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Changes in Matter
Chemical Changes
Chemical changes occur as a result of a chemical reaction, transforming the chemical properties of a substance. When one set of chemicals (the reactants) is converted into another set of chemicals (the products), we call it a chemical reaction. Evidence that a chemical reaction has occurred may include a change in the physical properties of a substance, such as a color change or the formation of a new material. It may also be observed as a change in energy, such as an increase or decrease in temperature or the emission of light.
Influencing Chemical Reactions
A chemist can influence the rate of a chemical reaction by changing the temperature of the reactants. Increasing the temperature usually results in a faster reaction, while decreasing the temperature usually results in slowing down the reaction rate.
Conservation of Mass and Energy
It is important to note that when a chemical reaction occurs, the new substances are made of the same atoms as the original substances; the atoms have simply been rearranged. This concept is known as the law of conservation of mass, meaning that atoms are not created or destroyed during a chemical reaction. Similarly, the law of conservation of energy states that energy is conserved throughout any physical or chemical process, including chemical reactions. Even when thermal energy is released during a chemical reaction, that energy was not created during the process; it was present the whole time, stored in the chemical bonds of the original substances.
The carbon cycle demonstrates the law of conservation of mass and the law of conservation of energy beautifully. The products of photosynthesis are the reactants of cellular respiration, and the products of cellular respiration are the reactants of photosynthesis. In addition, energy that is used during photosynthesis to make glucose is the energy that is released as ATP during cellular respiration. Through this cycle, we can see that both mass and energy are conserved.
The law of conservation of mass can be demonstrated using chemical equations. A chemical equation shows the reactants (the chemicals that are present before a chemical reaction) on one side of the equation and the products (the chemicals that are present after the chemical reaction) on the other side of the equation. These chemicals are represented by chemical formulas, using element symbols with coefficients (large numbers shown before an element or compound) and subscripts (small numbers shown after an element). An example of a chemical equation is shown below, representing the chemical reaction involved in photosynthesis:
This equation shows that 6 molecules of carbon dioxide (6CO2) combine with 6 molecules of water (6H2O). With the energy provided by sunlight, plants can turn these chemicals into a glucose molecule, or sugar (C6H12O6) and 6 molecules of oxygen (6O2). It is important to note that the numbers of each element are equal on both sides of this equation. There are 6 carbon atoms present in the reactants on the left, and 6 atoms of carbon in the products on the right. Similarly, there are 12 oxygen atoms on both sides and 12 hydrogen atoms on both sides. Because no atoms were added or removed (you might say that no atoms were created or destroyed), this equation shows that photosynthesis follows the law of conservation of mass.
WHY DO CHEMICALS REACT?
Physical Changes
Though physical changes can be an indication that a chemical change has occurred, they should not be confused with chamical changes. A physical change is any change in the physical properties of a substance that occur without changing the chemical structure of the substance. In other words, the atoms that make up the substance remain bonded in the same way as they were before the change. An example of a physical change would be a piece of paper being folded or torn. This is considered a physical change because it is still “paper”; the molecules are unchanged. However, if the paper is burned, the chemical structure of the paper molecules changes when the paper turns to ash. So, burning the paper would be considered a chemical change.