September 15 - Physical Changes vs. Chemical Changes

Today we learned all about physical and chemical changes!

The lesson started with a quick review of the homework, which included identifying types of reactions, balancing chemical equations, and utilizing phase symbols. You can read all about these fun topics in our previous blog entry from September 13th (scroll down)! We also completed a little more balancing practice. 

Shortly after, we started a discussion about the ways in which matter can change. Despite what the title of this post may lead you to think, there are actually more that two types of changes. In fact, there are three categories that nearly all changes can be broken down into:

• Physical changes
• Chemical changes
• Nuclear changes

Physical changes, the first of the three types, involves only a change in state or shape. For example, crumpling paper changes its shape but does not change its chemical nature. Boiling water and smashing cars are two other examples. This fine gentleman also demonstrates a physical change:

Contrasting physical changes are chemical changes. In a chemical change, new substances are often formed. Depending on the specific reaction, certain chemical properties may change, such as the conductivity or acidity. Here's the chemical equation for the decomposition of nitrogen triiodide:

2 NI₃ (s) → N₂ (g) + 3 I₂ (g) + energy

As you've probably guessed, this exothermic reaction is a chemical change. Be careful, though! Some changes can be deceiving. For example, dissolving table salt in water involves the breaking of chemical bonds but is often described as being a physical change. If you remove the water through evaporation, you end up with your original substance; sodium chloride! Some examples of chemical change include the burning of wood, the digestion of food, and the formation of rust on iron. In class, Mr. Doktor demonstrated the chemical change that occurs between lead (II) nitrate and potassium iodide. Two transparent solutions reacted to form a precipitate (lead (II) iodide) and a salt (potassium nitrate). Below you can find the balanced chemical equation and a video of the reaction.

Pb(NO₃)₂ (aq) + 2KI (aq) → PbI₂ (s) + 2KNO₃ (aq)

Often times, a change in state (like one from solid to gas) is confused as a chemical change. However, the chemicals involved will not change. When a change/transition between states occurs, it can be called a phase change. This graph, showing the relationship between heat and temperature, demonstrates the peculiar phase changes of H₂O:

 Here's another nifty diagram to help you learn about phase changes:

Posted by Michael.