For example, take a ball of metal, roughly the size of a baseball, compressed from raw metal. Compare this to a baseball made of paper. The baseball made of metal has a much greater weight to it in the same amount of volume. Therefore the baseball made out of metal has a much higher density. The density of an object will also determine whether it will sink or float in a particular chemical.
Any substance with a density lower than that will float, while any substance with a density above that will sink. Viscosity is defined to be the resistance to deformation of a particular chemical substance when a force is applied to it. In the example below, one can see two cubes falling into two different test tubes. The upper substance shows a violent reaction to the dropping of the cube.
The lower substance simply engulfs it slowly without much reaction. The upper substance has a lower viscosity relative to the lower substance, which has very high viscosity. One may even think of viscosity in terms of thickness.
The substance with more thickness has higher viscosity than a substance that is deemed "thin. The follow are all indicators of chemical reactions. For further information on chemical reactions, please refer to Chemical Reactions. A change in temperature is characteristic of a chemical change.
During an experiment, one could dip a thermometer into a beaker or Erlenmeyer Flask to verify a temperature change. If temperature increases, as it does in most reactions, a chemical change is likely to be occurring.
This is different from the physical temperature change. During a physical temperature change, one substance, such as water is being heated. However, in this case, one compound is mixed in with another, and these reactants produce a product. When the reactants are mixed, the temperature change caused by the reaction is an indicator of a chemical change. However, physical changes can be exothermic or endothermic. The melting of an ice cube, which is endothermic, is a change in a physical property and not composition.
Thus, it is a physical change. A change in color is also another characteristic of a chemical reaction taking place. For example, if one were to observe the rusting of metal over time, one would realized that the metal has changed color and turned orange. This change in color is evidence of a chemical reaction. However, one must be careful; sometimes a change in color is simply the mixing of two colors, but no real change in the composition of the substances in question.
When two or more compounds or elements are mixed and a scent or odor is present, a chemical reaction has taken place. For example, when an egg begins to smell, a rotten egg a chemical reaction has taken place. This is the result of a chemical decomposition. The formation of a precipitate may be one of the most common signs of a chemical reaction taking place. A precipitate is defined to be a solid that forms inside of a solution or another solid. Precipitates should not be confused with suspensions, which are solutions that are homogeneous fluids with particles floating about in them.
For instance, when a soluble carbonate reacts with Barium, a Barium Carbonate precipitate can be observed. For further information, please refer to Classification of Matter. The formation of bubbles , or rather a gas , is another indicator of a chemical reaction taking place. When bubbles form, a temperature change could also be taking place. Temperature change and formation of bubbles often occur together.
For example, in the following image, one can see a gas spewing. This is the formation of a gas. However, most reactions are much more subtle. For instance, if the following reaction occurs, one may notice Carbon Dioxide bubbles forming.
If there is enough Hydrochloric Acid, bubbles are visible. If there isn't, one can't readily notice the change:. The viscosity image is also free to be duplicated as per permission of author on Wikipedia. This is one of the gray areas of chemical change and physical change. Although the salt has dissociated into Sodium and Chloride ions, it is still salt in water. Salt, initially is actually just a conglomerate of sodium and chloride ions and by dissociating them, just the arrangement of the ions has changed.
Print Share: Facebook Twitter Email. Students are introduced to the concepts of chemical change and physical change in elementary school. These concepts are covered in greater detail as students progress through middle school and high school. A chemical change occurs when substances combine the reactants to form new substances the products as atoms are rearranged.
Common evidences of a chemical change include a change of color, odor, temperature, the formation of a gas, or a precipitate. A physical change occurs when there is a change in physical properties of a substance but not chemical compostion. Common physical changes include melting, change of size, volume, color, density, and crystal form. The classic baking soda and vinegar reaction provides evidence of a chemical change due to the formation of a gas and a temperature change.
Students tactually experience the formation of a gas as carbon dioxide fills up the balloon and sense a change of temperature.
As students complete this activity they will immediately notice the production of the gas. However, it may be necessary to ask the students to observe the temperature of the substance in the bottle after the reaction. It will be colder than before. Preparation: The instructor should gather the materials for this lab.
Prepare a station for each group of 2 students by placing the required materials in a cafeteria tray or bin. Prepare the lab procedure See attached. An unexpected color change or release of odor also often indicates a chemical change. For example, the color of the element chromium is determined by its oxidation state; a single chromium compound will only change color if it undergoes an oxidation or reduction reaction.
The heat from cooking an egg changes the interactions and shapes of the proteins in the egg white, thereby changing its molecular structure and converting the egg white from translucent to opaque. The best way to be completely certain whether a change is physical or chemical is to perform chemical analyses, such as mass spectroscopy, on the substance to determine its composition before and after a reaction.
Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:. Skip to main content. Introduction to Chemistry. Search for:. Physical and Chemical Changes to Matter. Learning Objective Identify the key features of physical and chemical changes.
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