Advanced #42

Solutes have different effects on properties like density and viscosity

Different solutes affect the properties of water in surprisingly different ways.

Scientific Explanation

When a substance dissolves in water, one might intuitively expect all physical properties to shift in the same direction — for instance, that a solute increasing density would also increase viscosity. In water, this is not the case. Different solutes can increase density while simultaneously decreasing viscosity, or vice versa.

This behavior is explained by the varying interaction of dissolved ions with the hydrogen bond network. In the Hofmeister series, ions are classified as kosmotropes (structure makers) or chaotropes (structure breakers). Kosmotropes such as sulfate (SO4 2-) or magnesium (Mg 2+) strengthen the surrounding water network and increase both density and viscosity. Chaotropes such as thiocyanate (SCN-) or iodide (I-) loosen the network and can decrease viscosity while still altering density.

The crucial point: one and the same solute can push different properties in opposite directions. Potassium chloride, for example, increases the density of water but decreases its viscosity.

Solute Effects on Water Properties Schematic showing how different solutes affect water properties differently. Kosmotropes (structure makers like sulfate) increase viscosity and density, while chaotropes (structure breakers like thiocyanate) decrease viscosity but may increase density differently. Solute Effects on Water Properties Kosmotropes (Structure Makers) Chaotropes (Structure Breakers) Viscosity ↑ Increase Density ↑ Increase Surface T. ↑ Increase e.g., SO₄²⁻, Mg²⁺ Na⁺, F⁻ Viscosity ↓ Decrease Density ↑/↓ Varies Surface T. ↓ Decrease e.g., SCN⁻, I⁻ K⁺, Cs⁺ Same solute can increase density but decrease viscosity — violating simple intuitions about dissolved substances
Kosmotropes (structure makers) and chaotropes (structure breakers) affect water properties in different ways.

Everyday Relevance

The Hofmeister series has great practical significance in biochemistry and pharmacy. Protein stability in solution depends on which salts are present: kosmotropes stabilize proteins, chaotropes can unfold them. This influences drug formulation, food preservation, and even the effectiveness of cleaning agents. Understanding these different ion effects is also essential in water treatment processes.