CBSE Class 9 Chemistry Practicals Worksheet

1 Chemical Reactions

INTRODUCTION

Laws of Chemical Combination
Since early seventeenth century, the scientists were trying to establish the basis of chemical combination between the atoms of elements to form different compounds. As a result of the vast researches carried out by them, they came to the conclusion that chemical combinations are based upon certain laws. These are known as laws of chemical combination. A number of laws have been proposed after thorough experimental studies. One of them is Law of Conservation of Mass.

Law of Conservation of Mass
We know that in a chemical reaction, the reactants combine to form the product. The law deals with the mass of the reactants and products involved during in a chemical reaction. It was given by a French chemist A. Lavoisier in 1774. He is known as the father of chemistry.
According to the law:

"The total mass of the products is equal to the mass of the reactants in a chemical reaction."
Or in other words, the law states that -
"The mass can neither be created nor destroyed in a chemical reaction"
In other words, the mass remains unchanged or conserved in a chemical reaction. The law is also known as "Law of Indestructibility of Matter".
For example:

Therefore, mass of reactants = mass of products.

Experiment 1

AIM
To verify the law of conservation of mass in a chemical reaction.

MATERIALS REQUIRED
Weighing balance, conical flask, small ignition tube, thread, cork to fit into the flask, \(100\text{ ml}\) beakers, barium chloride and zinc sulphate.

THEORY
The law of conservation of mass states that, "mass can neither be created nor destroyed in a chemical reaction". In other words "the total mass of the products is always equal to the total mass of the reactants in a chemical reaction".

METHOD
1. Prepare a \(5\%\) solution of barium chloride and \(5\%\) of sodium sulphate in two beakers separately.
2. Take \(5\text{ ml}\) of solution (\(5\text{ ml}\)) of sodium sulphate in the conical flask.
3. Take a little amount of (\(2\text{ ml}\)) of barium chloride in a small ignition tube. Hang the ignition tube into the flask carefully and make sure that the solution do not mix with each other.
4. Put a cork on the flask.
5. Weigh the flask with all the contents carefully. Let it be \(W_1\).
6. Now tilt and swirl the flask such that the solution of barium chloride and sodium sulphate mix with each other.
7. Weigh the flask again. Let it be \(W_2\).

OBSERVATION
1. A reaction takes place between barium chloride and sodium sulphate and the solution forms a white precipitate.
2. White precipitate of barium sulphate is formed.
3. We observe that \(W_1 = W_2\).

Chemical Equation

Explanation
When aqueous solution of barium chloride is mixed with an aqueous solution of sodium sulphate, a white precipitate of barium sulphate is formed. An analysis of the products formed shows that in an exchange between the reactants, i.e., ion has been exchanged by sulphate ion. As such no loss or gain in mass in expected. In other words, the mass remains conserved.

RESULT
The law of conservation of mass is verified.

PRECAUTIONS
1. Wash all the apparatus and clean them properly before use.
2. Take a small amount of \(\text{BaCl}_2\) and \(\text{Na}_2\text{SO}_4\) solution carefully.
3. Place the ignition tube containing aqueous solution of \(\text{Na}_2\text{SO}_4\) gently in the conical flask so that the two solutions do not mixed.

VIVA VOCE QUESTIONS

Question 1. Who stated the law of conservation of mass?
Answer: Antoine Lavoisier.
In simple words: Antoine Lavoisier discovered that mass is neither created nor destroyed during chemical reactions.

Exam Tip: Remember Antoine Lavoisier as the father of modern chemistry and the proponent of this law.

Question 2. State the law of conservation of mass.
Answer: The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.
In simple words: Total mass before and after a chemical reaction remains exactly the same.

Exam Tip: Always quote the definition verbatim in exams to secure full marks.

Question 3. What happens in the conical flask when we tilt and twist it?
Answer: A white precipitate of \(\text{BaSO}_4\) is formed.
In simple words: The two separate solutions mix together and react chemically.

Exam Tip: Mention the formation of the white precipitate as the visible evidence of the reaction.

Question 4. Why should we put a cork on the mouth of the flask?
Answer: To prevent any reactant or product from escaping.
In simple words: A closed system ensures no matter enters or leaves during the experiment.

Exam Tip: A closed system is mandatory to accurately verify mass conservation.

Question 5. Does the mass of the flask and its content change?
Answer: No, mass of the reactants and products is conserved.
In simple words: The total weight remains constant before and after the reaction.

Exam Tip: Conclude that \(W_1 = W_2\) to confirm the experimental result.

Multiple Choice Questions

Question 1. In a chemical reaction, the sum of masses of the reactants and products remain unchanged. This is called
(a) Law of conservation of mass
(b) Law of multiple proportion
(c) Law of constant proportion
(d) None of the options
Answer: (a) Law of conservation of mass
The total mass before and after a chemical reaction remains conserved.
In simple words: Mass is always conserved in chemical reactions.

Exam Tip: Associate conservation of mass directly with unchanging total mass.

Question 2. The law of conservation of mass states that
(a) Mass can be created or destroyed or created nor destroyed
(b) Mass can neither be created nor destroyed
(c) Mass can be created
(d) None of the options
Answer: (b) Mass can neither be created nor destroyed
Matter remains indestructible during chemical changes.
In simple words: Total mass stays constant.

Question 3. In the reaction, barium chloride reacts with sodium sulphate to form
(a) white precipitate of sodium chloride
(b) white precipitate of barium sulphate
(c) white precipitate of lead sulphate
(d) none of the options
Answer: (b) white precipitate of barium sulphate
The reaction yields insoluble barium sulphate as a white precipitate.
In simple words: Barium sulphate forms a solid white suspension.

Exam Tip: Barium sulphate precipitate is a classic textbook observation.

Question 4. In the reaction between barium chloride and sodium sulphate,
(a) ion can be exchanged
(b) remain the same
(c) remain unequal
(d) none of the options
Answer: (a) ion can be exchanged
Double displacement involves the mutual exchange of ions.
In simple words: Ions swap partners between compounds.

Exam Tip: Double displacement reactions are driven by ion exchange.

Question 5. During the reaction of barium chloride and sodium sulphate
(a) exchange of ion of chloride and sulphate takes place
(b) exchange of ion of chloride and sodium takes place
(c) exchange of ion of sulphate and barium takes place
(d) none of the options
Answer: (d) none of the options
The correct exchange is between barium ions and sodium ions with their respective anions (chloride and sulphate swap partners to form barium sulphate and sodium chloride).
In simple words: Barium and sodium ions exchange their respective negative ions.

Exam Tip: Pay close attention to which specific ions pair up.

Question 6. The law of conservation of mass was stated by
(a) A. Lavoisier
(b) Democritus
(c) W. Proust
(d) Louis Pasteur
Answer: (a) A. Lavoisier
Antoine Lavoisier formulated this foundational chemical law in 1774.
In simple words: Lavoisier established the law of conservation of mass.

Exam Tip: Link Lavoisier with mass conservation and modern chemistry.

Answers