Question

Mark the correct statement for viscosity. (a) Greater the viscosity, more slowly the liquid flows. (b) Viscosity increases with increase in temperature, (c) Hydrogen bonding and van der Waals forces decrease the viscosity. (d) Viscosity is the measure of ease with which a liquid flows.

Short answer

(a) and (d) are correct. Greater viscosity implies slower liquid flow, and viscosity does measure the resistance to flow in fluids.

Step-by-step solution

Understanding Viscosity

Viscosity refers to a fluid's resistance to flow. The higher the viscosity, the greater the resistance and therefore, the slower the fluid flows.

Effect of Temperature on Viscosity

For liquids, as the temperature increases, typically the viscosity decreases because the increased thermal energy allows molecules to move more freely past one another.

Role of Intermolecular Forces in Viscosity

Stronger intermolecular forces, such as hydrogen bonding and van der Waals forces, generally increase a liquid's viscosity as they hold the molecules closer together, making it harder for them to slide past each other.

Defining Viscosity

Viscosity is indeed a measure of how resistant a liquid is to flow, which reflects how much friction there is between layers of the fluid.

Evaluating Each Statement

(a) is correct because higher viscosity means slower flow. (b) is typically incorrect for liquids, as viscosity usually decreases with increasing temperature. (c) is incorrect because hydrogen bonding and van der Waals force increase viscosity. (d) is a correct definition of viscosity.

Key concepts

Understanding Intermolecular Forces

The concept of intermolecular forces is vital when studying the properties of liquids, including viscosity. These forces are the attractions that occur between different molecules. There are several types of intermolecular forces, including hydrogen bonding, dipole-dipole interactions, and van der Waals forces, which include London dispersion forces.

The strength of these forces depends on the nature of the molecules involved. For example, hydrogen bonds are significant when molecules contain a hydrogen atom attached to a highly electronegative atom such as oxygen, nitrogen, or fluorine, leading to a very polar bond. This polarity results in a strong attraction between adjacent molecules. Van der Waals forces, although weaker than hydrogen bonds, also contribute significantly to the overall intermolecular force, especially in nonpolar molecules.

Role in Viscosity: When intermolecular forces are strong, they hold molecules tightly together, creating resistance when the molecules try to slide past each other. Thus, substances with strong intermolecular forces tend to have higher viscosities. Conversely, if the intermolecular forces are weak, the molecules slide past each other more easily, resulting in a lower viscosity.

Temperature Effects on Viscosity

Temperature can dramatically affect the viscosity of a liquid. When we heat a liquid, we are essentially adding energy into the system. This added thermal energy gives the molecules more speed, allowing them to overcome the intermolecular forces holding them together more readily.

As the temperature increases, the average kinetic energy of the molecules also increases. With more kinetic energy, the molecules can break free from each other’s attractive forces with greater ease, which decreases the friction between the layers of the fluid that is responsible for its resistance to flow.

Practical Implications: This relationship between temperature and viscosity is crucial in many practical applications. For example, in engines, the viscosity of oil must decrease at higher temperatures to ensure proper lubrication. Conversely, for substances like chocolate or syrup, maintaining a certain level of viscosity when heated is essential for consistency in culinary applications.

Liquid Flow Resistance

Liquid flow resistance, or viscosity, is an important characteristic of fluids that describes how readily they flow. To understand this property, imagine a series of flat, horizontal layers of liquid stacked one on top of the other, moving parallel to each other. Viscosity corresponds to the internal friction between these layers as they glide across one another.

Higher viscosity means more friction or resistance, leading to slower movement of the liquid. For example, honey has high viscosity and flows much slower than water, mainly due to its greater internal friction. Factors impacting this resistance include not only the intermolecular forces discussed earlier but also the size and shape of the molecules and the temperature of the liquid.

Measuring Viscosity: Viscosity can be measured using various instruments, one of which is the viscometer, which can gauge the liquid’s resistance to flow under specific conditions. This measurement is crucial in industries like automotive, where the viscosity of lubricants must be controlled to ensure proper engine function, and in food production, where the texture and spreadability of products are affected by viscosity.