Chapter 4: Problem 10

Explain the formation of scum when hard water is treated with soap.

Short Answer

Expert verified

Scum forms when calcium and magnesium ions in hard water react with the fatty acid salts in soap to create insoluble substances.

Step by step solution

Understand Hard Water

Hard water contains minerals such as calcium and magnesium ions. These ions are responsible for the hardness of water.

Composition of Soap

Soap is typically composed of sodium or potassium salts of fatty acids. When soap is dissolved in water, it forms ions.

Reaction Between Hard Water and Soap

When soap is added to hard water, the calcium and magnesium ions react with the fatty acid salts in soap to form insoluble substances.

Formation of Scum

The insoluble substances formed from the reaction between the calcium and magnesium ions in hard water and the fatty acid salts in soap are known as scum. This scum does not dissolve in water and can be seen as a precipitate.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

hard water

Hard water contains high amounts of minerals, especially calcium and magnesium ions. These minerals are picked up by water as it flows through soil and rock. While hard water is not harmful to health, it can cause various practical problems in daily use. For example, it makes it harder to get soap to lather and can leave deposits on fixtures and pipes, shortening their life span.

soap composition

Soap is mainly made from sodium or potassium salts of fatty acids. When soap is mixed with water, it breaks into individual ions. These salts react with water to help clean and remove dirt and grease. But in hard water, the soap ions encounter calcium and magnesium ions, leading to different reactions.

calcium and magnesium ions

Calcium (Ca²⁺) and magnesium (Mg²⁺) ions are primarily responsible for water hardness. When soap is added to hard water, these ions interact with the soap molecules. This interaction is key to scum formation, as these ions can make it difficult for soap to work properly in cleaning.

insoluble substances

When calcium and magnesium ions in hard water react with soap, they form insoluble substances. These substances, often called scum, are insoluble in water and appear as a precipitate. Scum can be seen floating on the water surface or sticking to surfaces like bathtubs and sinks. This makes cleaning less effective and can leave unpleasant residues.

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Explain the formation of scum when hard water is treated with soap.

Short Answer

Step by step solution

Understand Hard Water

Composition of Soap

Reaction Between Hard Water and Soap

Formation of Scum

Key Concepts

hard water

soap composition

calcium and magnesium ions

insoluble substances

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