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I'm writing a book based on a dream I had, but after some research I can't find too much on a specific reaction I want. The main character mixes a clear liquid and yellow powder which hisses and bubbles creating a lot of yellow gas. The gas was slightly toxic, but he was exposed for around a minute and nothing super dangerous happened.

Are their any chemicals that make this reaction and includes a powder and liquid? The colors don't have to match, but I would prefer if the gas had some visible color so it's easier for me to describe.

Haybug
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    This might fit the bill. You're not doing it IRL, so the safety concerns are minimal. You might employ some literary freedom for the liquid part: ethanol is used here, but only very sparingly. You could also stick with vinegar and baking soda and just add some turmeric spice to the baking soda - it'll be a yellow mixture and maybe (?) produce a gas. Try experimenting. – Todd Minehardt Jun 13 '21 at 18:09
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    The reaction of ammonium dichromate described by Todd Minehart is spectacular. But it uses a substance which is cancerigen, And so it is forbidden to buy it in many countries. On the other hand, it produces a huge amount of green ashes, whose volume is much greater than the original volume of ammonium dichromate. It produces y gas which is colorless, nitrogen. – Maurice Jun 13 '21 at 19:21
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    The number of colored gas is not huge. Chlorine $\ce{Cl2}$ is pale green. But it is toxic : it was used during World War I as a poison gas. $\ce{NO2}$ is brown, but it is corrosive and toxic. Iodine $\ce{I2}$ gives violet vapors, but it must be a little heated. And this gaz is also corrosive. So your dream is probably impossible to realize. Sorry for being negative ! – Maurice Jun 13 '21 at 19:32

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Some pretty interesting ideas in chemistry have been preceded by dreams. A good mystery novel could be the beginning of a career.

Well, the colored gases are listed on Wikipedia (also colors of some solids):

enter image description here

Considering first your specification of slightly toxic, I would aim for a small quantity, which suggests that we need intense color. Iodine is one of the best: very purple, very visible. It needs a bit of heat to make it gaseous, but not much: a few crystals dropped on a very warm surface will generate a lot of purple. Not really hissing; perhaps this isn't exactly what you want. You could drop a few iodine crystals into a dish or cup of hot water...

Then there is NO$_2$. While the picture says dark brown, I think deep orange red is more accurate. It's toxic, but you can see it and smell it, so it may not be a terrible hazard because you can get away from it - again, in small amounts. Not like the explosion in Lebanon (Ref 2). I was in the lab when we made some NO$_2$ - accidentally. A clear liquid (a 5-gallon bucket of 25% HNO$_3$) was treated with what was thought to be sodium nitrate, but the 50# bag of solid was only partially filled, and was torn so it read SODIUM NITR&)E, or something similar. It was yellowish - thought to be somewhat aged - but it was really sodium nitrite, NaNO$_2$. About 5 pounds got dropped into the bucket. That was followed by a large POOF - not really an explosion, but suddenly a HUGE red cloud of NO$_2$ appeared. Fortunately, there was another exit to the lab, and no one was even slightly injured. It took a few hours to ventilate the lab. Now, on a small scale, any clear acid will react with NaNO$_2$ to produce orange-red NO$_2$. It's heavier than air, so it can be fairly easily contained in a beaker if your liquid (acid) is only a small amount at the bottom. So it could be dangerous, but can be handled safely.

enter image description here

While all the other gases could also be handled safely, they do seem more hazardous to me - but in a book you can always specify the precautions you want to emphasize.

Ref 1. https://en.wikipedia.org/wiki/Color_of_chemicals

Ref 2. https://www.bbc.com/news/explainers-53664064

James Gaidis
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  • (+1) Nice answer, but iodine is now problematic, as per the DEA in the USA, because people making illicit drugs use it. Both nitrogen dioxide and bromine vapor are way too toxic, so no good options. – Ed V Jun 13 '21 at 19:37
  • @Ed V: Perhaps the plot depends on not-so-good options, like finding stocks of illegal or hazardous chemicals in the basement of the school/hospital/courthouse. – James Gaidis Jun 13 '21 at 20:57