Chemical Reactions

Chemical equations show the ratio of reactants and products involved in a chemical reaction. For example, 2 H₂ + O₂ → 2 H₂O. How do balanced equations satisfy the Law of Conservation of Mass?

• The number of reactant molecules equals the number of product molecules.

• Every atom present in the reactants is also present in the products.

• The number of reactant molecules adds up to the number of product molecules

• The energy stored in bonds before and after the reaction is the same.

Keesha performed a chemical reaction and the products looked quite different from the reactants. She knew the amount of matter had not changed due to the law of conservation of mass. According to the law of conservation of mass, what happens in a chemical reaction?

• Matter is not created or destroyed and the same amount of mass remains.

• A new kind of matter is created and the amount of mass either increases or decreases.

• Matter is not created or destroyed and the amount of mass may change after the reaction.

• New matter is created and changes form so that the mass of the reactants and the products will almost be equal.

In a science lab, two students use a balance to measure the mass of a box containing a small crystal. The mass of the box is 12 grams (g). One of the students accidentally drops the box when removing it from the balance, and the crystal breaks into 20 pieces. What happens to the mass of the box and crystal after the crystal is broken?

• The mass remains the same.

• The mass cannot be determined.

• The mass increases by a factor of 20.

• The mass decreases by a factor of 20.

The picture shows an experiment in which a student mixed two clear liquids together.

The student can conclude that a chemical

change occurred because

• The temperature did not change during the experiment.

• A solid formed when the two liquids were mixed.

• The mass did not change during the experiment

• One substance was more dense than the other.

Mr. Carmen asked his class to observe the following events.

· steel wool rusting

· wooden splints burning down to ashes

· sandstone crumbling when tapped with a hammer

· an antacid tablet releasing carbon dioxide bubbles in water

Which of the events produced a physical change?

• steel wool rusting

• wooden splints burning down to ashes

• sandstone crumbling when tapped with a hammer

• an antacid tablet releasing carbon dioxide bubbles in water

Which process involves only a physical change?

• a nail rusting

• an egg cooking

• a candle burning

• an ice cube melting

Jesse takes a blue cup and fills it with 250 mL of water at 20 °C. He then fills three additional cups, using water at a different temperature for each cup, as shown in the table below.

TEMPERATURE OF WATER IN CUPS

Cup Color Water Temperature (°C)

Blue 20

Green 90

Orange 60

Red 5

Jesse then drops an antacid tablet into each of the cups and watches the water fizz. He notices that the water in each cup fizzes at a different rate.

Rank the cups of water according to the speeds at which they are most likely to fizz, from slowest to fastest.

• blue, green, orange, red

• green, orange, blue, red

• red, blue, orange, green

• Red, orange, blue, green

Read the following passage about Alex and look at the diagram.

Alex is decorating for a nighttime party with plastic glow sticks. The sticks are activated by forcefully bending them, mixing the chemicals inside the stick, and producing

bright light. Alex puts some of the sticks inside at room temperature, and some outside in the cold, snowy weather.

After the party, which glow sticks are most likely to still be glowing brightly?

• The glow sticks inside will be glowing brightly because chemical reactions are slower at high temperatures.

• The glow sticks inside will be glowing brightly because chemical reactions are faster at high temperatures.

• The glow sticks outside will be growing brightly because chemical reactions are faster at cold temperatures.

• The glow sticks outside will be growing brightly because chemical reactions are slower at cold temperatures.

Mario takes a cold can of soda out of a refrigerator. He sets the cold can on a table in the room and leaves it there for a few hours. Which of the following best describes what will occur?

• The cold from the soda can will reduce the temperature of the air in the room.

• Heat from the air will move into the soda can until both reach the same temperature.

• The soda in the can will change state and temperature as it releases cold molecules into the air.

• The soda can will absorb heat until it becomes warmer than the temperature of the air in the room.

A change of state may happen when energy as heat is transferred to or from a substance. Which change of state could occur as energy is added to a substance?

• gas --> solid

• liquid --> solid

• gas--> liquid

• liquid--> gas