If one were to adjust the positive temperature in an experiment, they would use a heating plate, or a stovetop.
How would one adjust negative temperatures in an experiment? (Not keeping the temperature constant, but decreasing and increasing the temperature at my will in specific amounts).
What I tried so far: "Spraying" liquid nitrogen at my work surface until it cools to the desired temperature. It is really inaccurate though.
Here's another idea: it's quite practical to pump heat just by passing electrical current through a device, which is usually just a plate with two wires:
I just love these things.
What's especially cool about them is that by simply reversing the current you reverse the heat flow! This just begs for a trivial control loop via thermocouple feedback.
A quick google query reveals commercial offers of heat baths built on this promising whopping 0.01 °C precision of regulation!
The elements are easily available in electronics shops, so with a good power supply it wouldn't be too hard to build your own low-cost precision heat bath.
Hope that helps!
For temperatures below room temperature cooling baths are used.
These baths consist of a solvent + something that has a low temperature already (ice, dry ice, or liquid nitrogen). A comprehensive list is given below:
Although all of these temperatures are technically achievable, in practice we rarely prepare most of them due to either cost (acetonitrile is quite expensive to be using for a cooling bath), toxicity (a benzene/CO2 bath is not ideal) or difficulty (a methanol/LN2 bath forms a very thick slurry which is difficult to work with).
Most reactions below room temperature are therefore conducted at 0 (ice/water) or at -78 (dry-ice/acetone), with in-between temperatures achieved by adding dry ice slowly to acetone until the desired temperature is reached.
In addition to using this kind of cooling baths, there are machines that will allow you to dial up a given temperature and they will cool the solvent to within a few degrees. These are incredibly useful but their size and cost restricts them from everyday use (most labs don't have one, and those that do won't have enough for everyone to use one everytime they need cryogenic conditions)
Temperature control for the home setup can be very difficult to control precisely. For high temperatures, heating your flask held within an oil bath is acceptable - never heat directly on a hot plate. One can achieve very accurate temperature regulation this way by adjusting the heater power.
For low temperatures, a cold bath may suffice, and NotNicolaou has provided a very nice answer in this regard, along with a very comprehensive table of components for reaching designated temperatures. However, the crux of your questions asks for a method that allows you to decrease and increase the reaction vessel temperature at will. Not easy. A cold bath is not likely to allow this, and so you need to engineer a solution based on how the 'real' labs do this.
A lot will depend on the scale of your reaction - micro vessels can be easily regulated with cooled gas flows. A regulated flow of evaporated liquid nitrogen is perfect for this, but not easy to implement. This is how most spectrometer temperature apparatus work, and is great for small vessels. If you are using a bigger reaction scale (anything in a round bottom flask for instance), as is most likely, then a liquid cooled bath is preferable.
To do this, you may need to consider having your cooling source external to your reaction bath, and applying a thermal regulation stage prior to passing it your reaction bath. Below is a rough schematic for this. Your cooling fluid is maintained at its lowest temperature (keep adding dry ice!). It is pumped through a heat exchange vector (such as a condenser) that has a high temperature medium (room temperature water, heated gas etc), and this heats your cooling medium to your desired temperature which is then passed into the reaction bath. This is ideally recycled back the source bath. The pump is typically a peristaltic pump design to prevent corrosion of the pump.
The critical element of the design is the heat exchanger, and some thought needs to be given to this. The simplest heat exchanger that the chemist uses is the condenser, although you might have to trial suitable diameters to manage heat exchange most effectively.
You can now regulate your reaction bath temperature up and down. You have two points from which to control the temperature of the final reaction bath: flow of cooling medium through the heat exchanger and the temperature and flow of the heat exchange medium. The slower the flow of your cooling medium through the heat exchanger, the greater the heat transfer. You can get very fine control of temperature using a setup of this nature, and it is essentially the same design of many temperature controllers.
The easiest - though not necessarily the cheapest - way to achieve a stable temperature below ambient is to use a refrigerated bath, together with a suitable bath fluid. Ethylene glycol - water mixtures can be used for moderately low temperatures; specialist silicone oils are better but more expensive.
If you can't conveniently immerse your experiment in an open-topped refrigerated bath then you need a circulating bath, which can pump the bath liquid around an external circuit. Cheaper ones have a single pressure pump so the external circuit needs to be closed, e.g. a jacketed vessel; more expensive ones can have balanced output and return pumps so they can circulate to an open container. With an open system you need to be aware of the possibility of siphoning when the circulator is switched off. A circulator should have the facility to connect an external temperature sensor, which could even be inside your reaction vessel, and the instrument will try and maintain the set temperature as measured by the external sensor. You may need to be aware of the possibility of condensation or icing, both on the chilled surfaces and in the bath fluid itself which may accumulate water over time.
Refrigerated baths aren't cheap, but they are fairly common pieces of equipment so if you are on a budget you could try looking for one at used equipment suppliers.
If you already have a temperature-controlled heated water bath or circulator then it may be more cost-effective to use it in combination with an immersion or flow-through chiller, which removes heat at an approximately constant rate allowing you to set a temperature below ambient on the heated bath. For a one-off or relatively short experiment you might be able to achieve a similar effect by running the circulating fluid through a loop immersed in a low-temperature mixture as described by NotNicolau - you'd just have to monitor the temperature of this mixture and top it up with the cold component as necessary.
