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Chapter 7: Problem 90

A scum forms on the surface of boiling hard water. What is this scum? Why does it form?

Short Answer

Expert verified
Scum is calcium and magnesium stearate, formed because calcium and magnesium ions react with soap in hard water.

Step by step solution

01

Understanding Hard Water

Hard water is water that has high mineral content, primarily calcium (Ca虏鈦�) and magnesium (Mg虏鈦�) ions. These minerals are dissolved in water as it passes through deposits of limestone and chalk.
02

Reaction with Soap

When soap is added to hard water, the calcium and magnesium ions react with fatty acids in soap to form an insoluble substance. This reaction can be represented by the formula: \[ 2C_{17}H_{35}COONa + Ca^{2+} ightarrow (C_{17}H_{35}COO)_2Ca + 2Na^+ \] where \((C_{17}H_{35}COO)_2Ca\) is calcium stearate, which is the precipitate or 'scum'.
03

Formation of Scum

The 'scum' is a precipitate known as calcium and magnesium stearate (soap scum), which is insoluble in water. It collects on surfaces such as sinks, bathtubs, and the surface of the boiling water where soap was added.

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Key Concepts

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

Calcium Ions
Calcium ions, represented as \( Ca^{2+} \), are key players in the composition of hard water. These ions naturally occur when water flows over and through mineral-rich areas like limestone and chalk. Calcium ions dissolve into the water, contributing to its hardness. This can result in the well-known problem of reduced soap lathering when washing. A significant amount of these ions can make water "hard" enough to cause deposits, called scales, in pipes and appliances, which can lead to decreased efficiency and the need for frequent cleaning or maintenance.
Magnesium Ions
Magnesium ions are another crucial component of hard water. They are represented by \( Mg^{2+} \) in chemical notation. Like calcium ions, magnesium also originates from the geological features the water encounters on its route, specifically rocks containing this mineral. Magnesium ions make it challenging for soap to form a proper lather, as they readily react with fatty acids, precipitating out of solution. This interaction is also responsible for creating deposits or scale, similar to calcium ions, which affect household and industrial systems alike.
Soap Scum
Soap scum is an annoying byproduct of the hard water and soap interaction. It forms when calcium and magnesium ions in hard water react with soap. The reaction produces solid particles rather than dissolving into water, resulting in a substance known as soap scum. This is composed primarily of calcium stearate and magnesium stearate.
This scum is a white, often slimy residue that accumulates on surfaces such as shower walls, sinks, or bathtubs. Not only is it unsightly, but it can also make cleaning more labor-intensive. Aside from its unattractive appearance, soap scum can trap bacteria and mold, potentially posing hygiene issues if not regularly removed.
Chemical Reactions
Chemical reactions occur whenever hard water and soap are combined. The key reaction occurs between the sodium salt of the fatty acids in soap and calcium or magnesium ions. This reaction can be summarized as:
  • Sodium soap + calcium ions = calcium soap (scum) + sodium ions

\[ 2C_{17}H_{35}COONa + Ca^{2+} \rightarrow (C_{17}H_{35}COO)_2Ca + 2Na^+ \]
In simple terms, the hard water ions form bonds with the soap's fatty acids, resulting in insoluble precipitates, or soap scum. These reactions highlight why using more soap won't necessarily yield a better cleansing effect in hard water areas, as most of it precipitates rather than acting as a cleaning agent.

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