EXPERIMENT 2 – A STUDY OF CHEMICAL CHANGES
INTRODUCTION
Chemistry is a science that investigates changes in matter. Chemical reactions are the changes
chemicals undergo. The changes you can observe are called “macroscopic changes. Often these changes,
such as color changes, the formation of a solid, or the formation of gas bubbles, are visible. Thus, though we
cannot see the atoms and molecules reacting, we can see indications that chemical changes have taken place.
Different atoms and molecules often react in different ways. Chemistry attempts to explain
macroscopic changes in terms of the behavior of atoms and molecules, that is, on the submicroscopic level.
You can use these different reactions to detect the presence of specific kinds of chemicals in mixtures.
PROCEDURE
Place a clear plastic sheet (or plastic wrap) over this page. Use the eye-droppers to put 1 drop of each
chemical on the X’s in the indicated spaces. For background contrast, view the drops on the black and white
background provided by the X’s. Record the results for each combination.
a.
b.
c.
d.
e.
f.
g.
X
X
X
X
X
X
X
NaHCO3
+
HCl
h.
HCl
+
Blue dye
i.
Blue dye
+
NaOCl
Then add 1
drop of HCl.
j.
NaOCl
+
KI
Then add 1
drop of
starch.
k.
KI
+
Pb(NO3)2
l.
Pb(NO3)2
+
CaCl2
m.
CaCl2
+
NaHSO4
Be Patient!
Some
chemical
reactions
are slow!
n.
X
X
X
X
X
X
X
NaHSO4
+
Na2CO3
Na2CO3
+
Phenolphthalein
Phenolphthalein
+
NaOH
NaOH
+
AgNO3
AgNO3
+
NH3
NH3
+
CuSO4
CuSO4
+
NaHCO3
Absorb this mixture
onto a white scrap of
paper or tissue and
tape it to the wall.
Look at it tomorrow.
RESULTS:
Reaction
Reactants
a.
NaHCO3 + HCl
b.
HCl + blue dye
c.
blue dye + NaOCl
c.
blue dye + NaOCl + HCl
d.
NaOCl + KI
d.
NaOCl + KI + starch
e.
KI + Pb(NO3)2
f.
Pb(NO3)2 + CaCl2
g.
CaCl2 + NaHSO4
h.
NaHSO4 + Na2CO3
i.
Na2CO3 + phenolphthalein
j.
phenolphthalein + NaOH
k.
NaOH + AgNO3
l.
AgNO3 + NH3
m.
NH3 + CuSO4
n.
CuSO4 + NaHCO3
Observation
QUESTIONS
1. Sodium hydrogen carbonate (sodium bicarbonate) is baking soda, NaHCO3. When HCl is added to
NaHCO3, carbon dioxide bubbles are formed. (a) What is the chemical formula of carbon dioxide?
(b) In what consumer product is carbon dioxide commonly found?
2. Which of the other mixtures also resulted in the formation of gas bubbles?
3. The body uses hydrochloric acid to help digest food.
(a) Where in the body is hydrochloric acid found?
(b) What color does blue food dye turn when HCl is added?
4. Sodium hypochlorite, NaOCl, is a common ingredient in household bleaches and cleansers. What
happens to the color of the blue dye when both HCl and NaOCl are added?
5. Potassium iodide, KI, is the source of iodine in iodized salt.
(a) What color is the KI + NaOCl mixture?
(b) What color does starch change to in the presence of KI and NaOCl?
6. A precipitate is a solid that forms upon mixing solutions.
(a) Which reaction produced a very bright yellow precipitate?
(b) Which reaction produced a distinctive “muddy” brown precipitate?
7. Review your results and list three different kinds of changes you saw in the lab that indicate that a
chemical reaction is occurring.
8. As you performed the lab, you read the safety information on the bottles of solution. Which of the
bottles contained the most dangerous solution you were given to work with today? Explain your
answer using information from the labels.