EasyChemistryHigh School (9–10)GeneralCarbon and Its Compounds › Reactions and Uses of Carbon CompoundsConceptual
You wash clothes in hard water. With soap, a grey scum forms and there is little lather.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
You wash clothes in hard water. With soap, a grey scum forms and there is little lather. With a detergent, there is plenty of lather and no scum. Explain the difference, and say why the soap is wasted.
Need a reminder? The key ideaShow
Soap is a sodium or potassium salt of a long-chain carboxylic acid. Fat soap glycerol. The tails go into the grease and the heads stay in the water, making micelles. In hard water, calcium and magnesium ions turn soap into scum. Detergents give no scum.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Hard water. It holds calcium and magnesium ions.
Step 2. Soap. These ions join the soap and make an insoluble salt, the scum. Soap used this way cannot clean.
Step 3. Detergent. Its calcium and magnesium salts dissolve, so no scum forms, and it lathers.
Answer: soap turns into scum with the calcium and magnesium ions, so it is wasted; detergents do not.
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