Specific Heat Capacity of Brass

Introduction

The specific heat of a substance is the amount of heat it can hold per unit mass. In this experiment, we place a brass weight in boiling water. When the weight is in the boiling water, it is gaining heat energy. After about 4 or 5 minutes, it will be heated evenly to the temperature of the water. It is at this point that it has gained all the heat that it can from the boiling water.

The brass weight can then be moved to the water in the calorimeter. It will then heat the water and the calorimeter.

Knowing the specific heat capacity of the water and calorimeter and the maximum temperature attained allows you to calculate the heat gained by them. This amount of heat gained must be the same as the amount of heat lost by the brass. This enables the specific heat capacity of brass to be calculated.

The key to understanding this experiment is the simple observation that the heat gained by the water and calorimeter is the same as the heat lost by the brass weight.

Heat gained by calorimeter and water = Heat lost by the brass weight

This is an expression that energy in a closed system is not created or lost. A fundamental principle of Physics.

The equation that we use to derive the specific heat capacity follows directly from this principle, see below for the equation.

The objective

The aim is to find Sb, the specific heat capacity of brass.

The apparatus

A glass beaker three-quarters filled with water
A brass weight (100 g)
A Bunsen burner, tripod, and gauze to keep the water boiling
A calorimeter made of copper with a known quantity of water
A digital thermometer
An orbital shaker

This shows the calorimeter with its thermometer on the agitator. There is a beaker of water containing the brass weight on a tripod and gauze being heated by a Bunsen burner.

The orbital shaker is optional, as it is sufficient to stir the water.

The variables

The time is the independent variable, and the temperature is the dependent variable.

The Physics

The heat gained by the water and the calorimeter equals the heat lost by the brass weight.

If the mass of the calorimeter is Mc, the specific heat capacity of brass is Sb, the mass of the water is Mw, the specific heat of water is Sw, the mass of the brass weight is Mb, the initial temperature of the water is Ti, and the final temperature of the water is Tf, then using the equation given in the Introduction:

McSb(Tf – Ti) + MwSw(Tf – Ti) = MbSb (100 – Tf)

The method

An outline of the method is to: heat the brass weight to a known temperature. Then to let it give up the heat it holds to some water in a calorimeter. We can then use the 'heat lost = heat gained' principle to work out the heat capacity of the brass weight.

Bring a beaker of water to boiling using a Bunsen burner. Use a loop of wire or string to lower the brass weight into the water.

Heat the brass weight to 100 oC by leaving it in the boiling water for 5 minutes. Then plunge it into the cold water in the calorimeter. Start the shaker to even out the heating of the water.

Note the temperature of the water before adding the weight and then at 15-second intervals.

Determine the highest temperature and therefore the heat gained by the water and calorimeter and the heat loss from the brass. A correction can be made to the directly observed maximum by noting that there is an inevitable heat loss from the calorimeter to the room. If we record the temperature for several minutes after the maximum has been reached, we can tell how much heat is being lost to the room per minute and use that to correct the observed maximum.

By far the easiest way of doing this is to use a spreadsheet program like Excel, then all the calculations can be done for you each time you change something. The spreadsheet is included with the full package.

Watch a video of the Specific Heat e-practical here.

This shows how to use it and how to collect the data.

Perform the experiment yourself, collect your own data, make mistakes and be able to correct them. The e-practical requires your browser the run WebGL 2 (usually found on Windows browsers, safari on iOS, and various Mobile browsers, test with https://get.webgl.org/webgl2/). This link is for students and evaluation only, schools should purchase a site licence.

Try it yourself

The e-practical requires your browser can run WebGL 2 (usually found on Windows browsers, safari on iOS, and various Mobile browsers, test with https://get.webgl.org/webgl2/).

The e-practical will run on laptops and desktops for PCs and Apple computers and will run on mid to high spec tablets.

Further Discussion

In the actual experiment, we would try to improve the accuracy by tapping the brass on the bench before plunging it into the water in the calorimeter to remove any hot water that might cling to it.

We would also probably stir the water in the calorimeter to ensure that the heating was even rather than use an electrical shaker.

Can you find any other suggestions for improving the accuracy of this experiment?

Sources of inaccuracy

If we don't heat the brass weight all the way through by not leaving it in the boiling water for long enough, then we will not have used all the heat capacity of the brass and our result will be inaccurate. You may need to perform the experiment more than once to establish how long is long enough.
The longer the brass is out of the heating water before being placed in the calorimeter the more heat will be lost to the room. A quick movement from boiling water to calorimeter is needed.
Any water clinging to the brass weight will adversely affect the results, this can be mitigated by knocking the weight on the bench to remove any water droplets..
Any heat lost from the calorimeter to the atmosphere will affect the result, so good insulation is key here.

This section is adapted from material developed by Dr Robert Lucas and is related to the book High School and Undergraduate Physics Practicals, published by CRC Press.