You may have heard someone use the figure of speech "slower than molasses in winter" to describe a process that occurs slowly. Explain why this is an apt idiom, using concepts of molecular size and shape, molecular interactions, and the effect of changing temperature.

When discussing the physical properties of substances, the molecular size and shape play a crucial role in determining how a substance behaves under different conditions. In the case of molasses, its molecules are composed of long, complex chains that affect how the molecule interacts with others of its kind. Larger molecules like those found in molasses have more surface area for intermolecular forces to act upon, which leads to increased viscosity.

Moreover, the shape of a molecule can impact how tightly they can pack together, further influencing the substance's overall consistency. The more irregular the shape, the less efficiently they can pack, which again, directly affects the viscosity and flow characteristics. This explains why molasses, with its large, irregular-shaped molecules, tends to be thicker and flows more slowly compared to substances with smaller, more uniformly shaped molecules.

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles (molecules, atoms or ions). They are one of the main factors affecting viscosity. Stronger intermolecular forces, such as hydrogen bonds and Van der Waals forces, mean that molecules have a greater attraction to each other and don't move past one another as freely.

In molasses, the significant intermolecular forces between its substantial molecules result in a high level of internal friction, which manifests as a high viscosity. This high level of stickiness and resistance to flow is why the phrase 'slower than molasses in winter' is such an accurate descriptor for a slow process.

Viscosity is a measure of a fluid's resistance to flow. It describes the internal friction of a moving fluid. A fluid with high viscosity resists motion because its molecular makeup results in a lot of internal friction when it is deformed, such as during the act of flowing. Viscosity is greatly influenced by both temperature and the intermolecular forces present within the fluid.

With regards to molasses, as temperature decreases, its already high viscosity increases even further, making it flow very slowly. This is due to the reduced thermal energy available for the molecules to overcome the attractive forces holding them together. The thick, syrupy character of molasses, therefore, becomes more pronounced in colder conditions, thus taking longer to pour or move, and lending itself perfectly to the expression of slowness.

Kinetic energy is the energy of motion. It is directly proportional to the mass and the square of the speed of the moving object. When it comes to molecular movements, kinetic energy refers to the energy that each molecule possesses due to its motion in the fluid. At higher temperatures, molecules have more kinetic energy, which means they move more rapidly and can overcome intermolecular forces more easily, resulting in lower viscosity.

The converse happens when the temperature drops; the kinetic energy of the molasses molecules decreases, and their movement becomes more sluggish. This reduction in molecular motion not only increases the viscosity of molasses but it also correlates to the saying 'slower than molasses in winter', as the kinetic energy of the molecules at this time is at a minimum, leading to the slowest possible flow for the syrup.