Question
\(Na_2CO_3(aq)+2HCl(aq)→2NaCl(aq)+H_2O(l)+CO_2(g)\)
A student combined two colorless aqueous solutions. One of the solutions contained \(Na_2CO_3\)
as the solute, and the other contained HCl . The chemical reaction that took place is represented by the equation above. What experimental result would be evidence that a chemical reaction took place when the solutions were combined?
A Bubbles formed when the two solutions were combined.
B The total volume of the mixture is close to the sum of the initial volumes.
C The resulting solution is colorless.
D The resulting solution conducts electricity.
▶️Answer/Explanation
Ans: A
A gas \(CO_2\) was produced from the chemical reaction between \(Na_2CO_3\) and \(HCl\). Its production will be evident by the formation of bubbles as the reaction proceeds. The formation of a gas is possible evidence that a chemical change has occurred.
Question
A chemistry teacher carried out several demonstrations, and students recorded their observations. For one of the demonstrations, a student concluded that a physical change took place, but not a chemical change. Which of the following observations could the student have made of the results of the demonstration?
A Two colorless solutions were combined, and the resulting solution was pink.
B When a solid was added to a liquid, sparks were produced.
C One piece of solid substance was changed into small pieces.
D When two solutions were combined, a precipitate formed.
▶️Answer/Explanation
Ans: C
Changes in phase, size, and shape are examples of physical changes. There is no evidence of a substance being transformed into a new substance.
Question
A student was given two clear liquids; a colorless liquid and a dark-blue liquid. The student was asked to combine the liquids in a beaker and record observations. Which of the following results, if true, would provide the best evidence that a chemical change took place when the liquids were combined?
A The resulting mixture was cloudy.
B The total volume of the mixture was equal to the sum of the initial volumes.
C The resulting liquid was light blue.
D The liquids formed two separate layers in the beaker.
▶️Answer/Explanation
Ans: A
The formation of a solid (precipitate) can result in the development of cloudiness, and the solid may eventually be visible at the bottom of the beaker. The formation of a precipitate is possible evidence that a chemical change might have occurred.
Question
\( M^{+}\) is an unknown metal cation with a +1 charge. A student dissolves the chloride of the unknown metal, MCI, in enough water to make 100.0 mL of solution. The student then mixes the solution with excess \(AgN0_3\) solution, causing AgCl to precipitate. The student collects the precipitate by filtration, dries it, and records the data shown below. (The molar mass of AgCl is 143 g/mol.)
Mass of unknown chloride, MCI 0.74 g
Mass of filter paper 0.80 g
Mass of filter paper plus AgCl precipitate 2.23 g
What is the identity of the metal chloride?
(A) NaCl
(B)KCl
(C)CuCl
(D)LiCl
▶️Answer/Explanation
Ans:B