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1) Keri has to identify a mysterious brown liquid in science class. She pours it through a filter, but the substance looks the same. Then she heats the substance and collects the water that evaporates. The water droplets look the same as the original substance. Keri concludes that since the substance cannot be separated it must be a compound. Is Keri correct?
A) No because compounds can be separated physically.
B) No because the substance could be an element.
C) Yes because water is a compound.
D) Yes because Keri tried to separate it and all mixtures can be separated physically.

2) Mrs. Smith wrote down a chemical equation on the board and stated that mass is conserved in the reaction. What did she mean?

A) There are equal numbers of atoms on both sides.
B) There are more atoms on the reaction side.
C) The number of atoms on each side changes.
D) There are more atoms on the product side

In this example, the same substance is present before and after, so this is the psysical change, just with different form or state of matter. Physical property can be observed and measured without any changes in molecular composition.

2) Answer is: A) There are equal numbers of atoms on both sides.

Conservation of mass (mass is never lost or gained in chemical reactions), during chemical reaction no particles are created or destroyed, the atoms are rearranged from the reactants to the products.

For example, balanced chemical reaction:

Pb(NO₃)₂(aq) + Li₂SO₄(aq) → PbSO₄(s) + 2LiNO₃(aq).

According to principle of mass conservation, number of atoms must be equal on both side of balanced chemical reaction.

There are two lithium atoms, one lead atom, one sulfur atom, ten oxygen atoms and two nitrogen atoms on both side of reaction.