Question

When a student was given a viscometer, the liquid was sucked with difficulty; the liquid may be: (a) benzene (b) toluene (c) water (d) glycerine

Short answer

The liquid is glycerine.

Step-by-step solution

Identify the Problem

The problem involves identifying a liquid based solely on its viscosity. The student found it difficult to suck the liquid using a viscometer, indicating that the liquid has a higher viscosity.

Understand the Viscosities of the Possible Liquids

It's important to know the relative viscosities of the potential liquids. They are roughly as follows: Glycerine > Water > Toluene > Benzene. As you can see, glycerine has the highest viscosity among the options.

Identify the Liquid

Based on the clues given in the problem, namely that the liquid was difficult to suck up with a viscometer, the liquid is most likely to be the one with the highest viscosity. Therefore, the liquid is glycerine.

Key concepts

Liquid Viscosity

Liquid viscosity is a measure of how resistant a liquid is to flow. A common analogy used to understand viscosity is to compare it to the 'thickness' of different fluids. For example, honey has higher viscosity than water because it flows much more slowly and has greater resistance to motion.

The exercise involving the viscometer directly relates to viscosity, as it gauges the ease with which the liquid can be moved, sucked, or poured. Liquids with high viscosity, like glycerine, are much harder to suck through a viscometer than those with low viscosity, such as benzene or water. This is because the internal friction between the molecules of a viscous liquid is greater, which corresponds to more resistance to flow.

Viscometry

Viscometry is the science of measuring the viscosity of liquids. Using an instrument known as a viscometer, you can find out how thick or thin a liquid is. There are different types of viscometers, including capillary, rotational, and falling ball viscometers.

In our textbook's described exercise improvement, the type of viscometer isn't specified, but the difficulty experienced while trying to suck the liquid through it leaned heavily towards it being a more viscous liquid. Understanding how to use and interpret results from viscometry tools is vital in fields such as lubrication, where the right viscosity of a fluid can mean the difference between a well-running machine and a breakdown.

Physical Chemistry

Physical chemistry is the branch of chemistry focused on understanding how matter behaves on a molecular and atomic level and how chemical reactions occur. Among its areas of study is the investigation of material properties, including viscosity, and how these properties change with varying conditions such as temperature or pressure.

The textbook problem illustrates the application of physical chemistry principles in determining the identity of a substance based on its physical properties—in this case, its viscosity. Knowing the relative viscosities of different substances is part of the foundational knowledge of physical chemistry, which allows for practical applications such as identifying unknown liquids. As students delve into the world of physical chemistry, they learn how to apply various principles and methods, like viscometry, to solve real-world problems.