Question

1. a student measured the mass of a candle before and after burning and found the mass to be lower after burning than before. how can this be best explained?

a. some of the candle wax was converted into light energy.
b. the law of conservation of mass does not apply to burning candles.
c. the masses were the same but the student did not measure accurately.
d. some of the products were gases which escaped and were not measured.

1. a scientist melted 25 grams of solid gold to form liquid gold. which statement best describes the mass of the liquid gold?

a. 25 grams because the liquid will take up the same volume as the solid.
b. 25 grams because the same number of atoms will be present in the liquid and solid.
c. slightly less than 25 grams because the density of the liquid will be less than the density of the solid.
d. slightly more than 25 grams because the kinetic energy of the atoms will be greater in the liquid than in the solid.

1. in a laboratory, selina added five drops of hydrochloric acid to a container with sodium bicarbonate (baking soda).

which observation helps her determine whether a chemical change has taken place?
a. formation of bubbles
b. formation of a mixture
c. change in the color of the sodium bicarbonate
d. change in the shape of the sodium bicarbonate

Explanation:

Question 10

• Option A: Mass is not converted into energy. Energy and mass are different concepts (mass - energy equivalence is a more advanced topic not relevant here).
• Option B: The law of conservation of mass applies to all chemical reactions. Burning a candle is a chemical reaction.
• Option C: There is no indication of measurement error. The mass difference is due to a chemical process.
• Option D: When a candle burns, it undergoes a chemical reaction. Some of the products (like carbon dioxide and water vapor) are gases that escape into the air and are not measured, which explains the mass decrease.

• Option A: Volume and mass are different physical quantities. Just because the volume may change (solid to liquid), it doesn't mean the mass is related to volume in this way.
• Option B: Melting is a physical change. The number of atoms (and thus the amount of matter) remains the same. Since mass is a measure of the amount of matter, the mass of liquid gold is 25 grams (same as solid gold).
• Option C: Density (\(

ho=\frac{m}{V}\)) change (due to volume change) doesn't affect the mass. Mass is conserved in physical changes.

• Option D: Kinetic energy is related to the motion of atoms (temperature - related in this context). It has no bearing on the mass of the substance.

• Option A: The formation of bubbles (gas evolution) is a strong indication of a chemical change. In the reaction \(NaHCO_{3}+HCl = NaCl + H_{2}O+CO_{2}\uparrow\), carbon dioxide gas is produced, which appears as bubbles.
• Option B: Formation of a mixture can be a physical process (e.g., mixing sand and water). It doesn't necessarily imply a chemical change.
• Option C: There is no color change for sodium bicarbonate in this reaction (\(NaHCO_{3}\) is white, and the products \(NaCl\) is also a color - less salt in solution).
• Option D: Change in shape can be a physical change (e.g., crushing a solid). It doesn't indicate a chemical reaction.

Answer:

D. Some of the products were gases which escaped and were not measured.

Question 11