A student collecting CaCO3 produced by the reaction of Na2CO3(aq) and CaCl2(aq) obtains a percent yield of 81%. Choose all of the following observations that could explain the low yield. a. The combined reactants were not stirred before filtering the precipitate.
b. The student did not completely dry the precipitate before weighing it.
c. The precipitate was not washed prior to drying.
d. A rubber policeman was not used to scrape precipitate from the beaker.
e. The filter paper was not wetted with water prior to filtering the precipitate.

a, d and e. are true.

Explanation:

The reaction that occurs is:

Na2CO3(aq) + CaCl2(aq) → CaCO3(s) + 2NaCl

In ideal conditions, the percent yield of the reaction must be 100%. All explanations about why the student could not collect all precipitate are right:

a. The combined reactants were not stirred before filtering the precipitate. Not stirring could not promote all the reaction. TRUE.

b. The student did not completely dry the precipitate before weighing it. If the student don't dry the precipitate, the mass of precipitate must be higher producing a percent yield > 100%. FALSE.

c. The precipitate was not washed prior to drying. Produce more mass. FALSE.

d. A rubber policeman was not used to scrape precipitate from the beaker. If the student doesn't collect all the precipitate the percent yield could be < 100%.. TRUE.

e. The filter paper was not wetted with water prior to filtering the precipitate. TRUE. If you don't wet the filter paper you can lose a part of precipitate from the walls of this one.

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.

it means there are 3 atoms of oxygen