Chemical Changes When cutting a piece of wood, you now know that the molecules of the wood stay the same. The only change that is occurring is a physical one because we are changing the length of the wood. Since length is a physical property measurement, it would naturally become a physical change. But what happens when that same piece of wood is placed in a campfire and burned for heat? Doesn’t the size change? Well, yes, the size does change. As a matter of fact, the size gets really small as the wood turns into ash. But what type of property deals with this kind of change? Is it still a physical property? Even though the size and shape are changing, it is no longer a physical property. Chemical Properties When matter has the ability to change into a different type of matter with different properties, we call the measurements of these changes chemical properties. Many different materials are described based on their ability to change into something else. The most common chemical properties are flammability and reactivity. Flammability looks very similar to a word that many of you have seen before: flame. In this case, flammability is the ability of a substance to burn or better yet, how easily something burns. Look back at our burning piece of wood, since it can burn, we could assign a flammability rating. The paper that you are reading would have an even higher flammability rating since it can catch fire easier than a piece of wood (if you have ever tried starting a campfire, you know what I mean). Water on the other hand cannot burn. For this reason, water would have a flammability rating of zero. More often than not, the flammability measurement is how many seconds it takes to catch on fire. All of the electronics and clothing you have purchased have a flammability rating. The simple reason is the government tries to minimize how easily something catches on fire to help improve your safety around the home. If you look at your electronics (especially near the battery), you will see a UL symbol. The symbol shows that the equipment has been tested by the Underwriters Laboratory. This laboratory is responsible for assigning flammability ratings for everything sold in the U.S. The second chemical property is called reactivity. Reactivity refers to the ability for two or more substances to combine and form a new substance. Some things react easily while some substances don’t react at all. If you remember our discussion of the noble gasses (Helium, Neon, and Argon for example), they are called noble because they don’t react with any other elements. Their reactivity would be zero. The group on the far left of the periodic table (Hydrogen, Lithium, Sodium, and Potassium for example) is highly reactive. Their reactivity would be really high since they can react with oxygen and burn easily. Chemical Changes Just like physical properties tell us when a physical change is happening, chemical properties can tell us if a chemical change will occur. In class, we described a chemical change as the molecules of a substance changing to form a new substance. This is actually the only definition we need to know (☺). Chemical changes form something new. As for chemical properties and chemical changes, they are different. When talking about the chemical properties of a substance, we are describing how easily something changes. A chemical change though is describing what happens when something changes. For example, take the elements hydrogen and oxygen. Recall that both are highly flammable. This is a chemical property that describes hydrogen and oxygen. When hydrogen and oxygen get near each other they form water. The process of turning these two gasses into water is a chemical change because the molecules changed. Let’s do another example. Go back to our burning wood. Wood has the ability to catch fire. This is a chemical property. When wood catches fire and turns into ash letting off lots of smoke, this is a chemical change. So again, chemical properties tell us how easily something can change while chemical changes tell us when something is different. Signs of Change Most of the time, it’s easy to see when something is going through a physical change. Chemical changes though require a little bit more effort as well as several more of our senses. We have already performed several experiments that were chemical changes. Take, for example, vinegar and baking soda. We saw bubbles form that filled a balloon. Gas formation (bubbles) is a great sign that a chemical change is occurring. Ever opened up a carton of milk and smelled something sour? This is also a chemical change, odor. What about our burning log? If you have ever been close to a campfire, you already know that it gives off heat. This is another sign that a chemical change is occurring. That same campfire also gives off light which is also a sign. One of the harder changes takes a little more time to show up. Take the Statue of Liberty for example. It’s made of copper and copper has a brownish color. So why is the Statue of Liberty green? This is another great example of a chemical reaction. Color change is a great chemical change indicator. The hard part though is trying to figure out if the color change is chemical or physical. Remember, we said physical changes that include color can be separated back out. Chemical color changes are inseparable. The one last sign that can show up in a chemical change comes from a word that makes you think of rain, precipitate. We use part of that word in precipitation to describe rain. Rain is a physical change since it is gas turning into a liquid. Precipitate though describes when small particles of solid are formed. One of the most common precipitates is rust. If you have ever left a scouring pad lying around in water for several days, it turns a nice brownish‐red color. Try using that scouring pad and small particles will break off in your hand. These small particles are the precipitate known as rust. In most cases, the precipitate also includes a color change. So chemical changes have lots of signs that help us know when they take place: 1. The formation of a gas. 2. Odor 3. Heat (both exothermic and endothermic) 4. Light 5. A color change that is inseparable 6. The formation of a precipitate. Chemical vs. Physical In the end, every change comes down to one of two categories. It’s either a physical change or a chemical change. If the molecules stay the same, it is physical. If the molecules change, it is chemical. Neither has anything to do with how easily they change back. Some would like you to believe that physical changes can be easily reversed. The last I checked broken glass can’t be easily turned back into a lamp. The same thing goes for cutting that tree down or wrecking a car. So don’t think of either type as being easily changed back. Think of how the molecules are changing as well as the signs you are seeing to determine which type of change you have. Guided Reading for Chemical Changes Directions: Read and highlight the front. Then use the information to answer the questions below. Define chemical properties? What are the two types of chemical properties? 1. 2. What is a chemical change? How are chemical properties and chemical changes different? What are the 6 different signs of a chemical change? 1. 4. 2. 5. 3. 6. What is the basic difference between chemical and physical changes? Other than those listed in the reading, name two chemical changes you have seen in your everyday life. 1. 2.