Question

example problems

1. a student mixes vinegar with baking powder, and bubbles quickly appear.

which statement best explains what happened?
a the substances only mixed physically.
b a new substance formed because a gas was released.
c the vinegar turned into a solid substance.
d only the temperature changed, so it was physical.

1. a student stirs sugar into iced tea. which two statements describe this change?

a the sugar dissolves but remains the same substance.
b a new substance forms because the tea changes flavor.
c this is a physical change since the sugar could be recovered.
d a gas forms, showing a chemical change.
e the sugar completely disappears, showing it became new matter.

1. which evidence best supports that a chemical change occurred?

a the mixture became cloudy but stayed the same temperature.
b heat was given off during the reaction.
c the mixture was hard to stir.
d the substance could be filtered back out.

1. the table shows results from an investigation.

trial materials combined observations
1 oil + water oil floated on top
2 yeast + sugar water bubbles formed and odor appeared
3 sand + salt both settled at the bottom
which trial most likely produced a new substance?
a trial 1
b trial 2
c trial 3
d all three

Explanation:

1. When vinegar reacts with baking - powder, a chemical reaction occurs producing carbon - dioxide gas, which indicates the formation of a new substance. Option B is correct as gas evolution is a sign of a chemical change.
2. When sugar is stirred into iced tea, it dissolves. Dissolving is a physical change as the sugar remains sugar and can be recovered. Options A and C are correct.
3. Heat being given off during a reaction is a strong indicator of a chemical change. Chemical reactions often involve energy changes. Option B is correct.
4. In trial 2, yeast reacting with sugar in water causes bubbles (gas formation) and an odor, which are signs of a chemical reaction and the production of a new substance. Option B is correct.

Answer:

1. B. A new substance formed because a gas was released
2. A. The sugar dissolves but remains the same substance; C. This is a physical change since the sugar could be recovered
3. B. Heat was given off during the reaction
4. B. Trial 2