ccks BIO 152
Popular in Biology of the organism
Popular in Biology
This 2 page Bundle was uploaded by Samuel Croteau on Friday May 20, 2016. The Bundle belongs to BIO 152 at Massachusetts College of Pharmacy & Health Sciences taught by Dr. Demasi in Fall 2016. Since its upload, it has received 19 views. For similar materials see Biology of the organism in Biology at Massachusetts College of Pharmacy & Health Sciences.
Reviews for ccks
Report this Material
What is Karma?
Karma is the currency of StudySoup.
You can buy or earn more Karma at anytime and redeem it for class notes, study guides, flashcards, and more!
Date Created: 05/20/16
Tutorial: Recognizing Chemical Change Page 1 When you change the shape of an object—whether by crumpling a piece of paper or by stretching a rubber band—the change in shape usually indicates a physical change in matter. The paper is still paper and the rubber band is still rubber. Other kinds of observable signals indicate that a chemical change has occurred. Since chemical changes actually happen on a particle-by- particle basis, we rely a lot on these visual clues to detect chemical changes. Keep in mind that in a chemical change, one or more new substances are formed. These substances have properties that differ from those of the original substances before the change. Page 2 Have you ever noticed the soap scum that forms in a bathtub or on shower walls? Maybe you’ve also noticed a white residue that appears on silverware when you don’t quite wash all the soapy water off before you set it out to dry? In both cases, the solid that forms indicates that a chemical reaction has taken place! In the case of soap scum, dissolved calcium and magnesium ions in water come in contact with dissolved soap molecules and combine to form an insoluble solid. Soap scum is an example of a precipitate. A precipitate is a solid produced during a chemical reaction between two or more solutions. The precipitate usually settles out of the mixture at the bottom of the beaker or flask. Can you see the settled precipitate in this image? Another great example of a precipitation reaction is the formation of bright yellow lead iodide solid by reacting a colorless solution of potassium iodide with a colorless solution of lead nitrate. Formation of a precipitate is one sign of a chemical change. Page 3 When water boils, you know that liquid water changes to the gas phase. When the water is at a rolling boil, as seen in the picture, the bubbles that form are pockets of water vapor. This is an example of the release of gas during a physical change. Since the gas has the same chemical composition as the liquid, no chemical change has occurred. Now consider the seltzer tablet experiment that you carried out at the beginning of this section. By adding the solid seltzer tablet to water (and knowing that the water wasn’t boiling), the formation of bubbles indicates that a chemical change is taking place. Sometimes gases, such as oxygen, carbon dioxide, and nitrogen, are released during chemical changes. However, be smart! The observation of a gas should also be accompanied by other observations to confirm that a chemical change, and not just a physical change, is occurring. Page 4 During some chemical changes, energy may be released, usually in the form of light or heat. A campfire is a good example of this. Wood burning is a chemical change, because both light and heat are released. An increase in temperature signals that heat has been released during the chemical change. A reaction that releases heat is called exothermic. In other cases, heat may be absorbed from the surroundings during a chemical reaction. Do you recall what happened to the temperature of the water in your seltzer experiment? If the temperature of the water decreased as bubbles began to form, the reaction was absorbing heat. A reaction that absorbs heat energy is called endothermic. Page 5 During some chemical changes, the new substance will have a different color than the substance from which it was formed. The rusting of metal is an example of a chemical change that produces new substances with different colors. A gray metal turns reddish-brown as iron reacts with oxygen in the air to form iron oxide compounds on the surface of the metal. Notice that the chain in the picture has some parts that are still gray and others that have turned color as the chain has © K12 Inc. AV011413 rusted. What other properties of the rusted and non-rusted areas appear to be different? Since rust has a different chemical composition than the original iron, the rust will have a unique set of physical and chemical properties. Another example of color change is autumn leaves that turn colors because of complex chemical changes occurring in plants. Page 6 You may have realized that for each signal of chemical change to occur, the chemical identity of the original substance has to change. It can take a lot of words to describe what is happening during a chemical change, so chemists use short hand. Let’s see how this works for the rusting of iron. In the rust example, the original substance, iron, reacts with oxygen in the air to form iron oxide. This chemical reaction can be summarized in a chemical equation. To say this out loud, you would say, “Iron plus oxygen yields iron oxide.” The right-pointing arrow means produces or yields. In every chemical reaction, the starting substances are called the reactants and the ending substances are called the products. To show that the total amount and kinds of matter are accounted for during the change, chemists often use symbols and sets of numbers to keep count of the amounts of each element present. We will look at how to write such equations in an upcoming unit. © K12 Inc. AV011413
Are you sure you want to buy this material for
You're already Subscribed!
Looks like you've already subscribed to StudySoup, you won't need to purchase another subscription to get this material. To access this material simply click 'View Full Document'