PS-4.6 Distinguish between chemical changes (including the formation of gas or reactivity with acids) and physical changes (including changes in size, shape, color, and/or phase)
Chemical changes: gas formation, Acids, Physical changes: phase change
We already know how to recognize a physical change. Phase changes, such as melting, freezing, boiling, evaporation and condensation, do not produce new substances. These are physical changes. For example, melt Iron and you still have Iron, Boil water and you still have H2O, it's just in the gas phase. Tear a piece of paper into smaller pieces and you still have paper, it's a physical change!
But, how can you tell when a chemical change is taking place? There are a number of indicators. The production of a gas is one. When Zinc metal reacts chemically with aqueous Hydrochloric acid, the products of this acid / metal reaction are aqueous Zinc Chloride and Hydrogen gas. The Hydrogen gas produced in this reaction is given off as bubbles that bubble up to the surface. The production of a gas is a good indicator that a chemical change is taking place. The production of a gas during a chemical reaction is clearly the result of a chemical change, not a physical change as in boiling.
Zn + 2HCl --> ZnCl2 + H2
Chemical reactions can also be exothermic or endothermic, giving off or taking in energy in the form of heat. The chemical reaction between Zinc metal and Hydrochloric acid shown above is Obviously exothermic because it gives off heat. Combustion of any kind represents a chemical change because combustion reactions are exothermic, they give off heat energy and light.
Another way to determine if a change is a chemical change is to look at what you start with and then look at what you end up with after the change. In a chemical change atoms are rearranged to produce new substances. Note that this statement states that atoms are rearranged, NOT destroyed. All the atoms that were present before the reaction are there after the reaction. In this chemical equation, the reactants are Sodium metal and Oxygen. The product of this reaction is Sodium Oxide, a totally new substance with a totally new set of physical properties.
Look at this chemical equation: 4Na + O2 --> 2Na2O
Chemical reactions produce products with new properties. For instance, Sodium is a soft, shiny metal, that is less dense than water. Sodium is highly reactive, so reactive, in fact, that it is never found in its pure form in nature. Oxygen is a colorless, odorless gas at room temperature. Oxygen is reactive in the presence of nonmetals and is required to support combustion. The product of the reaction shown above is a relatively inert ( non-reactive) white solid with a high melting temperature; Sodium Oxide. In simple terms, this chemical reaction does just what all chemical reactions do. All chemical reactions rearrange atoms to produce new products with new chemical and physical properties.
Look at the properties of Sodium and Oxygen before they react chemically, and compare them to the Sodium Oxide produced in the reaction. The product, Sodium Oxide has a completely different set of properties than the reactants.
Summary: Things that indicate if a chemical change is taking place:
A. Does the change produce an evolution of a gas?
B. Does the change release heat or take in of heat? ( exothermic or endothermic)
C. Does the change produce a new substance in the form of a precipitate?
D. Does the change result in new substances with new chemical formulas and new properties?
E. Does the change produce new substances with a distinctive odor?