Physical Changes Why learn about measurement? Many of the measurements we have been learning fall into a special category called physical properties. What makes them a physical property is that we do not destroy what we are measuring to learn something about it. For example, when we measure the length, width, and height of a cube, this is a physical property. Recall, that our measurements didn’t end with just a ruler or meter stick. We have been using triple beam balances to find mass, graduated cylinders and overflow canisters to find volume, and thermometers to measure phase changes and temperatures not to mention we use the combination of mass and volume to find density. All of these tests because they do not destroy what we are measuring fall into the physical property category. Now, if we start burning things, although fun, this would no longer be a physical property. There are other special physical property tests that we do not discuss but are equally important. For example, when salt or sugar dissolves in water, this is a special physical property called solubility because the salt and sugar aren’t chemically changing into something else. Other properties such as the ability to be pulled into a wire (ductility) and the ability to be rolled or pounded into a thin sheet (malleability) are other types of physical properties. For example, corn has the ability to be smashed into tortillas and tacos which taste great. The aluminum in pop can comes from rolled and smashed aluminum. Both of these display the property of malleability. As for ductility, the easiest example is copper wire. All of the wiring in your house that delivers your electricity was formed into wires thanks to copper having ductility. So why are we learning about all of the different measurement types? Because these measurements help us better understand something that you discussed in elementary school, physical changes. Physical Changes So what is a physical change? We’ve been defining it as “no change in the molecular structure of a material.” This is totally correct, but there is also another way to define physical changes that uses the idea of physical properties. A physical change is a change that affects one or more physical properties of a substance. For example, if we cut a tree in half, we change the height of the tree. Height is a physical property. Since we are changing a physical property, it must be a physical change. The same thing is true when something melts. Take an ice cube. If we let it melt, the shape changes as well as the volume. These are all physical properties that change which means melting is a physical change. What about those special cases we mentioned under physical properties such as solubility and malleability? One of the solubility examples we mentioned was salt dissolving in water. When something dissolves, it can be separated back out. For example, paint salt water on a piece of black paper and let the water dry. What happens? Chances are you will notice that the salt is still on the paper even though the water has disappeared. So is anything changing during Physical Changes? Well, the simple answer is not really. Other than those physical properties we talked about above, everything else is staying the same. When you look at the atoms and molecules, they are unchanged. Think about when water ice turns to liquid water. If the molecules of water (H2O) broke apart, we would have oxygen (O) gas and hydrogen (H) gas. Hydrogen gas is highly explosive while oxygen gas is extremely flammable. This would give new meaning to “dying of thirst.” (☺) Recognizing Physical Changes So if we look at all of the physical properties and think about how they can change, we can figure out when something goes through a physical change. For example, when we cut the tree in half above, we changed its shape and size. When we smashed the corn to make tortillas or tacos, we changed the shape and size as well as the appearance of the corn. When the ice cube melted, it went through a phase change. All of these examples help us determine how to recognize a physical change. The same can be said of salt dissolving in water. Since the salt molecules don’t really change, (nor do the water molecules), when something dissolves, it’s also a physical change. Dissolving isn’t as easy to figure out though as changing shape, size, and appearance because we can’t really see that the salt molecules haven’t changed. So unless you go through the effort of separating out salt from water, it wouldn’t easily be known. The effort to separate things comes into play for many things that we think are changing chemically. For example, when you add cherry Kool‐
Aid powder to water to make a yummy tasty drink, can you separate the Kool‐Aid back out from the water? If you’ve never tried this, the answer might surprise you that Kool‐Aid can be separated from the water meaning this is a physical change. So in the end, we have several basic examples we can use to help describe physical changes: 1. Any change in shape, size, or appearance. 2. When a substance dissolves and can be separated back out. 3. Any phase change (melting, freezing, evaporation, and condensation to name a few). By watching what happens during an experiment and then comparing our observations to the list above, we can easily figure out if something is a physical change. Guided Notes for Physical Changes Directions: Read and highlight the front. Then use the information to answer the questions below. Define Physical Property Name 4 different tests that we have used in science that are physical properties. 1. 2. 3. 4. What is the definition of a physical change we have been using in class? Define physical change using the term “physical property” in the definition. What happens to the molecules of a substance during a physical change? Why is adding a color dye to water only a physical change? What are three ways we can identify physical changes during a lab experiment? 1. 2. 3. Give two examples of a physical change other than those listed in the reading. 1. 2.