Chemical Reactions

Science, Grade 6


Table Of Contents: Chemical Reactions

1. Chemical and Physical Changes of Matter
A physical change involves a change of state or form of a substance while its chemical properties remain the same. Examples of physical changes include crumpling a piece of paper, melting an ice cube, and breaking a glass bottle. Changes in physical properties do not produce new substances and are only concerned with energy and states of matter. On the other hand, a chemical change takes place at the molecular level and occurs when one or more substances are changed into new substances with different properties. Examples of chemical changes include burning a piece of paper into ashes, cooking an egg, rusting of iron, and mixing an acid with a base to produce water and salt.
2. Chemical Reactions
Chemical reactions occur every day all around you, and even within you. Chemical reactions occur in plants during photosynthesis and in the engines of automobiles. A chemical reaction is a series of chemical changes in which one or more substances are converted to one or more different substances. Substances are either chemical elements or compounds. An original substance that is involved in a chemical reaction is called a reactant, while the substance(s) produced is called a product. The electron configuration of atoms plays an important role in how elements interact with each other and form chemical bonds. The ease with which an atom will form chemical bonds determines its ability to undergo chemical reactions.
3. Evidence of Chemical Reactions
During a chemical reaction, atoms can combine to form molecules, molecules can break apart to form atoms, or molecules can react with other molecules to form new substances. Chemists look for specific evidence to determine if a chemical reaction has taken place. This evidence can include the formation of gas, color change, formation of a precipitate, change in temperature, or the emission of light. If one or more of these changes are observed and a new substance has formed, then a chemical reaction has occurred.
4. Chemical Bonds
A chemical bond is an attraction between atoms that holds them together in a molecule. During a chemical reaction, new substances are produced as the original bonds of the reactants are broken. Atoms are then rearranged and new bonds are formed between the atoms of the products. For example, what happens when hydrogen and chlorine molecules are combined? The bonds that hold the chlorine and hydrogen atoms together break, and then a new bond is formed between the chlorine and hydrogen atoms.
5. Chemical Formulas
Chemical formulas are a shorthand way to represent the types of atoms and their numbers in an element or a compound. The atoms of each element are represented by one or two letters. When more than one atom of a specific element is found in a molecule, a subscript is used to indicate this in the chemical formula. For example, the chemical formula for water, H₂O, represents that two atoms of hydrogen have joined together with one atom of oxygen to form a molecule of water.
6. Chemical Equations
A chemical reaction can be expressed as a chemical equation. A chemical equation is a statement of a chemical change using symbols. Consider the reaction between carbon and oxygen gas to form carbon dioxide. The starting substances, known as reactants, are carbon and oxygen gas. The final substance formed, known as a product, is carbon dioxide. To write the chemical equation for a reaction, first you write the symbols for the reactants, then an arrow, followed by the symbols for the products. The “plus” (+) sign separates the reactants or products from one another. The arrow indicates the direction of the reaction or “to yield.”