Advanced #66

Water has an unusually high viscosity

The viscosity of water is higher than expected for such a small molecule.

Scientific Explanation

Viscosity measures a liquid’s resistance to flow. For a molecule weighing only 18 daltons, water exhibits an unusually high viscosity: 0.89 millipascal-seconds at 25 degrees Celsius. Based on molecular weight alone, one would predict a value of about 0.3 millipascal-seconds — water is approximately three times more viscous than expected.

The explanation, once again, lies in hydrogen bonds. When water flows, molecules must slide past one another. This requires continuously breaking and reforming hydrogen bonds. Each water molecule is engaged in an average of about 3.5 hydrogen bonds at any moment, each lasting only a few picoseconds. Although the bonds constantly break and reform, they measurably slow down the flow.

For comparison, acetone (molecular mass 58 daltons) has a considerably lower viscosity (0.31 millipascal-seconds) than water despite being more than three times heavier. This demonstrates the dominant influence of hydrogen bonding on transport properties.

Water Viscosity Comparison Bar chart comparing viscosity at 25 degrees C for water (0.89 mPa s), ethanol (1.07 mPa s), acetone (0.31 mPa s), and the expected value for water based on molecular weight alone (about 0.3 mPa s). Water is nearly 3 times more viscous than expected. Viscosity (mPa·s) 0 0.5 1.1 0.89 Water ~0.3 Expected (by MW) 1.07 Ethanol 0.31 Acetone ~3× higher than expected Viscosity Comparison at 25 °C
Viscosity comparison: water is about three times more viscous than its molecular weight would suggest.

Everyday Relevance

Water’s high viscosity influences many biological and engineering processes. In the human body, it determines the flow resistance of blood plasma and thereby affects blood pressure. In nature, it governs how quickly nutrients disperse through water bodies and how organisms navigate through water.

The relatively high viscosity is also noticeable when stirring a spoon through water — it offers more resistance than one would expect from such a lightweight molecule. For industry, viscosity affects pump requirements, piping design, and the efficiency of heating and cooling systems.