7.2 Managing Variables

• The method for burning food:

  Measure the mass of food with a top pan balance before burning. Make sure the volume of the water is 20cm (cubed) by using a measuring cylinder. Burn the food and put it under a boiling tube filled with 20cm (cubed) water. Wait till the food stops burning and measure the mass of food. The difference in mass shows how much food is burnt. Then check the temperature of the water with a thermometer. The more energy the food releases the more the water temperature rises.
• Safety implications of the burning food practical:

  • Do not touch fire or open flame
  • Wear goggles
  • Point the mouth of the tube away from yourself
  • Never heat a liquid in a closed container
• How to calculate the amount of energy in food:

  Energy (joules)= mass of water (g) x specific heat capacity of water 4.2(J/°Cg) x temperature change (°C) Calculate the energy content =Energy released as calculated (J) / Mass of sample food (g)
• The results of the burning food practical:

  After the food has stopped burning, measure the mass of the food. The difference in the mass of food from the start to end shows how much food is burnt. The temperature of the water is increased because the energy from the food is released as heat. Then you can calculate the amount of energy in the food.
• Equipment needed for the burning food practical :

  Thermometer, boiling tube, mounted needle, measuring cylinder, top pan balance
• Identify 3 variables in the burning food practical:

  Independent variable: type of food

  Dependent variable: temperature rise of water

  Control variable: volume of water and mass of food
• Know the units for calculations related to the burning food practical

  • mass of food (grams)
  • volume of water (cm cubed)
  • temperature (°C)
  • Specific heat capacity of water (4.2 J/°C)
  • Energy (joules)
• Independent variable:

  a variable you change in an experiment
• Dependant variable:

  a variable you measure in an experiment.
• Control variable:

  a variable that stays the same in an experiment.
• How many times must you measure the temperature of the water to work out which food has more energy?

  We need to measure it twice. Once before and once after burning the food.
• What will tell us which food has more energy?

  The larger difference in temperature between before and after will allow us too see which food has more energy.
• What is one idea on how to measure the energy in foods?

  We burn the food and use the heat to warm up water. The more energy the food releases, the more the water's temperature will rise or increase.
• What equipment should you use to measure the temperature?

  thermometer
• What equipment should you use to measure the volume of water?

  measuring cylinder
• What equipment should you use to measure the mass of food?

  top pan balance
• What else could affect the water's temperature rise besides the types of food we are testing?

  The volume of water
• What could be the problem if we use too little water?

  It could boil away before we collect our results.
• What could be the problem if we use too much water?

  The temperature rise would be too small to measure accurately.
• What is a sensible volume of water?

  20cm (cubed)
• How did Eniola check her method before comparing different foods?

  She tested the same food but different masses (0.5g & 1.0g) and compared the temperature rise for both.
• How did Eniola know her method worked?

  When she doubled the mass of food, the temperature rise doubled.
• Why must Eniola be careful?

  She had to be careful not to burn herself because the apparatus got very hot.
• Think of some safety tips you could give Eniola to prevent her hurting herself?

  • Do not touch fire or open flame
  • Wear goggles
  • Point the mouth of the test tube away from yourself
• What were her two problems she found in her method?

  • hard to get pieces of food with an exact mass and it took time
  • not all food burnt
• How did she overcome these problems in her method?

  She measured the mass of the food at the start and the end.
• How could she calculate the temperature rise of 1 gram of food?


  temperature rise (°C)/ mass of food used (grams)