International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

What can increase the viscosity of a fluid?

1. Increasing its temperature

2. Adding salt

3. Increasing the number of carbon atoms

4. Decreasing its pressure

The correct answer is: Increasing the number of carbon atoms

Increasing the number of carbon atoms in a fluid, particularly in the context of hydrocarbons like oils, generally leads to an increase in viscosity. As the carbon chain length increases, the molecules become larger and more complex, resulting in greater intermolecular forces such as van der Waals forces. These increased forces make it more difficult for the molecules to move past one another, thereby raising the viscosity of the fluid. In contrast, increasing temperature typically decreases viscosity, as higher temperatures provide energy that allows molecules to overcome intermolecular attractions more easily. Adding salt may influence the viscosity of solutions but does not inherently increase viscosity in all cases; for certain fluids, it might even decrease it. Decreasing pressure usually has a negligible effect on the viscosity of liquids, since liquids are incompressible.