Which of the following does not decrease with rise in temperature? (a) Density (b) Surface tension (c) Vapour pressure (d) Viscosity

Understanding how temperature affects the density of substances is essential in various scientific fields and applications. Density, which is mass per unit volume, tends to decrease as temperature increases. This phenomenon occurs because substances (typically gases and liquids) expand when heated, and since the mass remains constant, the volume increases, resulting in lower density.

Imagine heating a balloon; when it expands, the air inside it becomes less dense. This principle explains why hot air balloons rise and why warm water floats on cooler water. It's an excellent example of thermal expansion impacting density. When working with this concept, it's crucial to note that while this is a general trend, there can be exceptions, such as water's behavior near its freezing point.

Surface tension is a fascinating property that describes the elastic tendency of a liquid's surface. It's caused by the attraction between the liquid's molecules, which creates a 'skin' that resists external force. As temperature increases, the kinetic energy of molecules increases, causing the cohesive forces that bound them together to decrease. Consequently, the surface tension of a liquid diminishes with higher temperatures.

Molecules within a liquid are surrounded by others and are attracted equally in all directions. However, surface molecules are not surrounded by the same number of neighbors, so their cohesive forces are more significant inward, creating surface tension. Remember that soap reduces surface tension, which is why it helps clean dishes so effectively by allowing water to spread and penetrate grime.

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid form. It's an important characteristic of liquids that becomes particularly relevant when discussing boiling points and evaporation. An increase in temperature energizes liquid molecules, enabling more to escape into the gaseous phase, thereby raising the vapor pressure.

Imagine a pot of water on the stove. As it heats up, bubbles form and rise. These bubbles are water vapor, and they indicate increasing vapor pressure within the water. When the vapor pressure equals the atmospheric pressure, the water boils. That's why water boils at lower temperatures at high altitudes where atmospheric pressure is lower.

Viscosity is a measure of a fluid's resistance to flow, often thought of as its 'thickness'. Honey and molasses, for example, have higher viscosities compared to water. When temperature increases, typically, the viscosity of a liquid decreases because the heat provides energy for molecules to overcome intermolecular forces and flow more easily.

Consider pouring syrup on pancakes. If the syrup is cold, it flows slowly and reluctantly. When warmed, it pours more smoothly, demonstrating reduced viscosity. Viscosity is critical in industries like lubrication, where oils must remain fluid under varying temperatures to protect machine parts.