Concentration scales and operating conditions:\nNormality (equivalents per litre) of a solution does not change appreciably with an increase in which variable?

Introduction / Context:
In analytical and process chemistry, normality N expresses equivalents of solute per litre of solution. Understanding what affects N helps avoid calibration errors and ensures consistency in titrations and dosing. While some variables clearly change normality, others have negligible effect under typical liquid-phase conditions.

Given Data / Assumptions:

• Normality N = equivalents of solute / litre of solution.
• Liquids are nearly incompressible; volume changes with pressure are very small compared with temperature or composition effects.
• Temperature significantly affects solution volume via thermal expansion.

Concept / Approach:
Any factor that changes the solution volume at fixed moles of equivalents will change N. Adding solvent (dilution) increases volume and decreases N. Adding solute increases the equivalents per litre and generally increases N. Temperature alters volume measurably; therefore N is temperature dependent. In contrast, pressure changes in the usual laboratory/plant range have negligible effect on liquid volume, so N remains essentially unchanged with pressure.

Step-by-Step Solution:

Verification / Alternative check:
Compare liquid isothermal compressibility (order of 10^-10 Pa^-1) to volumetric thermal expansion coefficients; temperature effects dominate over pressure for liquids in standard conditions.

Why Other Options Are Wrong:

• Temperature: affects volume ⇒ changes N.
• Adding solute: increases equivalents ⇒ changes N.
• Dilution: increases volume ⇒ decreases N.

Common Pitfalls:
Confusing normality with molality (mol/kg solvent). Molality is independent of temperature and pressure because it uses mass, not volume.

Final Answer:
Pressure.