How Do You Choose Cookware?

Transcription

1 Cookin' Chem Activity 6 How Do You Choose Cookware? GOALS In this activity you will: Explore the concept of specific heat capacity. Experimentally determine the specific heat capacity of various substances. Distinguish between materials used in cookware. What Do You Think? You want to fry some eggs. There are many choices for cookware. Pots and pans can cost anywhere from a few dollars to hundreds of dollars. Some frying pans are steel with a copper bottom, some are steel with a steel bottom, and some are iron. Which cookware would you choose to fry an egg and why? Record your ideas about this question in your log. Be prepared to discuss your responses with your group and the class. Investigate In this activity, you will investigate and measure the heating ability of different materials, and decide which ones are suitable for cookware. For each material to be tested, the general procedure involves taking a hot object and placing it in cold water. You then measure the final temperature of the object and water. Recognizing that the heat energy gained by the water is equal to the heat energy lost by the object, you can calculate the specific heat of the object. You want to investigate whether all materials will bring the water to the same final temperature. 560

2 Activity 6 How Do You Choose Cookware? 1. Before proceeding, make a list of what measurements you will have to record in the experiment. 2. Your teacher will have a large container of boiling water with the different metal samples submerged and sitting on a screen support. a) Record the temperature of the boiling water. This will be the starting temperature of the substance. 3. Carefully measure 200 ml of water and pour it into a double foam cup. a) Record the beginning temperature of the water in the cup. 4. Using crucible tongs, quickly lift a metal sample out of the boiling water and transfer it to the double foam cup. Try not to transfer any of the hot water. Gently agitate the cup to distribute heat. a) Record the highest temperature that the water reaches. 5. Remove the metal sample from the foam cup and after drying it off, determine the mass of the sample. a) Record the mass of the metal sample in your log. 6. Now that you know a procedure, you can create an investigation for the other samples. You may want to use a data chart similar to the one shown below to keep track of all the measurements. 7. The Law of Conservation of Energy states that the energy gained by the cold water must be equal to the energy lost by the hot sample. You know how to calculate the energy change by using the equation: Q mcδt You can calculate this Q gain for cold water because you know the mass of the cold water, the specific heat of water (1 cal/g ºC), and the change in temperature. You also know the mass of the sample and the change in temperature of the sample. The unknown is the specific heat c of the sample. Knowing all other values, you can solve for this unknown. a) If you are confident with your algebra skills, calculate and record the specific heat of each sample. Skip Step 8. Safety goggles and a lab apron must be worn at all times in a chemistry lab. Use extreme caution handling the hot metal. Wash hands and arms thoroughly before leaving the lab area. Substance Mass (g) Start temp. of substance Start temp. of cold water End temp. of substance and water 561

3 Cookin' Chem Temp. change of water Heat absorbed by water (lost by substance) (cal) Substance tested Mass of substance (g) Temperature change of substance Specific heat capacity of substance (cal/g ºC) Safety goggles and a lab apron must be worn at all times in a chemistry lab. 8. If you need more assistance, continue with the steps shown below. a) Make a table similar to the one shown above to record your calculations. b) Determine the temperature change of the water in the polystyrene cup and record it in the chart. c) Use the heat equation to determine the amount of heat the water absorbed from the hot object used. heat (mass of water) (specific heat of water) (temperature change of water) or Q mcδt Specific heat of water is 1 cal/g ºC. d) Since the heat energy from the hot metal was gained by the water, the quantity of heat energy lost is identical to the amount gained. Knowing the quantity of heat the material lost, the mass of the material, and the temperature change it underwent, the specific heat of the material can be calculated: c Q mδt The temperature change of the material is from the boiling water temperature to the high temperature of the cold water after the hot substance has been transferred to it. The units for your specific heat will be in cal/g ºC or cal g 1 ºC 1 9. Answer the following questions in your log. a) Which substance had the lowest specific heat capacity? Which had the highest? b) Explain why you would prefer a substance with a higher or lower specific heat capacity for cookware. 562

4 Activity 6 How Do You Choose Cookware? SPECIFIC HEAT CAPACITY AND COOKWARE When you reheat pizza that has been wrapped in aluminum foil and remove the food from the oven, the aluminum foil cools quickly while the pizza stays hot. It is the low heat capacity of aluminum that allows it to cool rapidly. Once you have the pizza out of the foil and are holding the slice by the crust, you might think it is safe to go ahead and bite into it. But when you do yow! It s hot! How can the tomato sauce be so much hotter than the crust if they were in the same oven together? The sauce has a lot of water in it and water has a higher heat capacity than the materials in the crust. So the sauce continued to hold the heat energy while the crust was releasing the heat energy more rapidly. Chem Words specific heat capacity: the amount of heat necessary to raise the temperature of one gram of a substance by one degree Celsius. The specific heat capacity (or specific heat) of a material is a measure of the amount of heat energy that is required to change the temperature of a given amount of substance by 1ºC. In this activity, you heated different materials to a known temperature and then used the materials to heat up some water. This process allowed you to determine the specific heat of the material using the heat equation: Q mcδt When a cook selects a pot to use, the most important consideration (besides the size of the pot) would be the material that the pot is made of. Some of the properties that might be considered are durability, cost, how well it holds heat (heat capacity), how evenly it is heated (thermal conductivity), and rust resistance. In this activity, you looked at the specific heat capacity of various materials. Silver has a low specific heat but would make a rather expensive and non-durable pot or pan. Copper also has a low specific heat and conducts heat well, so some pots have a copper base to allow a uniform temperature when heating. This is a desirable property as most stoves either use a circular flame (like gas ranges) or a spiraling heating element (like electric ranges), which deliver the heat in a non-uniform manner. 563

