Conservation of Mass in Chemical Reactions

Which statement provides evidence that mass is conserved in the reaction NaOH + HCl → NaCl + H₂O?

• A) Two new substances are formed in the products.

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

• C) There are two hydrogen atoms in the reactants and two hydrogen atoms in the products.

• D) A chlorine atom pairs with a sodium atom in the products, and a chlorine atom pairs with a 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 principle of conservation of mass.

In the chemical reaction NaOH + HCl → NaCl + H₂O, the number of hydrogen atoms in the reactants is ____.

• one

• two

• three

• four

Answer explanation

In the reaction NaOH + HCl, NaOH has one hydrogen atom and HCl has one hydrogen atom. Therefore, the total number of hydrogen atoms in the reactants is 1 + 1 = 2.

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

• 62 g

• 98 g

• 36 g

• 100 g

Answer explanation

The correct amount of sulfuric acid used in the reaction is 98 g, which is based on the stoichiometry of the reaction and the molar mass of sulfuric acid (H2SO4).

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

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

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

• C) A liquid was sprayed into the air and a sweet smell was perceived.

• D) 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.

Fill in the blank: In a chemical reaction, the mass of the reactants is _______ the mass of the products.

• A) greater than

• B) less than

• C) equal to

• D) not related to

Answer explanation

In a chemical reaction, according to the law of conservation of mass, the mass of the reactants is equal to the mass of the products. Therefore, the correct answer is C) equal to.

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

• Vapor forms over boiling water.

• A solid forms when a clear solution freezes.

• A solid forms when two clear solutions are mixed.

• Sugar crystals form on the sides of a pot boiling with 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 do not demonstrate a chemical change.

A solid is formed when two clear solutions are mixed. This is an indication that a chemical reaction has occurred. Fill in the blank: A solid that forms from two clear solutions is an example of a ____ chemical.

• reaction

• solution

• mixture

• evaporation

Answer explanation

A solid forming from two clear solutions indicates a chemical reaction, as new substances are created. Therefore, the correct term to fill in the blank is 'reaction', highlighting the occurrence of a chemical change.

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 suggests that energy is being released or absorbed, which is a hallmark of chemical changes.

Fill in the blank: During the laboratory investigation, after adding a piece of steel wool to the beaker, the next step is ____.

• Pour 150 milliliters of vinegar into a beaker.

• Place a thermometer in the vinegar and record measurements.

• Observe and record thermometer measurements.

• Remove the steel wool from the beaker.

Answer explanation

After adding steel wool, the next logical step is to observe and record thermometer measurements to monitor any temperature changes resulting from the reaction with vinegar.

Based on the observations of mixing two liquids, 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 transformation has occurred at the molecular level, unlike mere physical changes such as shape or volume.