Which property describes the thickness of how well particles stick together?

The correct answer is viscosity, which refers to the measure of a fluid's resistance to deformation and flow. In the context of how well particles stick together, viscosity describes the internal friction of a substance, indicating how easily it can flow when subjected to stress. In a liquid, higher viscosity means that the molecules experience stronger intermolecular interactions, which makes it more resistant to flow. This results in a thicker, stickier fluid where particles are more bonded together, influencing how the fluid behaves under various conditions.

Density, on the other hand, measures the mass of a substance relative to its volume and does not inherently concern the interactions between particles in terms of flow. Solubility indicates how well a substance can dissolve in a solvent, which is unrelated to how particles stick together in a viscous substance. Polarity refers to the distribution of electrical charge in molecules, contributing to molecular interactions but not directly measuring how thick or resistant a substance is to flow.