What are Calories? - Definition & Explanation

Instructor: Nathan Crawford

Nathan, a PhD chemist, has taught chemistry and physical science courses.

In this lesson, the concept of the calorie is defined as a measurement unit for energy. A brief history of the development of the calorie is included in the lesson as well as examples of unit conversions and basic calculations with this unit.

A Brief History

'Try our 100-Calorie snacks for that healthy energy boost!' reads a generic advertisement on a website. Have you ever wondered just what defines a Calorie? What do calories measure? Why do advertisers write the word with a capital C?

The concept of the calorie has been around for quite some time. The original concept can be traced to the work of Antoinne Lavoisier, the father of modern chemistry, and his work with the famous scientist Pierre-Simon Laplace in the 1780s. Lavoisier developed an apparatus to measure the amount of energy produced by living organisms.

Anotione Lavoisier
Anotione Lavoisier

Lavoisier ice calorimeter used to measure energy from organisms
Lavoisier ice calorimeter used to measure energy from organisms

As time passed, the concept of the calorie was refined during the 19th century to apply to any measure of energy. Specifically, the calorie was defined as the amount of energy required to increase the temperature of 1g of water by one degree Celsius.

The Calorie Defined and Its Presence in Foods

Think of warming a single milliliter of water by 1 degree Celsius. You would not even be able to notice the change in temperature! Since the calorie is based on the effect of energy absorption on a specific substance (water), this unit is not included in the Systemme Internationale (SI), also known as the metric system, which is the system that uses the joule (J) as the standard unit for energy.

The energy content of foods in the United States is listed as 'Calories' on the nutritional information found on the packaging. The spelling of the word calorie with a capital 'c' actually represents the kilocalorie, or 1000 calories. Therefore, the healthy snack containing 100 Calories contains 100,000 actual calories!

Nutritional information containing Calories
Nutritional information containing Calories

Unit Conversions Using Calories

You might encounter many situations in science courses where the calorie is not appropriate to express the result of a calculation. In these cases, you will need to employ a technique known as the factor-label method, also known as dimensional analysis. This technique involves the use of a conversion factor, an equality that shows the relationship between two or more units. The conversion factor is arranged in such a way as to cancel the unit that is being changed.

As an example, consider the 100 Calorie (Cal) snack mentioned in the very first part of this lesson. If you wanted to know the actual amount of energy in calories, you could easily move the decimal to the right three places, since 1Calorie = 1kilocalorie = 1000 calories (cal). If moving the decimal seems confusing, use the factor-label method using the conversion factor of 1Cal = 1000 cal. The 100 Cal value is placed over one, and the conversion factor is arranged to ensure the 1 Cal is in the denominator position. Practice this calculation on your own by using the Calories found in the nutritional labels of your favorite foods.

Conversion of Calories (kcal) to actual calories
Conversion of Calories (kcal) to actual calories

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