Bar
How Many Calories? Ask Mendel's Mother

B My son wants you to see if he is eating too much. Can you help him?

You must investigate how many calories there are in a typical dinner at the Brno Monastery. To do this you first must determine just how many calories there are in each of the food items. When you know this you must then calculate how many calories there are in the quantity of each food item consumed.

For example, it my son eats a piece of salmon, say 124 grams, you must first determine how many calories there are in one (1) gram of salmon, and then multiply your answer by 124 to find out the total number of calories.

When you have done this for all the items in the meal, you can then add them all together and find out the total number of calories in the whole meal. Easy, huh?

O.K. Where are you going to start?


B

Using the Calorimeter

Theory: This piece of equipment allows the investigator to measure the amount of potential energy in any sample of material that can be burnt in air (actually the oxygen in the air).

Calorimeter

As the sample burns, a chemical reaction takes place that liberates the available energy in the form of heat. The heat moves away from the chemical reaction and is absorbed by the surrounding water.

As the water absorbs the released heat the molecules in the water move faster and faster. Their kinetic energy is increased, and the investigator can measure this increase in kinetic energy using a thermometer.

By definition, a calorie is the amount of energy needed to raise the temperature of one gram of water from 14.5 to 15.5 degrees centigrade. Although it is not strictly accurate, this definition is often abbreviated to ".. the energy needed to raise the temperature of one gram of water by one degree centigrade."


B If it takes one calorie to raise the temperature of one gram of water one degree centigrade then it will take 10 calories to raise the temperature of 10 grams of water by one degree centigrade.

If:

T1 = the starting temperature of the water.

T2 = the final temperature of the water.

Then, the number of degrees centigrade through which the water was raised is:

Degrees raised = T2 - T1

For example, if the water temperature was raised from 15 degrees centigrade (the T1 value), and the final temperature is 73 degrees centigrade (the T2 value), then the "Degrees raised" is T2 - T1 or 73 - 15, which is 58 degrees.


B If the amount of water in the calorimeter was one gram, then it will take 58 calories to raise the temperature by this amount (from 15 to 73 degrees centigrade). But if the amount of water in the calorimeter was 2 grams, then it would take 2 x 58, or 116 calories to make the water reach the same temperature.

So, if:

V = amount of water (in grams).

The formula for calculating the number of calories involved is:

Number of calories = (T2 - T1) x V

It is possible, therefore, to measure the amount of energy released from the burning sample in the calorimeter. A known amount of is placed in the equipment, the sample is burnt and the number of degrees centigrade the temperature of the water is raised is measured.


B

What to Do

First, use the calorimeter simulation to determine the number of calories in one gram of each of the foods under investigation.

To do that,

  • choose an amount of water for the calorimeter; then

  • choose a food; then

  • decide on an amount in grams.

  • When all is ready, start the experiment.

Write down all your results, and use the method described above to calculate the total number of calories that were released from the food.

Then, divide this number by the number of grams of food you used, and you will have arrived at the calorific value for that particular substance. Write this down.

When you have done this for each food, go back to the table that showed just how much of each food my son eats. Multiply the calorific value of each food with the amount eaten and write down these results.


B Finally, add up all your results and get the total for the meal.

Was it too much or too little for one meal?


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© 1998 Professor John Blamire
Science at a Distance