Experiment: Cleaning action of soaps and detergents

Once you have prepared soap, the next Form 5 practical asks what actually makes soap and detergent clean, and why detergent works where soap fails. In this experiment we compare how soap and detergent remove grease, and how each behaves in soft water and in hard water. This guide gives the method, the observations to expect, the reasoning that connects them to molecular structure, and the science process skills the practical papers reward.

Aim

To investigate the cleaning action of soap and detergent on a greasy cloth, and to compare how soap and detergent behave in soft water and in hard water.

Apparatus and materials

• Soap solution and detergent solution of the same concentration
• Distilled water (soft water)
• Hard water (water containing dissolved calcium or magnesium salts)
• Small pieces of cloth stained with the same amount of cooking oil or grease
• Test tubes with stoppers, and a test-tube rack
• Measuring cylinder
• Beakers and glass rod
• Cooking oil for the grease stain

Procedure

1. Prepare several small pieces of cloth stained with the same amount of cooking oil, so each starts equally greasy.
2. Label three test tubes: distilled water only, soap solution, and detergent solution.
3. Add the same volume of each liquid to its test tube.
4. Place a greasy cloth into each test tube, stopper the tube, and shake each the same number of times with the same strength.
5. Observe how clean each cloth becomes and compare the three.
6. To test hard water, put the same volume of soap solution into one test tube and the same volume of detergent solution into another, then add the same volume of hard water to each.
7. Stopper and shake both tubes equally, and compare how much lather (foam) forms and whether any scum appears.
8. Record which liquid removes the grease best, which lathers well in hard water, and where a scum forms.

Expected observations

In distilled water alone, the greasy cloth stays oily and little grease is removed. In the soap solution and in the detergent solution, the grease is lifted off and the cloth becomes much cleaner, and a lather forms. Soap and detergent both clean far better than water on its own. In the hard-water test, the detergent still produces a good lather and cleans well, but the soap produces little lather and a white, insoluble scum appears in the tube. Describe these qualitative differences, grease removed by soap and detergent but not by water, and scum with little lather from soap in hard water while detergent still lathers, rather than inventing numerical amounts of foam.

Inference and conclusion

A soap or detergent molecule has two parts: a long hydrocarbon tail that dissolves in oil and grease (the hydrophobic part) and an ionic head that dissolves in water (the hydrophilic part). During washing, the tails bury themselves in the grease while the heads stay in the water. Agitation breaks the grease into small droplets, each surrounded by molecules with their heads pointing outwards, so the droplets are held in the water and rinsed away. This is why both soap and detergent clean far better than water alone. In hard water, the calcium and magnesium ions react with soap to form an insoluble scum, using up the soap before it can clean, which is why soap lathers poorly and leaves a scum in hard water. A detergent does not form an insoluble salt with these ions, so it still lathers and cleans. The conclusion is that soap and detergent clean by the same mechanism, but detergent has the advantage in hard water.

Science process skills (Paper 3 style)

Stating a hypothesis. A suitable hypothesis: detergent produces more lather and cleans better than soap in hard water because it does not form an insoluble salt with calcium and magnesium ions.

Identifying variables. For the hard-water comparison, the manipulated variable is the type of cleaning agent (soap or detergent), the responding variable is the amount of lather (and whether scum forms), and the controlled variables are the volume and hardness of the water, the concentration of the agent and the way each tube is shaken.

Operational definition of cleaning action. Cleaning action is defined operationally as how completely the grease is removed from the cloth under the same shaking.

Fair testing. State how the test is made fair: same amount of grease on each cloth, same volume and concentration of liquid, and the same number and strength of shakes.

Making an inference. From the scum and poor lather with soap in hard water, infer that soap reacts with the ions in hard water to form an insoluble salt.

Communicating. Write a conclusion that explains cleaning in terms of the hydrophobic tail and hydrophilic head, and explains the hard-water result in terms of the insoluble scum.

Safety precautions

• Wear safety goggles, so that splashes of soap or detergent solution do not get into the eyes.
• Stopper the test tubes firmly before shaking, so the contents do not spray out.
• Point the mouth of the test tube away from people and yourself while shaking, to avoid splashes.
• Wipe up spills of oil or solution at once, so no one slips.

Common errors

• Using different amounts of grease. If the cloths start with different amounts of oil, the comparison is unfair; stain each with the same amount.
• Shaking the tubes unequally. More shaking removes more grease; shake each tube the same number of times with the same strength.
• Different concentrations of soap and detergent. Use the same concentration, or the comparison tells you about concentration rather than about soap versus detergent.
• Confusing scum with lather. In hard water, the white solid that forms with soap is scum, not foam; describe it correctly as an insoluble scum.
• Forgetting the water-only control. Without a distilled-water-only tube there is nothing to compare against, so include it to show water alone cleans poorly.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we make sure students can link every observation to structure, the two-ended molecule for cleaning, and the insoluble scum for the hard-water result, because Paper 2 and Paper 3 both reward the explanation, not just the observation. The cleaning action of soaps and detergents is a core content standard in the Consumer and Industrial Chemistry chapter of SPM Chemistry, and it follows directly from the soap you prepared by saponification. This comparison also trains the fair-test discipline assessed in the Paper 3 practical (Paper 3 is a practical test assessing science process skills): one variable changed, everything else held the same, and a conclusion that gives the reason.