A sample of 'soft soap' contains
- A.Caesium
- B.Potassium
- C.Calcium
- D.Magnesium
Correct answer
Explanation
The correct answer is B, Potassium. Soft soaps are potassium salts of fatty acids, while ordinary hard bar soaps are sodium salts.
Soap is made by saponification, heating an oil or fat with an alkali. With sodium hydroxide the product is a hard soap, used as laundry and bathing bars. With potassium hydroxide the product is softer, more soluble in water and milder on the skin, so potassium soaps are used in liquid soaps, shaving creams and shampoos. Glycerol is formed as a by-product in both cases. The cleaning action is the same: the long hydrocarbon tail holds the grease and the ionic head holds the water, forming micelles that wash away.
A is wrong because caesium is a rare and costly alkali metal that is not used to make soap.
C is wrong because calcium salts of fatty acids are insoluble; they are the scum that forms when soap meets hard water.
D is wrong for the same reason: magnesium soaps are insoluble scum, not a usable soap.
Exam tip: sodium soap, hard; potassium soap, soft; calcium and magnesium soaps, scum in hard water.
Practice Questions
View allWhich one of the following frequency ranges is sensitive to human ears ?
- A.0-200 Hz
- B.20-20,000 Hz
- C.200-20,000 Hz only
- D.2,000-20,000 Hz only
Show answer
The normal human ear hears sound from about 20 hertz to 20,000 hertz, the audible range. Frequency, measured in hertz, is the number of vibrations per second, and it decides the pitch of a sound: the higher the frequency, the shriller the note.
Read the full explanation →The frequency of ultrasound waves is
- A.less than 20 Hz
- B.between 20 Hz and 2 kHz
- C.between 2 kHz and 20 kHz
- D.greater than 20 kHz
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- A.Coke
- B.Propane
- C.Petrol
- D.Wax
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Read the full explanation →Which of the following sets of elements has the same valency?
- A.Na, Mg, Ca
- B.Na, Mg, Al
- C.Mg, Ca, K
- D.Mg, Ca, Ba
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Read the full explanation →Which one of the following is the correct order of the valencies of elements Ne, Si, N and Mg?
- A.Ne < Mg < N < Si
- B.Si < N < Mg < Ne
- C.Ne < N < Si < Mg
- D.Mg < Ne < N < Si
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Read the full explanation →