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Practice each question, then open the answer or worked solution.
What change will you observe if you test soap with litmus paper (red and blue)?
Solution: When soap is dissolved in water due to the formation of alkaline NaOH or KOH, the solution is alkaline. The solution changes the colour of the red litmus to blue, but in the soap solution, the blue litmus remains blue.
What is hydrogenation? What is its industrial application?
Solution: Hydrogenation is a process or a chemical reaction between hydrogen and other compounds. It is usually done in the presence of catalysts. For example, nickel, palladium or platinum. Hydrogenation is used mainly to saturate organic compounds.
Which of the following hydrocarbons undergo addition reactions: C2H6, C3H8, C3H6, C2H2 and CH4?
Solution: Unsaturated hydrocarbons undergo addition reactions. C3H6 and C2H2 are unsaturated hydrocarbons which undergo addition reactions.
Give a test that can be used to differentiate between saturated and unsaturated hydrocarbons.
Solution: The bromine water test is used to differentiate between the unsaturated compounds (like alkenes and alkynes) and the saturated compounds. For this purpose, bromine is used in the form of bromine water. A solution of bromine in water is called bromine water. Bromine water has a red-brown colour due to the presence of bromine in it. When bromine water is added to an unsaturated compound, then bromine gets added to the unsaturated compound, and the red-brown colour of bromine water is discharged. So, if an organic compound decolourises bromine water, then it will be an unsaturated hydrocarbon (containing a double bond or a triple bond), but saturated hydrocarbons (alkanes) do not decolourise bromine water.
The bromine water test is performed to differentiate between the unsaturated compounds (like alkenes and alkynes) and the saturated compounds. When bromine water is added to an unsaturated hydrocarbon, the red-brown colour of the bromine solution is discharged. So, if there is dis-colouration, then the compound will be an unsaturated hydrocarbon.
Explain the mechanism of the cleaning action of soaps.
Solution: There are so many impurities and dirt mixed in water, and most of all, the dirt does not dissolve in the water. Soap molecules are a combination of salts such as sodium or potassium. The molecules are of a long chain of carboxylic acids. So, when the carbon chain is dissolved in oil, and the ionic end is dissolved in the water, the soap starts cleansing and trapping the dirt. When this happens, the soap molecules form structures called micelles that are used for capturing the oil droplets, and then the other end is the ionic faces. This will then form an emulsion in water and help in dissolving the dirt or impurities when the clothes are washed.
The soap molecules have different properties at different ends. The first end is the hydrophilic end, which dissolves in the water and is attracted towards the water, and the second one is the hydrophobic end, which is dissolved in the hydrocarbons and is repulsive to water. The hydrophobic tail aligns itself along the surface of the water because it is not soluble in the water.