Question

Why is mercury a more suitable substance to use i a barometer than water?

Short answer

Mercury is more suitable for use in a barometer because of its high density and low vapor pressure. The high density means the height of the liquid column is convenient and manageable, unlike water which would require a very tall and impractical column.

Step-by-step solution

Understanding the Properties of Water and Mercury

Start by checking the characteristics of water and mercury. Water is a common liquid with a known density (approximately 1,000 kg/m³ at room temperature). However, it also has a high vapor pressure, meaning it evaporates easily (which can affect the accuracy of a barometer). On the other hand, mercury has a much higher density (around 13,600 kg/m³) and a very low vapor pressure.

Analyzing the height of the Liquid

Barometers work by balancing the weight of a column of liquid against atmospheric pressure. With water's lower density, the height of the water column needed to balance standard atmospheric pressure would be approximately 10 meters (which is impractical for a portable, readily readable instrument). Due to mercury's much higher density, the height of the mercury column is conveniently around 760 millimeters (or .76 meters) under the same conditions which is more manageable.

Effects of Vapor Pressure

Finally, consider the effect of vapor pressure. The higher vapor pressure of water could lead to condensation within the barometer, which could impact its accuracy especially if the condensation occurred in the vacuum chamber above the water column. Mercury's low vapor pressure means it doesn't readily evaporate at normal temperature and pressure, making it more stable and accurate.

Key concepts

Density of Mercury and Water

When it comes to understanding the suitability of a liquid for use in barometers, a critical factor to consider is the liquid's density. Density, which is mass per unit volume, significantly influences how high a liquid must rise in a tube to counteract atmospheric pressure.

Water has a density of approximately 1,000 kg/m³ at room temperature, which is quite low compared to mercury's substantial density of around 13,600 kg/m³. This vast difference explains why a water-filled barometer would need a tube almost 10 meters tall to balance the atmospheric pressure, while a mercury barometer achieves the same with a tube only about 760 millimeters tall. The compact size of the mercury barometer is not just more practical but also enhances its precision and ease of use.

Vapor Pressure in Liquids

Another significant aspect of a barometer's liquid is its vapor pressure, or the pressure exerted by a vapor in thermodynamic equilibrium with its condensate at a given temperature. In simpler terms, it's how much a liquid tends to evaporate at a given temperature, affecting both the operation and the accuracy of the barometer.

Water, with its relatively high vapor pressure, easily vaporizes, which can lead to condensation inside the barometer's tube. This condensation could potentially alter the internal volume and disrupt the measurement. Conversely, mercury's low vapor pressure means it is less prone to evaporation at normal temperatures, maintaining consistency in the barometric readings and providing reliable measurements without the interference from vapor-related issues.

Atmospheric Pressure Measurement

The fundamental objective of a barometer is to measure atmospheric pressure, which is the force exerted onto a surface by the weight of the air above that surface. Measuring this pressure accurately is essential for various scientific and meteorological purposes, including weather prediction and understanding of altitude-related changes in the air.

A barometer operates by balancing the weight of a column of liquid against the atmospheric pressure. The height of this liquid column is directly related to the atmospheric pressure - with a decrease in atmospheric pressure resulting in a lower column, and an increase in pressure resulting in a higher liquid column.

Practicality of Mercury in Barometers

Mercury, due to its high density and lower vapor pressure, enables the construction of more compact and reliable barometers, able to provide precise atmospheric pressure readings that are integral for scientific analysis and understanding our environment.