Pdf fillable | Chemistry homework help
- Give one example of a physical change that you encounter in your everyday life. NOTE: You may NOT use any of the examples listed in question #2 above!
Physical and Chemical Changes Virtual Lab
INTRODUCTION AND THEORY:
Matter and Its Physical and Chemical Changes Matter is defined as any material that has mass and occupies space. If we find that a sample of matter can be divided into more than one component using some means of physical separation, then we know our sample of matter must be a mixture. If we are unable to separate our sample of matter into different components, then it must contain only one type of material referred to as a pure substance. Thus, mixtures are made up of two or more pure substances. There are many different ways to physically separate a mixture. Each separation method is based upon some physical change. Examples of physical changes are: An example of a physical separation method is distillation, which separates pure substances based upon their differences in boiling point. Ethanol can be separated from a fermentation mixture (containing water, other organic chemicals, and non-volatile debris) using distillation. When you brew coffee, you are using gravity filtration to separate the soluble chemicals in the ground coffee beans from the insoluble grounds left in the filter paper. The means of separation here is the physical property of solubility in hot water. The key concept here is that physical separations, distillation, gravity filtration, etc., do not alter the chemical identity of any chemicals. The products after separation are the same chemicals that were initially used to make the mixture. This is very different from chemical changes (or “chemical reactions”) which result in formation of at least one new chemical substance. During a chemical change, some chemical bonds of the starting material(s) are broken, and some new chemical bonds are formed. At least one of the original chemicals must be used up (consumed), and at least one new chemical substance will be formed (produced). When two materials are added together, they may or may not undergo a chemical change. If a chemical change occurs, there often (but not always!) will be some sort of obvious evidence that a chemical reaction has taken place. Some examples of indications of a chemical change are: When you are asked to describe a material and its appearance before an experiment, you should include as much information as possible. You should describe its physical state (i.e., solid, liquid, or gas) and color. In addition, you might want to include the material’s opacity if it’s a solution/liquid (whether it is clear or cloudy), a description of its shape or texture if it’s a solid, a temperature estimate (is it warm or cool?), and whether it is uniform in appearance or has distinct regions. For example, if you were describing a chocolate chip cookie, you would observe that it was an approximately round-shaped solid with one flat side, composed of mostly tan-colored grainy material, and includes distinct smooth dark brown regions. If the cookie was fresh from the oven, you could note that it was warm to the touch, but cooling as it sat on the counter/cooling rack. Heterogeneous and Homogeneous Mixtures If no chemical reaction occurs when we combine two or more materials, then the result will be some sort of mixture. There are two different types of mixtures: homogeneous and heterogeneous. A homogeneous mixture is one in which the material is visually uniform in appearance throughout and has uniform chemical composition. An example would be if you dissolved some sugar in water, forming a uniform, clear and colorless solution of sugar water. Once the sugar has completely dissolved and dissipated, any sample of the mixture which you remove would have the same appearance; a clear, colorless solution. The mixture would also show the same chemical composition; the same proportions of sugar and water would be found in each sample you remove. Note that the process of dissolving the sugar in water does not break or form any new chemical bonds. We could physically separate this mixture by allowing the water to evaporate, and the sugar would be recovered intact as the non-volatile residue. In contrast, a heterogeneous mixture has distinct regions with (usually) different appearance and (always) different chemical composition. For instance, oil and vinegar salad dressing separates into two distinct layers – a layer containing the oil floating on top of the bottom layer containing the vinegar. If the oil and vinegar are similar in color, then the two layers may have similar “appearance”, but they clearly are chemically distinct!