration. At the time I was using a Hewlett-Packard 5890 gas chromatograph once I had a sample ready to analyze. I used to be doing these half a dozen at a time lined up in a water bath.
The Kuderna-Danish and three ball Snyder sounds like something you’d have to go to a really specialized bakery/brothel to obtain, but every scientific supply house has them.