Competency #7

Answer and Explanation

There are several observations that you could make. In no particular order these include:

  1. A gas product is made. This is either observed as bubbles or “smoke”
  2. A solid product is made. In aqueous reactions, this product is called a precipitate.
  3. There is a color change. This can be colorless compounds becoming colored, color disappearing, or color changing from one to another.
  4. There is a temperature change. If a chemical reaction produces heat it is called exothermic (and you feel heat coming off the reaction.) If a chemical reaction uses up heat it is called endothermic (and you feel the surroundings getting colder).
  5. There is a change in the pH. pH is a measure of a chemicals acidity, and changes in this level mean that an acid/base reaction is taking place.

Not all changes that matter undergoes are chemical reactions. In particular, phase changes are not considered chemical reactions. Phase changes are processes like a liquid becoming a gas, or a liquid becoming a solid. When water freezes to ice, this is physical change not a chemical reaction because the chemical formula for the product and reactant are the same.

Thus, although it is acceptable to write H2O (l) H2O (g) this is not a chemical reaction. Rather it is a description of a physical change.

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