Question

When a liquid is heated (a) surface tension increases (b) surface tension is lowered (c) viscosity increases (d) surface tension is not effected

Short answer

Option (b) surface tension is lowered.

Step-by-step solution

Understanding the Concept

Surface tension is a property of liquids that arises from the cohesive forces between molecules at the surface of a liquid. It generally decreases with an increase in temperature because higher temperature causes increased molecular motion, weakening the cohesive forces.

Evaluating the Options

Review each option to determine which is affected by heating a liquid: (a) Increasing surface tension implies stronger cohesive forces, which contradicts the molecular movement. (b) Lowering surface tension means weaker cohesive forces due to increased molecular activity. (c) Increased viscosity implies a thicker or denser fluid, which typically does not happen with just heating. (d) 'Not affected' implies no change, which contradicts the effects of heating.

Choosing the Correct Answer

Based on the understanding that heating a liquid generally decreases the surface tension, option (b) 'surface tension is lowered' is the expected result.

Key concepts

Cohesive Forces

Cohesive forces are the attractive forces between molecules of the same substance, in this case, within a liquid. These forces are responsible for holding the molecules together and give liquids their shape and volume. In the context of surface tension, cohesive forces refer to the interaction between molecules at the surface of a liquid. This creates a 'skin' on the liquid surface, which is why some small objects, like a needle, can float if carefully placed on water. When cohesive forces are stronger, the surface tension is higher because the molecules at the surface cling more tightly to each other.

• This phenomenon is critical in processes that depend on liquid behavior, such as flow in biological systems or the manufacturing of materials.
• Stronger cohesive forces usually indicate more intermolecular bonding, like hydrogen bonds in water, which contributes to various physical properties of liquids.

Understanding cohesive forces helps explain why surface tension decreases with temperature. As temperature increases, molecular energy increases, leading to more vigorous movements, reducing the strength of these cohesive forces.

Temperature Effect on Liquids

Temperature plays a significant role in influencing the properties of liquids. As temperature increases, the molecules within a liquid gain kinetic energy, which affects how they interact with each other. This increase in thermal energy can alter various physical properties, predominately surface tension and viscosity.

• As molecules move more vigorously with rising temperature, the cohesive forces among them weaken. This results in a decrease in surface tension, making the liquid's surface less tight or structured.
• Higher temperatures mean that molecules are further apart on average, aiding them in overcoming cohesive forces more easily.

The impact of temperature is crucial in applications like evaporation, where lower surface tension promotes faster liquid transition to gas. Similarly, in industrial processes like casting metals, temperature control ensures desired material properties.

Viscosity in Liquids

Viscosity is a measure of a liquid's resistance to flow. In simple terms, it describes how 'thick' or 'thin' a liquid is. Viscosity depends on the internal friction between molecules, which is affected by the temperature and the cohesive forces within the liquid.

• Generally, with an increase in temperature, viscosity decreases. This is because the increased kinetic energy allows molecules to move past each other more readily, facilitating easier flow.
• Imagine honey at room temperature; it's thick and flows slowly. Heating it reduces its viscosity, making it flow more easily.

Temperature's effect on viscosity is particularly significant in engineering fields, affecting everything from lubrication systems in machinery to how fluids are transported through pipelines. A lower viscosity at higher temperatures can be beneficial or detrimental depending on the situation, thus understanding this relationship is vital for controlling fluid behavior effectively.