Answer:
is a reactant; it is present before the reaction occurs.
Explanation:
In a chemical reaction the chemical formulas written before the arrow are described as reactants as they react together to form products which are written after the arrow.

Thus
and HCl are reactants here whereas
,
and
are products.
Explanation:
It is often desirable to determine the mass percent of elements in a given compound.
To determine the mass percent of elements:
- Evaluate the formula mass of the compound. This is done by summing the atomic masses of the atoms in the compound together.
- The mass percentage is determined by pacing the mass contribution of each element or group to the formula mass of the compound and multiply by 100.
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Explanation:
impurities affect both melting point and boiling point
in the case of melting point it lowers the melting point
so I guess one of the samples Ie contained impurities