Warren G. Harding High School chemistry students recently performed several experiments to determine whether changes in the composition of an original substance were physical or chemical.
In chemistry, the term change can refer to both physical and chemical changes. In the simplest sense, a physical change is a change in the form of the original substance. A chemical change is a change in the composition of the original substance. A chemical change is also called a chemical reaction.
Chemists have developed a list of common signs that may indicate the occurrence of a chemical change. These include:
- Bubbles of gas appear
- A precipitate (solid) forms
- An unexpected color change occurs
- Gain or release of energy (heat or light)
- A change in volume occurs
- A change in electrical conductivity occurs
- A change in melting point or boiling point occurs
- A change in odor or taste occurs
- A change in a distinctive chemical or physical property occurs; not easily reversed
Physical changes occur when objects undergo a change that does not change their chemical nature. A physical change involves a change in physical properties. Physical properties can be observed without changing the type of matter.
Examples of physical properties include: texture, shape, size, color, odor, volume, mass, weight, and density.
Change of state (ie. solid to liquid):
- Creation or separation of a mixture (including homogeneous mixtures, where the solute may not be visible)
- Physical deformation (cutting, denting, stretching, etc.)
- Physical relocation (moving an object)
Some examples of physical and chemical changes:
- If a piece of paper is cut up into small pieces, it is still paper. If you add water to a piece of string a chemical or physical reaction will not take place.
- Mixing sugar with water to dissolve sugar in the water. However, if one baked a cake with flour, water, sugar, and other ingredients, new substances would appear. Chemical reactions occur in the baking process, and the changes are chemical changes.
The students were using this information and performing several different experiments to determine whether they were physical or chemical changes.
