Science 8 Properties of Matter | TEKS 8.6E, 7.6BC, 6.6C

An equation for a chemical reaction is shown. NaOH + HCl → NaCl + H₂O Which statement provides evidence that mass is conserved in the reaction?

• Two new substances are formed in the products.

• Water is one of the products, but not one of the reactants.

• Two hydrogen atoms are in the reactants, and two hydrogen atoms are in the products.

• One chlorine atom pairs with one sodium atom in the products, and one chlorine atom pairs with one hydrogen atom in the reactants.

Answer explanation

Choice C shows that the number of hydrogen atoms is the same in both reactants and products, demonstrating that mass is conserved in the reaction. This balance of atoms confirms the law of conservation of mass.

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NGSS.MS-PS1-5

A chemical equation for the reaction between calcium hydroxide (CaOH)2 and sulfuric acid H2SO4 is shown. The masses of some of the reactants and products are given in grams.

How many grams of sulfuric acid were used in the reaction?

• 62 g

• 98 g

• 36 g

• 100 g

Answer explanation

In the reaction between calcium hydroxide and sulfuric acid, the balanced equation shows that 1 mole of H2SO4 reacts with 1 mole of Ca(OH)2. The molar mass of H2SO4 is 98 g, confirming that 98 g of sulfuric acid were used.

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NGSS.MS-PS1-2

NGSS.MS-PS1-5

Which statement describes an action that most likely resulted in a chemical reaction?

• A pan of water was heated on a stove, and steam formed.

• A liquid was mixed with another liquid, and a precipitate formed.

• A liquid was sprayed into the air, and a sweet odor was smelled.

• A child blew through a wand filled with soap, and bubbles were produced.

Answer explanation

Choice B describes a chemical reaction because the formation of a precipitate indicates that new substances have been created from the mixing of two liquids, which is a hallmark of a chemical change.

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NGSS.MS-PS1-2

NGSS.MS-PS1-5

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.

Answer explanation

A solid forming when two clear solutions are mixed indicates a chemical reaction, known as a precipitation reaction. The other options describe physical changes, not chemical reactions.

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NGSS.MS-PS1-2

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

• The steel wool changed shape during step 6.

Answer explanation

The increase in temperature of the vinegar during step 4 indicates a chemical reaction, as it shows that energy is being released or absorbed, which is a hallmark of chemical changes.

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NGSS.MS-PS1-2

NGSS.MS-PS1-5

Students mixed two liquids in a beaker and listed their observations. Based on these observations, which statement contains the best evidence that a chemical reaction occurred?

• There is a change in shape.

• There is a change in volume.

• The two liquids mix into a solution.

• The two liquids form a new substance.

Answer explanation

The best evidence of a chemical reaction is when the two liquids form a new substance, indicating that a chemical change has occurred, unlike mere physical changes like shape or volume.

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NGSS.MS-PS1-2

• The mass increases.

• The mass decreases.

• The mass remains constant.

• The mass first decreases and then increases.

Answer explanation

According to the law of conservation of mass, the total mass of reactants equals the total mass of products. Since no mass is lost or gained during the reaction, the mass remains constant.

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NGSS.MS-PS1-2

NGSS.MS-PS1-5

Which statement best describes evidence that a chemical reaction occurs as a cake bakes?

• The water in the cake batter evaporates.

• The cake rises as gas bubbles form in the baking cake.

• The cake takes the shape of the container in which it is baked.

• The water in the cake batter is used to keep the cake moist as it bakes.

Answer explanation

The statement about the cake rising due to gas bubbles forming indicates a chemical reaction, as it shows the production of gas and a change in the cake's structure, unlike the other options which describe physical changes.

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NGSS.MS-PS1-2

The chemical formula for methane is CH₄. Which statement describes the atoms present in FOUR molecules of methane?

• Four molecules of methane would contain 1 atom of carbon and 4 atoms of hydrogen.

• Four molecules of methane would contain 4 atoms of carbon and 1 atom of hydrogen.

• Four molecules of methane would contain 4 atoms of carbon and 16 atoms of hydrogen.

• Four molecules of methane would contain 16 atoms of carbon and 16 atoms of hydrogen.

Answer explanation

Each molecule of methane (CH₄) contains 1 carbon atom and 4 hydrogen atoms. Therefore, four molecules contain 4 carbon atoms (1x4) and 16 hydrogen atoms (4x4). Thus, the correct statement is that they contain 4 carbon and 16 hydrogen atoms.

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NGSS.MS-PS1-1

Sodium sulfate (Na2SO4) is used to produce many products. Which elements are represented in the formula AND how many atoms of each element are represented in the formula?

• A) Sodium (Na) - 2, Sulfur (S) - 1, Oxygen (O) - 4

• B) Sodium (Na) - 1, Sulfur (S) - 1, Oxygen (O) - 4

• C) Sodium (Na) - 2, Sulfur (S) - 2, Oxygen (O) - 4

• D) Sodium (Na) - 2, Sulfur (S) - 1, Oxygen (O) - 2

Answer explanation

In the formula Na2SO4, there are 2 sodium (Na) atoms, 1 sulfur (S) atom, and 4 oxygen (O) atoms. Therefore, the correct choice is A) Sodium (Na) - 2, Sulfur (S) - 1, Oxygen (O) - 4.

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NGSS.MS-PS1-1