Measuring the energy in food by calorimetry (triple)

How could anyone measure the energy in a biscuit, when energy will not sit on a set of scales? Gram for gram, packets say a chocolate digestive holds about twice the energy of white bread. The tempting answer is to weigh the food: the heavier piece must hold more. But a slice of bread can outweigh a biscuit and still hold less energy per gram. Those label figures go back to experiments that burned foods and measured the heat they gave off. First the apparatus that does the burning, then the sum that makes the comparison fair.

Carbohydrates, proteins and lipids are where the energy in food is stored. Burning a food releases that energy as heat. The school apparatus is plain. A boiling tube holds a measured mass of water, with a thermometer standing in it. Underneath, a small piece of food sits on a mounted needle, ready to be set alight. Weigh the food first. Take the water's starting temperature. Set the food alight under the tube. Then read the highest temperature the water reaches. It works because the heat from the burning food goes into the water. You cannot read energy directly, but you can read a temperature rise. Most students already know what this method is for. One examiner's report puts it this way. Marks were obtained for the idea that it measured the energy content or calories in a food and most candidates gained the mark for this question. Now, does all of that heat end up in the water? If not, where does it go? No. Some heats the air around the flame, some rises past the tube, and some warms the glass, so it never reaches the water. That means a school result always comes out lower than the true value. It is an underestimate. That is what a lid and insulation are for: they cut the heat that escapes, and stop water evaporating away. One examiner's report records what happened when students had not met this apparatus. Candidates who were unfamiliar with calorimetry experiments gave answers such as the lid prevents the entry of bacteria or unwanted substances. These responses did not gain credit. Follow the heat instead. A lid keeps heat and water in; it has nothing to do with germs.

The sum starts with the temperature change, written on its own line before anything else: the end temperature take away the start temperature. Two foods are burned under twenty grams of water each. Food A has a mass of nought point five grams. Food B has a mass of one point two grams. Under food A, the water goes from twenty-one to thirty-three degrees C. Under food B, it goes from twenty-two to forty degrees C. Which food holds more energy, gram for gram: A or B? Start with the change. For A, thirty-three take away twenty-one is twelve degrees. For B, forty take away twenty-two is eighteen degrees. Now divide by the mass. Twelve divided by nought point five is twenty-four degrees per gram. Eighteen divided by one point two is fifteen degrees per gram. Food A holds more energy per gram. A student says food B must hold more energy, because its water got hotter. Now spot the flaw: what is wrong with that student's reasoning? B's piece was more than twice as heavy, so more heat came off it. A fair comparison needs the rise per gram, and per gram, A wins. One more trap is using the end temperature. Forty is not how much the water warmed; eighteen is. Another report, about a calculation of this kind, puts it this way. A majority of candidates correctly calculated the energy content of the food. The most common error was to use the end temperature in place of the change in temperature. Writing start, end, change before any division is the habit that prevents it.

A school result always falls short of the packet value, so the useful question is which change brings it closer. Three students suggest a change. The first says put a lid on the boiling tube. The second says use cold water instead of room-temperature water. The last says use a bigger piece of food. Which change brings the result closest to the packet value, and why do the other two fail? The lid, because it keeps in heat that was escaping and stops water evaporating. Cold water changes where the reading starts, not how much heat escapes. A bigger piece gives a bigger rise, but per gram the answer is the same, and a similar share of the heat still escapes. Your body releases this same energy without flames, slowly, by respiration. The video on respiration, aerobic and anaerobic, covers how.

Calorimetry, checked from memory: new readings first, and the biscuit at the end. Water starts at eighteen degrees C and ends at thirty. What number goes into the sum? Twelve: thirty take away eighteen. The change, never the end temperature. Now the lid is left off. Will the result be too high or too low, and why? Too low. More heat escapes into the air, so less of it reaches the water. One more: half a gram of biscuit warms the water by fifteen degrees. How much per gram? Fifteen divided by nought point five is thirty degrees per gram.

This one's yours if start, end, change and the per-gram step run without looking, so mark it with a thumb. If not, you are in good company: nobody gets the per-gram step first time, so save it and come back.

That completes our chapter on enzymes, digestion and metabolism. Next chapter: Transport in and out of cells. Hopefully that's next in your feed, but if not, search for ScholaFly B I oh four, oh oh.

Every other video is on scholafly.com too, easy to search by video code or by keyword. Whatever you're revising, go there and have a look, or, watch the next video.