8.5 E. Evidences of Chemical Change

Students performed a lab investigation on chemical reactions. The students put on goggles and plastic gloves and then followed these steps.

Which observation would indicate that a chemical reaction occurred?

• The vinegar took the shape of the container in step 1.

• The steel wool sank into the vinegar in step 3.

• The temperature of the vinegar increased during step 4.

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• The steel wool changed shape during step 6

Which of these observations is an indication that a chemical reaction has occurred?

• Steam forms above boiling water.

• A solid forms when a clear solution is frozen.

• A solid forms when two clear solutions are mixed.

• Sugar crystals form on the sides of a boiling pot of sugar water.

Some students in a chemistry lab conducted an investigation in which they added four different solid substances to separate beakers of water. They stirred the mixtures for one minute and then recorded their observations in the table below.

Which substance most likely caused a new substance to be formed when mixed with water?

• Substance 1

• Substance 2

• Substance 3

• Substance 4

A scientist performed four investigations using eight different liquids. In each investigation, the scientist combined two of the liquids under a fume hood and recorded observations in the table below.

In which investigation is it least likely that the liquids reacted chemically?

• 1

• 2

• 3

• 4

A student obtains two strips of magnesium, Mg, ribbon that are each 3 cm long. One strip of magnesium is placed in a test tube containing 5 mL of water, and the other strip is placed in a test tube containing 5 mL of hydrochloric acid, HCl. Both liquids are at room temperature. The student’s observations are recorded in the table.

Which statement is not supported by the student’s observations?

• A chemical reaction takes place between magnesium and hydrochloric acid.

• A gas is released in Test Tube 2.

• The substances in both test tubes are reactive only at high temperatures.

• Energy is released in the reaction involving hydrochloric acid.

A silver ring reacts with compounds containing sulfur in the air to form silver sulfide, a black substance that makes up the tarnish on the surface of silver objects. To remove the tarnish from the ring, students placed it in a pan lined with aluminum foil and added hot water. Baking soda was added to the hot water and stirred. Students made observations about the process.

Which observation of this process provides evidence of a chemical reaction?

• Hot water heated the aluminum foil.

• The liquid solution changed color.

• The pan was lined with aluminum foil.

• The hot water cooled.

A scientist spilled a few drops of dilute hydrochloric acid (HCl) on a lab table. For safety purposes, the scientist sprinkled some baking soda onto the spill.

Which observation would provide the best evidence that a chemical reaction occurred?

• The baking soda and hydrochloric acid combined, and bubbles formed.

• The baking soda absorbed the hydrochloric acid.

• Some of the baking soda dissolved in the hydrochloric acid.

• The hydrochloric acid evaporated, leaving only the baking soda.

During an investigation, a student combined the two clear, colorless solutions shown below.

The student observed that the temperature changed from 25°C to 23°C when the solutions were combined and that a white substance rapidly formed and settled to the bottom of the container. What most likely happened to produce these results?

• One of the original compounds came out of solution.

• Some of the water froze into ice crystals.

• The solutions reacted chemically.

• Rapid evaporation of water occurred, leaving a solid.

Coal contains carbon and other elements. Carbon dioxide forms when coal burns in the presence of oxygen. Which of these is the best evidence that a chemical reaction occurs when coal burns?

• The shape of the coal changes.

• Oxygen is present.

• A new substance is produced.

• Coal is made up of more than one element.

Nitrogen dioxide is a gas that can be generated by emissions from vehicles and factories. It can also be generated by natural sources, such as forest fires, lightning, and microbial activity in soil.

The equation for producing nitrogen dioxide is shown below.

Which of these provides evidence that a chemical reaction occurs?

• A gas is present.

• The total number of atoms remains unchanged.

• The state of matter remains the same.

• A brown gas is produced.