A beaker contains a solution of calcium hydroxide $\left( Ca ( OH )_2\right)$ in water. A student places a straw in the beaker and then breathes into the solution through the straw. As the student breathes into the solution, a white precipitate forms. What is the most likely composition of the precipitate?
A. $H _2 O$
B. $CaCO _3$
C. $CH _3 OH$
Answer (2)
Carbon dioxide from the student's breath reacts with calcium hydroxide in the solution.
The reaction produces calcium carbonate and water: C a ( O H ) 2 ( a q ) + C O 2 ( g ) → C a C O 3 ( s ) + H 2 O ( l ) .
Calcium carbonate is insoluble and forms a white precipitate.
The most likely composition of the precipitate is C a C O 3 .
Explanation
Problem Analysis We are given a solution of calcium hydroxide ( C a ( O H ) 2 ) in water. When a student breathes into the solution through a straw, a white precipitate forms. We need to determine the most likely composition of the precipitate from the given options: H 2 O , C a C O 3 , and C H 3 O H .
Identifying the Reaction When the student breathes into the solution, the carbon dioxide ( C O 2 ) in their exhaled breath reacts with the calcium hydroxide ( C a ( O H ) 2 ) in the solution. This reaction produces calcium carbonate ( C a C O 3 ) and water ( H 2 O ). The balanced chemical equation for this reaction is:
Understanding the Products C a ( O H ) 2 ( a q ) + C O 2 ( g ) → C a C O 3 ( s ) + H 2 O ( l ) Calcium carbonate ( C a C O 3 ) is insoluble in water, which means it forms a solid precipitate. Water ( H 2 O ) is already present in the solution, so it will not form a precipitate. Methanol ( C H 3 O H ) is not produced in this reaction.
Conclusion Therefore, the most likely composition of the white precipitate is calcium carbonate ( C a C O 3 ).
Examples
This reaction is a classic example of how carbon dioxide can react with alkaline solutions to form insoluble carbonates. A similar process occurs in nature, where carbon dioxide in the atmosphere reacts with calcium hydroxide in limestone formations, leading to the formation of calcium carbonate deposits. This principle is also used in various industrial processes, such as the production of cement and the removal of carbon dioxide from flue gases.
The white precipitate formed when the student breathes into the calcium hydroxide solution is calcium carbonate (CaCO3), which results from the reaction between carbon dioxide from breath and calcium hydroxide. The chemical equation for the reaction is Ca(OH)2 + CO2 → CaCO3 + H2O. Therefore, the answer is B. CaCO3.
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