Fluid properties – At standard lab conditions, what is the (dynamic vs kinematic) viscosity value commonly quoted for pure water at 20°C?

Introduction:
Viscosity quantifies a fluid's internal resistance to motion. Two closely related measures are used in practice: dynamic viscosity (mu) and kinematic viscosity (nu). Many exam questions ask for the standard reference value for water at 20°C, a temperature often used for property tables and laboratory calibration.

Given Data / Assumptions:

• Temperature is 20°C (approximately room temperature).
• Fluid is pure water at 1 atm.
• We distinguish dynamic viscosity mu (in Pa·s or poise) from kinematic viscosity nu (in m^2/s or stokes).

Concept / Approach:

Dynamic viscosity mu measures shear stress per unit shear rate, while kinematic viscosity nu = mu / rho measures momentum diffusivity. In SI, mu is in Pa·s (N·s/m^2) and nu in m^2/s; in CGS, mu is in poise and nu in stokes. For pure water at 20°C, tabulated values are mu ≈ 1.002 mPa·s ≈ 0.01002 poise = 1.002 centipoise (cP) and nu ≈ 1.004 × 10^−6 m^2/s = 1.004 centistokes (cSt) because rho ≈ 998 kg/m^3. Examinations typically expect the canonical statement “water has a viscosity of about one centipoise at 20°C.”

Step-by-Step Solution:

Verification / Alternative check:

Property tables and handbooks list water at 20°C with mu ≈ 1.0 cP and nu ≈ 1.0 cSt. If the prompt explicitly asked for kinematic viscosity, “one centistroke” would be correct. Here the safest conventional choice is the dynamic value, one centipoise.

Why Other Options Are Wrong:

One stoke: 1 st = 10^−4 m^2/s; far larger than water's nu.One centistroke: This is nu, not mu; ambiguous unless specified.One poise or ten centipoise: Ten times too large for water at 20°C.

Common Pitfalls:

Mixing dynamic and kinematic viscosity units or forgetting that “poise” and “stoke” are CGS units. Also, quoting values at 25°C (slightly lower viscosity) instead of 20°C can cause small numeric drift.

Final Answer:

one centipoise