When substances react, the atoms involved in the
reactants are rearranged, forming other products. Students have
learned that the physical and chemical properties of the newly
formed substances (products) are different from the physical and
chemical properties of the original substances (reactants).
Students in grade eight will learn that
it is the underlying arrangement of the atoms in the
reactants and products and the energy needed or released during the
rearrangement process that explain chemical reactions. Understanding chemical reactions is essential
because they constitute, directly or indirectly, a large portion of
the discipline of chemistry.
Students need to be able to distinguish a
chemical change from a physical change. In a physical change one or
more physical properties of the material are altered, but the
chemical composition (i.e., the arrangement of the atoms in
molecules) remains the same. In a chemical change the atoms are
rearranged to form new substances with different chemical and
physical properties. Students must be familiar with the periodic
table and the names and symbols of the chemical elements.
In grade one students are prepared for the idea
of chemical reactions when they learn that the properties of
substances can change when they are mixed, cooled, or heated. In
grade three they learn that when two or more substances are
combined, a new substance may be formed with properties that are
different from those of the original materials. In grade five they
learn that during chemical reactions, the atoms in the reactants
rearrange to form products with different properties.
The study on reactions begun in grade eight will
support future studies about conservation of matter and
stoichiometry as well as work on acids, bases, and solutions.
Students will go beyond studying reactions and their
reactant/product relationships to work with the rates of reaction
and chemical equilibrium. Students should be able to envision a
chemical equation at the atomic and molecular levels. They should
“see” the number of reactant atoms and molecules in the equation
coming together and by some process rearranging into the correct
number of atoms and molecules that form the products. This important
conceptual skill helps students to keep track of all the atoms.
This standard focuses on changes that occur when
atoms and molecules as reactants form product compounds with
different chemical properties. Teachers may have students perform
simple reactions or demonstrate the reactions for students.
5.b The conservation of matter is a classical
concept, reinforcing the idea that atoms are the fundamental
building blocks of matter. Atoms do not appear or disappear in
traditional chemical reactions in which the constituent atoms and/or
polyatomic ions are simply rearranged into new and different
compounds. Conservation of atoms is fundamental to the idea of
balancing chemical equations. The total number of atoms of each
element in the reactants must equal the total number of atoms of
each element in the products. The total number of atoms, hence the
total mass, stays the same before and after the reaction.
There are several ways to teach and assess
students’ understanding of the concept of conservation of mass in
chemical reactions. Weighing reactants before and products after a
reaction shows that mass is neither gained nor lost. However,
experimental errors are possible; the most common one is not
sufficiently drying the products before weighing. One simple
demonstration of the concept that atoms (or matter) are conserved in
chemical reactions in which mass might appear to be lost is to
determine the combined mass of a small, sealed container filled
one-third with water, the screw-on cap, and one-quarter of an
effervescent tablet. After the piece of tablet is dropped in the
water, the container is immediately sealed. When the fizzing has
stopped, the combined mass of the sealed container and the tablet
should remain the same. After the seal is broken, much of the carbon
dioxide gas formed by the reaction escapes, and the mass of the
container and its contents decrease.
Students should also be taught to balance simple
chemical equations. This step reinforces the idea that atoms do not
appear or disappear in chemical reactions and, therefore, that
matter is conserved.