Lesson 2
Home Up Safety Precautions Lab Techniques Observing Chemical Reactions Chemical Reactions Creating Materials in New Phases Chemical Reactions Creating Materials with New Colors Chemical Reactions Creating or Using Heat Energy Describing Chemical Reactions Distinction Between Chemical and Physical Changes Quick Quiz on Chemical Reactions

 

Chemical Reactions Creating Materials in New Phases

First Example

Reaction of silver nitrate with hydrochloric acid [1rx2AgCl.JPG]I'll add a few drops each of silver nitrate solution and hydrochloric acid solution to one another and see what happens. (Exercise 2.a.) Notice that as the two solutions come in contact you get a milky white color. This is called a precipitate. It is actually a solid. There are little tiny bits of solid particles that form as the two chemicals come in contact with one another. After a bit, you will be able to see that the solid material settles out to the bottom.

Second Example

Reaction of sodium bicarbonate solution with hydrochloric acid [1rx3CO2.JPG]Next we add a few drops each of sodium bicarbonate solution and hydrochloric acid solution. (Exercise 2.b.) Notice that bubbles are formed. If you don't see them when you mix the solutions together, try it again--little, tiny bubbles.

 

Third Example

Reaction of solid sodium bicarbonate with hydrochloric acid [1rx4CO2.JPG]Next we work with solid sodium bicarbonate and add some hydrochloric acid to it. (Exercise 2.c.) Again, notice the bubbles. Also, depending on how much of the solid sodium bicarbonate we use, it might all disappear. If we use quite a bit, then we would need more hydrochloric acid in order to have all of that disappear.

 

Comments

Notice that there is something common to all three of these reactions. In each case we ended up with a phase that wasn't there before. Actually, we ended up with a new material that was in a phase that wasn't there before. For example, the silver nitrate solution and the hydrochloric acid solutions are both liquids. Mixing them together forms a solid. There is still liquid there, but there is also a solid that wasn't there before. In the second and third examples, the new material was a gas. Specifically, it was carbon dioxide bubbles. So the new material is in a phase that wasn't there before, and consequently, we were able to observe its presence. On the other hand, if we were to mix two liquids together and get a new material that is also a liquid, we may not be able to see that. But if the new material is in a phase that is not already present, then we will be able to detect that it is being formed. Also, when you do Part C, you may note that the solid that you start with may disappear.

Either now or when you are in the lab, do Exercises 2-4 to see the reactions first hand. After you finish, be sure to clean up the test tubes or spot plate that you just used.