5 Cookin' Chem Chem Words alloy: a mixture of two or more metals. Aluminum is known for its heat conduction properties. Also, the process by which it is made makes the aluminum up to 30% harder than stainless steel. This process results in an extremely smooth gray surface, which although it is not non-stick, is very easy to clean. On the other hand, the U.S. Consumer Product Safety Commission (CPSC) warns that placing aluminum cookware (or stainless cookware with an aluminum core) on high heat may cause it to melt. When aluminum cookware that is empty (or nearly empty) is placed on high heat, it can boil dry. If a consumer picks up aluminum cookware that has boiled dry, the molten aluminum can drip onto the consumer s arms, hands, legs, or feet causing severe burns. The oldest known metal for cooking is iron, which is never used in its pure form but mixed with other compounds to yield the alloys cast iron and stainless steel. An alloy is simply a mixture of two or more metals. Cast iron has a carbon content of greater than 2%. It takes a long time to heat up, but then it will stay hot for a long time. Foods that require high heat are best cooked in cast iron. Because iron is a reactive metal, one disadvantage of its use is that acidic foods should not be cooked in an iron pot for long periods of time. Aluminum would have this problem as well. Checking Up 1. Why does aluminum foil cool down faster than a piece of pizza? 2. Suppose you take a piece of pizza out of the oven. Will the crust and the sauce on top stay at the same temperature? 3. Why is copper used in cookware? Stainless steel is a strong alloy of iron that contains carbon and also chromium or sometimes nickel. The advantages of stainless steel are that it is rust resistant, and it is quick to heat up. It can withstand high temperatures, but does not distribute heat well. In the oven, food cooks more quickly in steel pans. 564

6 Activity 6 How Do You Choose Cookware? What Do You Think Now? At the beginning of the activity you were asked: Which cookware would you choose to fry an egg and why? Would you change your original choice of frying pan material for cooking an egg? If so, on what basis would you make the change? What does it mean? Chemistry explains a macroscopic phenomenon (what you observe) with a description of what happens at the nanoscopic level (atoms and molecules) using symbolic structures as a way to communicate. Complete the chart below in your log. How do you know? How were you able to calculate the specific heat of an object? How was conservation of energy used in your strategy? Why do you believe? MACRO NANO SYMBOLIC In the activity, a hot object was placed in cool water. Describe the temperature changes of the object and the water. Any kitchen will have various utensils that are made of different materials for different uses. Items with a low heat capacity include certain metal pots and pans, or glass baking dishes. Why might a stainless steel pan have a copper bottom? Why should you care? When objects change temperature, the average kinetic energy of their molecules change. Describe how collisions of fast-moving molecules of one substance and slow-moving molecules of another substance can lead to a common final temperature. Providing your audience with the chemistry behind a chef s choice of cookware will certainly add to the cooking show. You may also choose to discuss why some foods cool quickly while others remain hot for some time. How could this be creatively described in your show? What equations can be used to calculate specific heat? 565

7 Cookin' Chem Reflecting on the Activity and the Challenge Knowing how well a material conducts heat is the first step in making a cookware choice. Heat conductivity is most important for pots and skillets used on the stovetop, where uniform heating helps to prevent hot spots that burn food before it s completely cooked. The low specific heat capacity of metals means that it does not take much heat to increase the temperature of the metal. Consider how information such as this might be used in your cooking-show segment. For instance, you could describe why a chef might choose a copper-coated steel pan over an iron skillet for frying eggs. Your cooking show may also point out how some materials from the oven quickly cool to room temperature while others stay hot. A good restaurant must ensure that all foods are the right temperature when the food is served. This may be something that you want to highlight in your cooking show. 1. If you had pans (of identical mass) made of each of the following metals Al, Cu, and Fe which would be the hottest after sitting on a burner for 1 min? Explain your answer. Material Specific heat (J/g ºC) copper iron aluminum glass stainless steel water a) If you had a g copper pot, how many calories of heat energy would it take to raise its temperature by 60.0ºC? b) How many calories of heat energy would it take to raise an iron pot with the same mass by 60.0ºC? 3. Why do confectioners use marble slabs to prepare candies and fudge? 4. When you pour hot water into a china cup, why should you have a spoon in the cup? 5. If you had water in either of the pots in Question 2, would the same amount of heat need to be supplied in order to bring about the same temperature change? 6. The heat capacity of a substance depends on: a) temperature only b) mass only c) temperature and mass d) mass and specific heat 566

8 Activity 6 How Do You Choose Cookware? 7. Consider the graph of temperature vs. time (minutes) showing the cooling curves of metals A and B. If 100 g of metal A loses 840 J of heat in the first 6 min, the specific heat capacity of metal A is about J/g ºC. a) 0.21 b) 0.84 c) 2.1 d) If the temperature of a certain mass of an aluminum alloy (which has a specific heat capacity of 0.8 J/g ºC) is lowered by 10ºC and the heat lost is 80 J, then the mass of the aluminum alloy is: a) 8 g b) 80 g c) 100 g d) 10 g 9. What properties would you want to see in your pots and pans that would make them good for cooking. Include a list of different materials that are used for cookware and rank the usefulness or suitability of these based on the properties that you listed. 10. Preparing for the Chapter Challenge You have investigated the specific heat capacity of various materials and Activity 1 led you to an understanding of conduction. You should now have some idea why certain materials are selected for use in cookware (and other kitchen utensils) due to their ability to hold different quantities of heat. Prepare a listing of kitchen utensils that have (a) high specific heat capacities; and (b) low specific heat capacities. Include the functions of the pots and pans in the list. Discuss with your group how this might be incorporated into your cooking segment. temperature C A B time (minutes) 567