Chemical Reactions Study Questions

Which of the following is not a sign of a chemical reaction?

• gas formation

• solid formation

• energy change

• state(phase) change

• Option 5

Answer explanation

A state (phase) change, such as melting or boiling, does not indicate a chemical reaction. In contrast, gas formation, solid formation, and energy changes are clear signs of a chemical reaction.

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NGSS.MS-PS1-2

In a chemical reaction

• The atoms of the reactants always stay together to form the products.

• The atoms of the reactants unbond, rearrange, and then rebond to form the products.

• New atoms are formed which combine to make the products.

• Some atoms disappear while others multiply to form the products.

Answer explanation

In a chemical reaction, the correct process involves the atoms of the reactants unbonding, rearranging, and then rebonding to form the products. This explains how substances transform during reactions.

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NGSS.MS-PS1-5

One clue of a chemical change is the formation of a precipitate. A precipitate is formed when

• Two liquids react and a gas is produced.

• A solid dissolves in a liquid.

• One liquid dissolves in another.

• Two liquids react and a solid is formed.

Answer explanation

A precipitate forms when two liquids react and produce a solid. This indicates a chemical change, as seen in the correct choice: 'Two liquids react and a solid is formed.' Other options do not result in a precipitate.

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How can you tell that the electrolysis of water produces a chemical change?

• It is a reversible reaction.

• It changes the composition of matter.

• It produces a change of state.

• It requires heat to produce changes.

Answer explanation

The electrolysis of water is a chemical change because it alters the composition of matter, breaking water (H2O) into hydrogen (H2) and oxygen (O2) gases, which are different substances from the original compound.

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If a student dissolves 50 grams of sugar in water and then lets the water evaporate, how much sugar will be left?

• 45 grams because some of the sugar gets destroyed.

• 65 grams because the sugar crystals are larger.

• 50 grams because the mass of the sugar stays the same.

• None of the sugar would be left.

Answer explanation

The correct answer is 50 grams because the mass of the sugar remains unchanged during the evaporation process. Sugar does not get destroyed or lost; it simply changes state from solid to dissolved and back.

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Some chemical reactions require a substance called a catalyst, like sulfuric acid. The main purpose of a catalyst is

• to warm up the reaction.

• to speed up the reaction.

• to create more reactants.

• to stop the reaction.

Answer explanation

The main purpose of a catalyst, like sulfuric acid, is to speed up the reaction without being consumed in the process. It lowers the activation energy required, allowing the reaction to occur more quickly.

Jaleesa draws a model of a chemical reaction between two substances. She uses a black circle to represent one kind of atom and a white circle to represent another kind of atom. Which model shows that a chemical reaction takes place?

• Model A

• Model B

• Model C

• Model D

Answer explanation

Model A shows a clear transformation of the black and white circles, indicating that a chemical reaction has occurred. The other models do not demonstrate this change, making Model A the correct choice.

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In a chemical reaction, mass is conserved. This means that:

• The mass of the reactants stays the same during a chemical reaction.

• The mass of the products stays the same during a chemical reaction.

• The type and number of atoms in the reactants equals the type and number of atoms in the products.

• The mass of the products is always twice the mass of the reactants.

Answer explanation

In a chemical reaction, mass conservation means that the type and number of atoms in the reactants must equal those in the products, ensuring that mass remains constant throughout the reaction.

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Which of the following is a chemical reaction?

• New atoms are formed when substances are mixed together.

• A solid material melts and becomes a liquid.

• Atoms of the reactants rearrange to form a new substance.

• Dissolving a substance in water.

Answer explanation

The correct choice, 'Atoms of the reactants rearrange to form a new substance,' describes a chemical reaction where bonds are broken and formed, resulting in new substances. The other options describe physical changes, not chemical reactions.

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Adam combined vinegar and baking soda inside the flask and balloon. During the reaction, the baking soda was used up. Also, gas was created and trapped inside the balloon. Before the reaction, the flask, the balloon, and the reactants had a mass of 300 g. What statement could be made about the total mass of the flask’s contents after the reaction?

• The mass is greater than 300g.

• The mass is less than 300g.

• The mass equals 300g.

• The mass cannot be determined.

Answer explanation

According to the law of conservation of mass, the total mass of the reactants equals the total mass of the products. Since no mass was lost or gained, the mass after the reaction remains 300g.

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NGSS.MS-PS1-2