Lab: Chemistry
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Source: Lara Al Hariri and Ahmed Basabrain at the University of Massachusetts Amherst, MA, USA
Soap is prepared using a saponification reaction, where a base catalyzes the hydrolysis of three ester groups of an oil, such as coconut oil. During saponification, hydroxide ions from the base attack the carbonyl group on the oil to form a ratio of three molecules of soap to one molecule of glycerol. The resulting soap molecule is a long carbon chain, which is hydrophobic, with a carboxylate ion at one end, which is hydrophilic.
In this experiment, you will prepare soap using coconut oil and sodium hydroxide, the base. As the reaction proceeds, you will observe the decrease in pH, which occurs as the sodium hydroxide is used up in the reaction.
Time of mixing (min) | Viscosity of solution | Number of phases | Color of solution | pH |
0 | ||||
10 | ||||
20 | ||||
30 | ||||
40 | ||||
50 | ||||
60 |
One drawback to soap is that the carboxylates form insoluble complexes with minerals found in tap water, such as magnesium, iron, and calcium. The soap scum you see in sinks and bathtubs is a buildup of these insoluble salts. If you dissolve equal amounts of soap and equal volumes of soft water, which has a low amount of minerals, and hard water, which has a high amount of minerals, more soap will stay in solution in soft water. So, the mixture of soap and soft water will be foamier and have more micelles than the mixture with hard water.
In the next part of the experiment, you will test the tolerance of your lab made soap to hard water by comparing deionized and tap water to a solution of CaCl2, which mimics extremely hard water. You'll then compare the properties of your soap to a commercially available liquid detergent.
pH | |
Lab-made soap in DI water | |
Commercial soap in DI water |
Initial volume of foam (mL) |
Final volume of foam (mL) |
ΔV (mL) | |
Lab-made soap in DI water | |||
Lab-made soap in tap water | |||
Lab-made soap in 1 M CaCl2 | |||
Commerical soap in DI water | |||
Commercial soap in tap water | |||
Commercial soap in 1 M CaCl2 |
Consider the reaction between the coconut oil and sodium hydroxide that forms glycerol and soap. Initially, the two phases are not miscible. The coconut oil is in the upper phase, and the sodium hydroxide is in the bottom, aqueous phase. Coconut oil has a melting point of 30 °C, so as the reaction proceeds under heat and stirring, the aqueous phase volume decreases as more sodium hydroxide reacts with the oil.
Sodium hydroxide has a pH of around 14, so the solution is initially around pH 14. After the reaction occurs, the pH drops from a pH of 13 to a pH of 12. Additionally, the turbidity and thickness of the reaction mixture gradually increases during the reaction as more coconut oil is hydrolyzed to soap and glycerol.
Next, compare the pH of the two soap solutions. The lab-made soap solution should have a pH in the range of 9 – 11. While the detergent solution should have a similar pH in the range of 8 – 11. Observe the foam level of the lab-made soap. The most foam is visible in deionized water due to the lack of multivalent cations. In contrast, there is less foam in the tap water because of the numerous cations present.
The lab-made soap formed the least amount of foam in the CaCl2 solution, which is expected as the soap molecules precipitate in the presence of Ca2+. The precipitate should be observed in the graduated cylinder.
The commercial detergent foamed similarly in the distilled water and tap water, and although there was less, it still foamed in the calcium chloride. This is because the detergent in the liquid soap is alkylbenzene sulfonate, which does not precipitate in the presence of multivalent cations.