(A) Why do we balance a chemical equation? Name and state the law that suggests the balancing of a chemical equation? Balance the following chemical equation: $Zn + H_3PO_4 → Zn_3(PO_4)_2 + H_2$ OR (B) Define a precipitation reaction. Give its example and also express the reaction that occurs in the form of a balanced chemical equation.

(A) We balance a chemical equation to satisfy the law of conservation of mass, which states that mass can neither be created nor destroyed in a chemical reaction. The law that suggests the balancing of a chemical equation is the law of conservation of mass. (B) A precipitation reaction is a type of chemical reaction in which an insoluble solid (precipitate) is formed from a solution. For example, when a solution of lead nitrate is added to a solution of potassium iodide, a yellow precipitate of lead iodide is formed. $Pb(NO_3)_2 + 2KI \rightarrow PbI_2 + 2KNO_3$

Question 273 marksShort AnswerSection C

$Zn + H_3PO_4 → Zn_3(PO_4)_2 + H_2$

Chemical Reactions and Equations

Chemical Reactions and Equations

Official answer

Pb(NO_3)_2 + 2KI \rightarrow PbI_2 + 2KNO_3

law of conservation of mass

balancing chemical equations

precipitation reaction

example

Marking scheme

1. $(A) 1 mark for explaining the law of conservation of mass$
2. $(A) 1 mark for balancing the chemical equation$
3. $(B) 1 mark for defining a precipitation reaction$
4. $(B) 1 mark for providing an example and expressing the reaction in the form of a balanced chemical equation$

Hint

$(A) Recall the law of conservation of mass and its significance in balancing chemical equations. (B) Understand the definition of a precipitation reaction and provide an example.$

Quick oral answer

$The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. This law is the basis for balancing chemical equations. A precipitation reaction is a type of chemical reaction in which an insoluble solid (precipitate) is formed from a solution.$

Common mistakes

1. $Forgetting to balance the chemical equation$
2. $Not providing a correct example of a precipitation reaction$

Analysis and explanation

(A) The law of conservation of mass is a fundamental principle in chemistry that states that mass can neither be created nor destroyed in a chemical reaction. This law is the basis for balancing chemical equations. (B) A precipitation reaction is a type of chemical reaction in which an insoluble solid (precipitate) is formed from a solution. The reaction between lead nitrate and potassium iodide is an example of a precipitation reaction.

Interesting facts

$The law of conservation of mass was first proposed by Antoine Lavoisier in the 18th century.$

$Precipitation reactions are commonly used in chemistry to prepare insoluble salts.$

Question details

Q27 of 39 questions

Board: CBSE
Class: Class 10
Subject: Science
Year: 2025
Section: Section C
Marks: 3
Type: Short Answer
Chapter: Chemical Reactions and Equations
Topic: $Chemical Reactions and Equations$
Bloom's level: Understanding
Syllabus topic: $Balancing Chemical Equations$

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Frequently asked questions

$How many marks does this question carry in CBSE Class 10 Science 2025?$

$This question carries 3 marks in the CBSE Class 10 Science 2025 examination.$

$Which chapter does this question come from in Science?$

$This question is from the chapter "Chemical Reactions and Equations" in the CBSE Class 10 Science syllabus.$

$What topic does this question cover in Science?$

$This question covers the topic "Chemical Reactions and Equations" from CBSE Class 10 Science.$

$What type of question is this in the CBSE Class 10 Science 2025 paper?$

$This is a Short Answer question from Section C in the CBSE Class 10 Science 2025 paper.$