C3bL5  Energy

Key Words

Calories - energy needed raise 1 g water through 1 ^oC.

Calorimetry - measuring the heat energy gained when burning food or fuel, or any other reaction.

Energy - ability to do a job of work.

Insulated - surrounded by a jacket made of a material that is a poor conductor of heat.

Obese - very fat.

Specific heat capacity - the energy needed to raise unit mass of material through unit rise in temperature.

Temperature - measure of how hot something is.

Grade E

We have food as our fuel to give us energy.  The energy value of food is given in calories, where 1 cal is the energy needed to raise the temperature of 1 gram water by 1 degrees Celsius.  1 cal = 4.2 J

In reality, the "food calorie" is 1000 calories, so a food saying that its energy is 1 calorie has an energy of 1000 cal (= 1 kcal) = 4200 J.

The daily energy requirements are:

• for men, 2500 kcal;
• for women, 2000 kcal.

Foods that have a lot of sugar, or oils, or fats, have a lot of energy.  Should you eat too much of these kinds of food, you can become overweight (tubby), or even obese.  Obesity leads to a lot of health problems, such as heart disease, diabetes, or cancer.

The basic reaction that drives our cells is:

glucose + oxygen  ® carbon dioxide + water + energy

The energy is used for running the body and allows us to do everything and anything.

Grade C

We can measure the energy coming from any chemical reaction using calorimetry. 

The simplest (and smelliest) way is use a burning piece of food to heat a known volume of water.  You record the temperature at the start and the temperature at the end.

Temperature rise = temperature at end - temperature at the start.

You can then use the equation for specific heat:

energy = mass of water × specific heat of water × temperature change.

From that, you can work out the energy per gram of food.

For a reaction in aqueous solution, we use the same idea, but using an insulated beaker called a calorimeter.

Chemists use very sophisticated equipment to measure energy changes accurately, but the principles are the same.

Grade A

There has been a lot of debate about junk food.  Recent government initiatives have encouraged schools to promote healthy eating, cutting down on convenience "foods", of which the ingredients can be quite stomach-turning.

Instead, fresh fruit and vegetables have been promoted.

A lot of the additives in processed food can also be harmful in some people:

• aspartame and acesulphame are two sweeteners found in "low calorie" drinks.  These can alter brain function and have even been associated with cancer.

• Monosodium glutamate can make some people feel ill.

• Excessive amounts of salt in food can raise blood pressure.