Unless you're a high-schooler building a nuclear fusion reactor, the hardest part of a science investigatory project often is coming up with a good idea. You want it to be cool yet feasible, novel but still useful.
That's why Ai-ni Bautista's science project on making liquid soap infused with guava leaf extract is so perfect — it's fun, easy, and fresh with a very practical outcome. This do-it-yourself soap will smell of lavender (and guava, of course) and even has health benefits for your skin.
Guava leaves contain phytochemicals that are astringent, allowing them to tone and tighten your skin. Guava leaves can also protect against ultraviolet radiation, a "major environmental factor" in "skin wrinkle formation and hyperpigmentation." The American Journal of Chinese Medicine even found that guava leaves' anti-inflammatory properties can be helpful in treating acne.
To make it even easier to ace your science investigatory project, we've deconstructed this chemistry experiment into step-by-step instructions. For the full video tutorial, be sure to check out the end of this article. Since the video has been made publicly available online, you can go right ahead and use it for your science fair or project — no need to ask permission!
This experiment will involve working with both sodium hydroxide and extremely high temperatures, so it will require safety precautions and definitely adult supervision. This guide will cover the important safety measures, but you can also read more about sodium hydroxide safety.
If the below information seems a little intimidating to you, you can try making hard soap instead of liquid soap. It's a little less involved, requires fewer ingredients and equipment, and is just as good as a science investigatory project. For details on that, check our full guide.
- about 50 guava leaves
- 16 oz. water
- 4 oz. sodium hydroxide (NaOH), also known as lye or caustic soda
- 20 oz. olive oil
- 8 oz. coconut oil
- a few spoonfuls of lavender-scented oil (or any other scent you prefer)
- a few drops of food coloring, any color you like
- vinegar (in case the lye comes in contact with skin)
- stove or other strong heat source, preferably an outdoor stove
- large pot
- three small containers (make sure they can all withstand boiling water!)
- safety mask
- safety goggles
- whisk or stick blender
- kitchen scale
- empty bottle or soap dispenser
Make sure that none of the equipment you use contains aluminum, tin, or zinc because lye will corrode all of those metals — and potentially produce highly flammable (read: dangerous) hydrogen gas. Stainless steel would be ideal. The sodium hydroxide may also produce fumes, so as much as possible, this experiment should take place outdoors or under a fume hood. If that isn't an option, make sure the room you use is well-ventilated (switch on the kitchen fan and open every window you can).
The following guide will make enough soap to fill a bottle of hand soap, so adjust your proportions accordingly if you want to make more.
Bring approximately 8 oz. of water (weigh it out using your scale) to a boil in your pot, then add about 50 guava leaves. Keep the water at boiling temperature, and stir occasionally.
After half an hour, take your pot off the stove, strain out the guava leaves, and transfer the guava leaf extract to a container. You can use a sieve or a pair of utensils.
At this stage, you're going to prepare a mixture of water, oil, and your chosen scent and food coloring (you're going to add the sodium hydroxide to this later, making your soap). The ratios we've provided here are based on a recipe that Ambra, a soap-making hobbyist from Iceland, posted on her blog, but you might want to tweak it with some trial-and-error to get the soap to the consistency you prefer most.
It's true. Oil and water don't mix — they're immiscible together. Heck, most definitions of "immiscible" even use oil and water as their illustration. That's why you usually need a chemical emulsifying agent to completely combine oil and water, but heating your oil-water mixture, which is our next step, will achieve a similar effect.
Add about 8 oz. of coconut oil and 20 oz. of olive oil to 8 oz. of water in your pot. For soap-making, measure all your reagents by weight, and not volume, because the density of different oils can vary significantly. The mixture should immediately begin to form micelles, bubble-like concentrations of oil, especially at the surface.
Cover and bring the mixture to a boil, removing the lid to stir occasionally. Your oil-water mixture should be bubbling from the heat, but better combined than before.
Stir, then add the guava extract. Stir again, and add scented oil, and then the food coloring. We suggest adding the food coloring in small doses while stirring, as it takes a while for the coloring to disperse and arrive at a settled color. Throughout this process, keep your mixture at a boil. Boil for 30 more minutes.
Saponification is a chemical reaction between an ester and an alkali, producing a carboxylate ion and an alcohol. It's also what we're about to do next! That's because saponification, in less scientific terms, is soap-making (the linguistic root of "saponification" is sapo, the Latin word for soap). The coconut and olive oil we're using contain esters, and the sodium hydroxide is an alkali. These react to form a carboxylate salt compound — soap!
Put on your goggles, gloves, and mask. Adding the sodium hydroxide to the mixture will cause the temperature of your mixture to skyrocket to temperatures of up to 200º F, according to the Soap Queen's safety guide.
It's not just the sheer heat of your mixture that's now dangerous. That spike in temperature means some water particles are going to reach boiling temperature, and that rising steam will carry with it trace amounts of unreacted sodium hydroxide, which is poisonous. If any sodium hydroxide makes contact with skin, pour vinegar over the affected area to neutralize the burning alkali.
Using your kitchen scale, weigh out 4 oz. of sodium hydroxide. Slowly and carefully add it to your mixture in very small amounts. Keep your face away from the mouth of the pot as you add the sodium hydroxide to avoid inhaling the fumes.
When the mixture is no longer producing steam, turn off the stove and remove the pot from the heat. Allow it to cool slightly, then stir with your electric blender. Stir for fifteen minutes if stirring by hand. The mixture will behave somewhat like egg whites, foaming up and thickening as your stir.
After stirring, allow the mixture to cool and settle into a more liquid form. If you would like a cleaner-looking soap, you can skim off the foam from the surface. Transfer your finished liquid soap to a bottle or dispenser using a funnel.
Voilà! You have your very own homemade soap.
If you want to watch the entire process in sequence to familiarize yourself with the experiment first, or just have the video handy for reference as you conduct your own experiment in DIY soap-making, be sure to check our Ai-ni Bautista's video tutorial below.
If you're inspired by the success of your soap and enjoyed the excitement of working with heat and sodium hydroxide, why not try other "explosive" science projects, like cooking up your own rocket fuel?