Evidence of Chemical Reactions

Which of the following examples shows that a chemical reaction has occurred? 

• A rock breaks into smaller pieces when it is struck with a hammer.

• Rust forms on a hammer that has been left outside to long. 

• A cup of water turns pink when a few drops of red food coloring are added. 

• A solid is formed when heat is removed from a sample of water. 

Answer explanation

Rust forming on a hammer indicates a chemical reaction, as it involves the transformation of iron into iron oxide due to exposure to moisture and oxygen. The other options describe physical changes, not chemical reactions.

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NGSS.MS-PS1-2

A student places a piece of a potato into a container of hydrogen peroxide. One observation the student makes is that bubbles form around the potato. The presence of bubbles might indicate the formation of a new substance because bubbles - 

• could show that a gas is forming

• always rise to the top of a liquid. 

• are almost always perfectly spherical.

• usually burst after a few minutes.

Answer explanation

The formation of bubbles around the potato indicates that a gas is likely being produced as a result of a chemical reaction between the potato enzymes and hydrogen peroxide. Thus, bubbles could show that a gas is forming.

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NGSS.MS-PS1-2

Four students performed four different investigations by combining different substances. Their results are shown in the table provided. In which experiment are new substances being formed?

• Experiment 1

• Experiment 2

• Experiment 3

• Experiment 4

Answer explanation

In Experiment 1, the combination of substances resulted in a chemical reaction, indicating that new substances were formed. The other experiments did not show evidence of new substance formation.

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NGSS.MS-PS1-2

Hydrogen peroxide breaks down to form water and oxygen gas. Which observation is evidence that a chemical reaction has occurred?

• The mass of the solution remains the same.

•  Phase changes are observed.

•  The color of the solution remains clear.

• The temperature of the solution increases. 

Answer explanation

The increase in temperature indicates that a chemical reaction has occurred, as energy is released during the breakdown of hydrogen peroxide into water and oxygen gas. The other options do not provide evidence of a chemical change.

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NGSS.MS-PS1-2

A clear liquid from a flask is poured into another clear liquid in a beaker. Which of the following results of this procedure would indicate a new substance was formed?

• The level of the substance in the beaker is higher after the other liquid is added.

• A yellow solid forms and then settles to the bottom of the beaker.

• Small white crystals are left behind in the flask that held the first liquid.

Answer explanation

A yellow solid forming and settling indicates a chemical reaction occurred, producing a new substance. The other options do not demonstrate the formation of a new substance.

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NGSS.MS-PS1-2

Which of the following is an example that includes evidence of a chemical reaction?

• A sample of zinc is placed in water and it settles to the bottom

• A solid block of ice is heated and it completely melts into a liquid. 

• Sugar that is burned gives off an odor and turns brown and then black.

• A tomato is placed into a blender and chopped up into smaller pieces.

Answer explanation

The correct choice is 'Sugar that is burned gives off an odor and turns brown and then black.' This indicates a chemical reaction, as new substances are formed and observable changes occur.

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NGSS.MS-PS1-2

In a laboratory experiment, a white powder is mixed with a red liquid, and the following observations are recorded.
How many of the observations suggest a chemical reaction has occurred?

• 1

• 2

• 3

• 4

Answer explanation

Three observations indicate a new substance was formed: a color change, the formation of a precipitate, and a temperature change. These are signs of a chemical reaction, confirming the creation of a new substance.

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NGSS.MS-PS1-2

Evidence of a chemical reactions is anything that shows - 

• a new substance has formed

• a solid dissolved in a liquid

• interaction between 2 gases

• a change in the state of matter

Answer explanation

Evidence of a chemical reaction indicates that a new substance has formed, which is the hallmark of such reactions. The other options describe physical changes or interactions, not the formation of new substances.

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NGSS.MS-PS1-2

A precipitate is a 

• liquid that forms when two solids are reacted

• a solid that forms when two liquids are mixed

• a gas that forms when vinegar and baking soda react

• a change of color in a solution

Answer explanation

A precipitate is defined as a solid that forms when two liquids are mixed, resulting in an insoluble product. This distinguishes it from other options, such as gases or color changes, which do not describe precipitate formation.

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NGSS.MS-PS1-2

If reaction starts with 20g of reactants it should produce 

• a total of 40g of products

• a total of 10g of products

• a total of 80 g of products

• a total of 20g of products

Answer explanation

According to the law of conservation of mass, the mass of reactants equals the mass of products. Therefore, if the reaction starts with 20g of reactants, it will produce a total of 20g of products.

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NGSS.MS-PS1-2

NGSS.MS-PS1-5