At standard temperature and pressure (STP), what is the approximate dynamic viscosity of pure water expressed in SI units?

Introduction:
Knowing the viscosity of water at reference conditions is fundamental for mixing, pumping, and mass transfer calculations. Dynamic viscosity at STP is frequently used to estimate Reynolds number, friction factors, and diffusional resistances for aqueous systems in bioprocess engineering and fluid mechanics.

Given Data / Assumptions:

• Dynamic viscosity symbol mu with SI unit kg·m^-1·s^-1 (equivalently Pa·s).
• Reference condition is near 20 °C and 1 atm.
• Water is Newtonian under these conditions.

Concept / Approach:
At approximately 20 °C, water's dynamic viscosity is close to 1 mPa·s. In SI base units this equals 0.001 kg·m^-1·s^-1. This benchmark enables quick back-of-the-envelope Reynolds number estimates when combined with typical densities around 1000 kg·m^-3.

Step-by-Step Solution:

Verification / Alternative check:
Engineering handbooks and property tables list water viscosity near 0.001 kg·m^-1·s^-1 at room temperature, decreasing modestly with higher temperature.

Why Other Options Are Wrong:

• 0.005 kg·m^-1·s^-1: Too high for water at STP.
• 0.003 kg·m^-2·s^-1: Unit is incorrect; dynamic viscosity is not per m^2.
• 0.1 g·cm^-1·s^-1: This equals 0.01 kg·m^-1·s^-1 after conversion, still too high.

Common Pitfalls:
Mixing kinematic viscosity (m^2·s^-1) with dynamic viscosity (kg·m^-1·s^-1). Always check units before plugging into correlations.

Final Answer:
0.001 kg·m^-1·s^-1