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Name: Sari
Status: student
Grade: 9-12
Location: IL
Country: USA
Date: Spring 2009

I have been trying to find some information on lye. My class made soap on a small scale but we are now trying to make it on a larger scale. Part of the saponification process is mixing lye in distilled water. When we did this on the small scale, there were no noticeable odor or fumes. When we did it on a larger scale, some people claimed to smell something and saw fumes upon mixing the two. Assuming that the ratio of lye to water was proportional, were the odor and fumes the appropriate signs of the reaction? If so, is it dangerous for anyone to be around these fumes? Also, besides not standing directly over the beaker and breathing in the fumes, are there any safety precautions that we can take to ensure that no one is hurt?

Lye is also known as caustic soda or Sodium Hydroxide. Its chemical formula is NaOH.

Caustic Soda dissolving in water is an extremely exothermic reaction - a great deal of heat is generated as the Caustic Soda is separated into its component ions On a small scale this means that the water will get warmer. On a larger scale enough heat can be generated to cause the water to boil - and this can be extremely dangerous.

When dissolving NaOH in water the following precautions should be observed to minimise the risk of boiling the solution.

Have Sodium Bicarbonate or vinegar available on hand to neutralise spills. Use a large container with a wide mouth - (I prefer one that would seem to be too large - e.g. a 2 litre container to mix 500mls. The wide mouth prevents the fountain effect in case you do boil the mixture.)

Use cool water - just less than body temperature - never hot or cold. Always add flakes to the water - never the other way around. Add the NaOH flakes a little at a time and stir at every step. DO NOT add all the flakes at once.

Be aware of the temperature of the mix - if the mix become hot then allow to cool before proceeding further. Caustic Soda is extremely corrosive. It should be regarded as more dangerous than most acid solutions.

If Caustic Soda is splashed or spilled on skin or eyes, wash with copious amounts of water - up to 30 minutes may be necessary for strong solutions.

The fumes can also be dangerous. If inhaled they can cause irritation of mucous membranes, or burns in the case of strong exposure. Always mix this in a hood with proper safety protection including goggles, apron, and gloves.

As a laboratory technician I am regularly called upon to mix chemicals and prepare solutions. I regard Caustic Soda as one of the most hazardous chemicals in the school store. A solution of caustic Soda will do significantly more damage to your skin than a similar solution of most acids - even hydrochloric. Just as it turns your lamb fat or olive oil into soap, it will convert the oils in your skin and the fats under the skin into soap - and that will result in major damage, not just in the immediate area of the spill, but in surrounding tissues as well. Please treat Caustic soda as extremely dangerous.

Nigel Skelton


Lye is sodium hydroxide or caustic soda. When it dissolves in water the energy released from the hydration of the Na+ and OH- ions heats up the solution. Although this may not boil the water as a whole the local heating around the dissolving NaOH can cause caustic fumes that irritate the lungs and throat if inhaled or can seriously damage the eyes if you get spray in them.

There are a couple of ways to avoid this. Firstly: add the NaOH (either a solid or a concentrated solution) to the water, not the other way around. Secondly: you can use ice/water mix instead of water or place the flask containing the water in an ice/salt bath to keep it really cold. Finally: there is no substitute for a fume hood, I always make up NaOH solutions in the hood, sash down with safety spectacles, lab coat and gloves on.

NaOH is one of the most common chemicals in the lab, but it is also responsible for some of the worst incidents. Please be careful when using it: NaOH in the eyes poses a real risk of serious eye damage. To be honest, making up of NaOH solutions from solid is best performed by your teacher or a qualified chemist.

Best wishes

Tom Collins


Let us assume you are working with solid NaOH. Dissolving NaOH in water is an exothermic process. The solution's temperature increases. If a student puts too much NaOH into too little water, the temperature can rise to the point where some of the solution begins to evaporate. I suspect your team mates have been observing this process. To avoid repeats, just add the NaOH more slowly into a larger volume of water.

All team members involved at the lab station should be wearing goggles and gloves. If aprons are available in your lab, put them on. It is best to use a hood. Be sure to clean up any spills/spatters with lots of water and baking soda.

Warren Young

One of the most dangerous aspects of diluting lye, which is Sodium Hydroxide (also called NaOH) is that it generates lots of heat. I have seen the dilution generate enough heat to melt through the bottom of a 5-gallon bucket and cause a dangerous spill.

The fumes you saw were probably related to the heat causing some rapid evaporation and generation of vapors.

There are many safety precautions that you need to take. First is the use of protective eyewear, gloves, and aprons for everyone handling the materials. Second, perform the dilution (and all chemical reactions) in a well ventilated area. Opening doors and windows is good, but performing reactions in a hood or outside when possible is much better. Third, to mitigate the heat problem, dilute the lye into the water a little bit at a time, letting it cool between each addition. Another method would be to start the dilution in ice water or to put the reaction vessel into an ice bath.

Ian Farrell

You did not indicate quantitatively what "small" and "large" scales were. Chemical processes often do not scale up linearly, as you discovered. Sodium hydroxide (lye) reacts vigorously with water generating a large amount of heat. Here are some precautions to take:

1. Full face shield (Looks like a welder's face shield, but clear and colorless. Available from any chemical equipment vendor.)

2. Do the experiment in a fume hood.

3. Have sodium bicarbonate on hand to neutralize any spills.

4. Use heavy duty nitrile rubber gloves and a full length rubber apron.

5. Use an ice bath to minimize the temperature rise.

6. Add the sodium hydroxide to the cold water slowly with continuous stirring.

7. Do NOT use any aluminum tools, etc. Aluminum reacts vigorously with sodium hydroxide. One of the reaction products is flammable hydrogen gas.

8. The odor you detected comes from tiny droplets of water containing some sodium hydroxide. This is a warning that the temperature is too high. That is, the addition of NaOH is too fast. Obviously, inhaling these droplets is not a good idea.

9. Ask yourself, "Is a large batch of this reagent really necessary?" If you cannot find a really valid reason, stick to the small batches. Lye is a dangerous chemical. I assume yours is a chemistry class. Are your students sufficiently prepared to handle such corrosive chemicals? There are of course legal liability issues.

10. The mist generated by mixing lye and water should be your "warning bell" that "new" things can start to happen when you "scale up" a chemical process.

11. My bottom line recommendation is not to try a large batch. Too many nasty possibilities can be lurking in the shadows.

12. A unit on the safe handling of toxic and/or corrosive chemicals might be a better lesson.

But do not actually use a toxic and/or corrosive chemical.

Vince Calder

The main issue you will run into in scaling up any reaction is that the energy involved in carrying out the reaction, such as heat produced, is proportional to the mass of the reacting materials. The surface area of the reaction, through which energy is transferred to or from the surroundings, is usually proportional to the 2/3 power of the mass of the reacting materials. This means that when the reaction is carried out on a larger scale, it does not transfer energy to the surroundings as quickly. If the reaction produces heat, it will tend to get hotter (attain a higher temperature) at large scale for this reason.

Dissolving lye in water releases much heat. When the reaction is scaled up, it gets even hotter. Most likely, what your students observed was a mist of lye solution in the air, produced by the heat from the lye solution.

These fumes are dangerous, because lye in water is dangerous. Their chief hazards will be from inhalation and eye contact, with some possibility of injury from skin contact as well. If the dilution is carried out slowly, so that the temperature of the solution is minimized, the hazards will be slight. If you are concerned, covering the vessel with a watch glass (or even a piece of paper or cardboard) will reduce exposure to the mist. Carrying out the dilution in a fume hood will provide even greater protection, but may be difficult logistically.

Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming

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