Introduction Or How Do You Know When You ve Made Something New? Remember that all matter has characteristic physical and chemical properties. Matter can also undergo physical and chemical changes. How do you know when a physical or chemical change has occurred? How do you know if you ve made something different from what you started with? You know your product is different from your reactants if it behaves differently from the reactants. Different molecules with different molecular structures will behave differently and have different properties. There are a number of tests that you can perform on your product to show for sure that it is different from what you started with. In this lab you will analyze whether a physical or chemical change has occurred. Your job will be to use your results as the evidence to say whether you have made a physical or chemical changes. As you complete the following activities, fill out the data sheet and answer the questions as you go. Procedure Answer the pre-lab questions on the Student Data sheet and then complete Activity #1. Activity 1: Baking Soda + Hydrochloric Acid [Safety note: Wear safety goggles at all times. If you spill any acid, report immediately to your teacher. Wash your hands when you are finished.] Part 1. 1. Obtain a clean, dry ceramic dish. Weigh the container and record the mass on the Data and Calculations Sheet. 2. Keeping the container on the balance, measure out 2.5 grams of baking soda (sodium bicarbonate) into the container. The sodium bicarbonate has the formula NaHCO3. 3. Place the container with the baking soda on your worktable. Obtain 5.0 ml of 6M hydrochloric acid. Caution: Handle this acid carefully. It can cause harmful burns if it touches your skin. 4. Using a dropper, slowly add drops of the hydrochloric acid to your baking soda. It should bubble. Continue adding the acid until all bubbling stops. Stir the mixture with a stick or glass rod to make sure all of the baking soda has reacted. What is responsible for the bubbling? 5. Take your container with all the chemicals in it to a spot designated by your teacher, where you will allow all the liquid in the container to evaporate. 6. When all liquid has evaporated, you should have a solid product. After the acid and baking soda react, you will have to make sure you have something new, not the original molecules or chemicals you started with. The chemical reaction will be very apparent, as will at least one product, a gas. You will be expected to produce evidence that you have, in fact, produced new chemicals that are different from what you started with. Part II: Testing Your Product to Show That You ve Made Something New 1. Find your dry sample from last week. Weigh the dry container with solid product and record the mass on the Data and Calculations Sheet. 2. In Part 1, the baking soda reacted with acid by bubbling. Take a small sample of your solid product. Place in in a spot plate. 3. Now obtain some more hydrochloric acid and add a few drops to a SMALL sample of the solid product. Does it behave the same as the baking soda?
4. Place a small amount of baking soda in a well on your spot plate. Add water, drop by drop, counting the drops. Stir occasionally with a toothpick. Continue adding water (and counting) until the baking soda appears to have dissolved. Make a note of the number of drops you added. Now, obtain a similar amount of your solid product from the dish, place it in a test tube, and add water drop by drop, counting as you go. Continue adding drops of water until you have added as much water as you added to the baking soda. Has all of the product dissolved? 5. Obtain a dropper bottle of Bromothymol Blue and one of Phenolphthalein from your teacher. These are indicators. Indicators change color to show when a specific chemical is present. 6. Place a small sample of your product and a small sample of baking soda in two different wells on your plate. Add a drop of Bromothymol Blue to each. Record what happens. 7. Repeat #6 but use Phenolphthalein. Record what happens. How can you use these results? Are there differences in the results for the baking soda and product? What can you conclude about the product, compared to the reactant, baking soda? Activity 2: Decomposition of Sucrose Ordinary table sugar (sucrose) is a substance known as a carbohydrate. What do you think is the meaning of hydrate in carbohydrate? Make a scoop/boat out of aluminum foil. Place a pea size amount of sucrose in the scoop/boat. Hold the scoop with forceps about ½ cm above the flame of a candle. Heat gently. Avoid placing the scoop into the flame because that causes sooting. Record your observations and answer the questions on the Student Data sheet. Activity 3: Drain Cleaner and Epsom salt We have many different household chemicals in our cupboards at home. Sometimes mixing these in the wrong way can cause potentially lethal gases or other unwanted results. For example, using bleach at the same time you use ammonia can produce toxic fumes. Some people have been overcome by the fumes and ended up in the hospital for simply cleaning their bathrooms! The best caution is to use cleaning chemicals separately unless you know a little chemistry! Place approximately 1 ml (1 dropper full) of 0.2 M sodium hydroxide liquid (Lye) in a small test tube. Drop some drops of Epsom salt solution into the test tube and observe. Record your observations on the Student Data Sheet. CLEAN UP - When you are finished with the lab clean up Wash out all cups, beakers, test tubes and other lab ware. There are test tube brushes at the sink. Leave things to air dry on the mats next to the sink. Put all your other material back into the lab bucket. Clean up your work space through away any paper or other garbage. Wipe off your work area with a damp sponge and dry with a paper towel.
Pre-lab questions: Student Data Sheet Physical and Chemical Reactions Name: 1. How can you tell the difference between physical and chemical changes? 2. List at least 4 signs that a chemical change is likely taking place. Lab Data and Analysis: Activity 1: Baking Soda and Hydrochloric Acid Data and Calculations Sheet Mass of dish Number of drops of H2O added to baking soda to dissolve g drops Mass of just the baking soda Number of drops of H2O added to product to dissolve 2.5 g drops Mass of dish and baking soda Observations bromothymol blue + baking soda g Mass of dish and products after evaporation Observations bromothymol blue + product Mass of product formed g g Observations baking soda + phenothalein: Observations adding hydrochloric acid to baking soda: Observations product + phenothalein: Questions: Testing your final product Does the product have the same properties as the original reactants? Use evidence from your testing to support your answer.
Compare the mass of your reactants before and the mass of the products afterwards. Are they the same? Should they be? Why should they be? Are they different? Should they be? Discuss why they might not be. (something to think about: What happened to the liquid? What happened to the gas? Do they have mass?) You have now tested your original reactants and products. Based on your results, did a physical or chemical change take place? Justify your answer (How do you know?). Activity 2: Decomposition of Sucrose Observations during heating: 1. What do you think the black residue is that was left after heating? 2. Perhaps you observed steam emitted during the decomposition. What do you think is the chemical formula for steam? 3. Do you think this is an example of a physical or a chemical change? Explain your thinking.
Activity 3: Drain cleaner and Epsom Salt Observations: 1. Do you think this is a physical or chemical change? What clues lead you to believe this? 2. List at least two other tests you could do that could confirm your conclusion.