Changes in Chemical Reactions

A solid, yellow substance formed when Mr. Simon poured two colorless liquids together at room temperature. Which statement best explains why this demonstrates a chemical reaction?

• The color of the liquids changed to yellow.
• One of the liquids dissolved in the other liquid.
• One of the liquids changed from a liquid to a solid.
• The yellow substance is a solid at room temperature.

The following are examples of physical properties except ____.

• ability to dissolve in water
• ability to react with oxygen
• density
• shape

A characteristic of matter that allows it to change to something new is a ____.

• chemical change
• chemical property
• physical change
• physical property

Ms. Pitt set up a classroom demonstration to show what happens when a sample of zinc metal is added to a flask containing hydrochloric acid (HCl). The diagram shows what the students in the classroom observed during the demonstration. The students made two other observations: The flask felt warmer after adding the sample. The sample became smaller over time. This is evidence of

• a physical change and an exothermic reaction
• a chemical change and an exothermic reaction
• a physical change and an endothermic reaction
• a chemical change and an endothermic reaction

A physical property is a characteristic of matter that can be observed or measured without changing the identity of the matter. Why is iron’s tendency to rust not a physical property?

• A new substance with new properties forms when iron rusts.
• Iron is magnetic.
• It cannot be observed.
• The identity of iron remains unchanged when it rusts.

Which statement explains the law of conservation of mass?

• Every reaction creates an equal amount of mass related to the amount of energy required for the reaction.
• Substances cannot be created or destroyed in a reaction.
• The total amount of mass is equal to the volume of both chemicals in the reaction.
• The total mass before a chemical reaction is the same as the total mass after the reaction.

When 5 g of solid zinc is placed into a beaker that holds 15 g of hydrochloric acid solution, bubbles of hydrogen form. The mass of the contents of the beaker after the reaction is less than 20 g. Which best explains the difference in mass?

• The solid zinc reacted, so some of its mass was lost during the reaction.
• The solid zinc did not react completely, so not all of the atoms of reactants could become part of the products.
• It is expected because some atoms are used up in each chemical reaction.
• The lower mass is a result of the hydrogen gas leaving the beaker.

Miss White did two demonstrations for her class. The students summarized the demonstrations and their observations in a table, as shown. Which statement best describes the two demonstrations?

• Both demonstrations showed chemical changes, because a gas was formed in each.
• Both demonstrations showed physical changes, because a change of state occurred in each.
• Demonstration 1 showed a chemical change because a new gaseous substance was formed, and demonstration 2 showed a physical change because liquid water became water vapor.
• Demonstration 1 showed a physical change because a new gaseous substance was formed, and demonstration 2 showed a chemical change because liquid

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