CBSE Class 9 Chemistry Experiments Worksheet

EXPERIMENT 8

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

MATERIALS REQUIRED
(i) Barium chloride (\(\text{BaCl}_2 \cdot 2\text{H}_2\text{O}\))
(ii) Sodium sulphate (\(\text{Na}_2\text{SO}_4 \cdot 10\text{H}_2\text{O}\))
(iii) Distilled water
(iv) Two beakers (150 mL)
(v) One beaker (250 mL)
(vi) Physical balance
(vii) Spring balance (\(0 - 500\text{ g}\))
(viii) Polythene bag
(ix) Two watch glasses of known masses
(x) Glass stirrer.

THEORY
Law of conservation of mass states that the mass remains conserved during a chemical reaction. In this experiment we shall verify the law of conservation of mass using a precipitation reaction. This reaction is considered as the simplest method to verify this law.

PROCEDURE
1. Pour 100 mL distilled water in two beakers (150 mL).
2. Using the physical balance and a watch glass of known mass, weigh 7.2 g of \(\text{BaCl}_2 \cdot 2\text{H}_2\text{O}\) and dissolve it in a beaker (150 mL) containing 100 mL distilled water.
3. Similarly, weigh 16.1 g of \(\text{Na}_2\text{SO}_4 \cdot 10\text{H}_2\text{O}\) in another watch glass of known mass and dissolve it in another beaker (150 mL) containing 100 mL distilled water.
4. Take the third beaker (250 mL) and weigh it using a spring balance and polythene bag (see Experiment no. 3 for details).
5. Mix both solutions of 150 mL beakers in the third beaker (250 mL). Mix the contents using a glass stirrer.
6. On mixing white precipitate of \(\text{BaSO}_4\) appears due to precipitation reaction.
7. Weigh of beaker containing the reaction mixture again to determine the mass of the precipitation reaction products.
8. Compare the masses of before and after the chemical reaction.

OBSERVATION
(i) Mass of 100 mL distilled water = 100.0 g
(The density of distilled water is 1 g/mL.)
(ii) Mass of \(\text{BaCl}_2 \cdot 2\text{H}_2\text{O}\) = 7.2 g
(iii) Mass of \(\text{BaCl}_2\) solution = 107.2 g
(iv) Mass of \(\text{Na}_2\text{SO}_4 \cdot 10\text{H}_2\text{O}\) = 16.1 g
(v) Mass of \(\text{Na}_2\text{SO}_4\) solution = 116.1 g
(vi) Total Mass of reactants (solutions of \(\text{BaCl}_2\) and \(\text{Na}_2\text{SO}_4\)) = 223.3 g
(vii) Mass of empty 250 mL beaker, \(m_1\) = 50.0 g
(viii) Initial mass of reaction mixture and empty beaker (before the precipitation), \(m_2 = (m_1 + 223.3\text{ g})\) = 273.3 g
(ix) Final mass of reaction mixture in the beaker after the precipitation, \(m_3\) = 273.3 g

INFERENCE
We see that within the reasonable experimental limits.
Initial mass (\(m_2\)) (of reaction mixture) = Final mass (\(m_3\)) (of reaction mixture)
Thus, the law stands verified.
The chemical reaction involved is
\(\text{BaCl}_2(\text{aq}) + \text{Na}_2\text{SO}_4(\text{aq}) \longrightarrow \text{BaSO}_4(\text{s}) + 2\text{NaCl}(\text{aq})\)
White precipitate
and more precisely
\(\text{Ba}^{2+}(\text{aq}) + \text{SO}_4^{2-}(\text{aq}) \longrightarrow \text{BaSO}_4(\text{s})\)

PRECAUTIONS
1. The spring balance should be held vertical while taking measurements.
2. Before making use of the spring balance it must be ensured that its pointer is at zero mark. If not then ask your teacher to help.
3. The readings of the spring balance should be noted only when its pointer comes to rest.
4. Mixing of barium chloride and sodium sulphate solutions be done slowly with constant stirring.

VIVA-VOCE

Question 1. Name any two laws of chemical combination.
Answer: (i) Law of multiple proportion.
(ii) Law of conservation of mass.
In simple words: The two fundamental laws governing how elements combine to form compounds are the law of multiple proportions and the law of conservation of mass.

Exam Tip: Memorize the five basic laws of chemical combination for board examinations.

Question 2. What is law of conservation of mass?
Answer: Law of conservation of mass states that the mass remains conserved during a chemical reaction.
In simple words: Mass can neither be created nor destroyed in a chemical reaction.

Exam Tip: Always state that total mass of reactants equals total mass of products in a closed system.

Question 3. State law of conservation of matter.
Answer: Matter can neither be created nor be destroyed. It can only be changed from one form to other.
In simple words: The total amount of matter in the universe remains constant.

Exam Tip: Distinguish clearly between conservation of mass and conservation of matter when asked.

Question 4. Is there any relation between law of conservation of mass and law of conservation of matter?
Answer: Yes. Law of conservation of mass is a special case of law of conservation of matter.
In simple words: Mass conservation is the practical application of matter conservation during chemical changes.

Exam Tip: Keep your answer direct and precise for one-mark questions.

Question 5. Who was the first scientist to verify law of conservation of mass?
Answer: Antoine Lavoisier.
In simple words: Antoine Lavoisier experimentally proved that mass does not change during chemical reactions.

Exam Tip: Remember Antoine Lavoisier as the father of modern chemistry.

Question 6. What do you mean by conservation of mass?
Answer: Conservation of mass means total mass of reactant is equal to the total mass of products during a chemical reaction.
In simple words: Reactant mass equals product mass always.

Question 7. In laboratory, mass is measured by which device?
Answer: Physical balance.
In simple words: A physical balance or electronic balance is used to measure mass accurately.

Exam Tip: Do not confuse mass measuring devices (physical balance) with force measuring devices (spring balance).

Question 8. What are the weights less than 1 gm called?
Answer: Fractional weight.
In simple words: Weights smaller than 1 gram used in a physical balance box are called fractional weights (milligram weights).

Exam Tip: Fractional weights are usually in milligrams (\(\text{mg}\)).

Question 9. In this experiment which reaction is used to verify law of conservation of mass?
Answer: Precipitation reaction between solution of Barium chloride and Sodium Sulphate.
In simple words: A double displacement precipitation reaction is chosen because it produces an easily measurable solid product.

Exam Tip: Mention the formation of insoluble barium sulphate precipitate.

Question 10. Why do we choose above reaction for verifying law of conservation of mass?
Answer: Because it is a very simple reaction to handle.
In simple words: It occurs instantaneously in aqueous solutions at room temperature without requiring complex apparatus.

Exam Tip: Simplicity and speed make precipitation reactions ideal for school laboratories.

MULTIPLE CHOICE QUESTIONS

Question 1. Who proposed law of conservation of mass?
(a) Ge lu sac
(b) Priestley
(c) Lavosier
(d) None of the options
Answer: (c) Lavosier
Antoine Lavoisier established the law of conservation of mass in 1789.
In simple words: Lavoisier proposed that mass is conserved in chemical reactions.

Exam Tip: Spelling variations like Lavosier or Lavoisier refer to the same scientist.

Question 2. Which one is a balanced chemical reaction?
(a) \(\text{BaCl}_2(\text{aq}) + \text{Na}_2\text{SO}_4(\text{aq}) \longrightarrow \text{BaSO}_4(\text{s}) + 2\text{NaCl}(\text{aq})\)
(b) \(2\text{NaCl}(\text{s}) + \text{BaSO}_4(\text{aq}) \longrightarrow \text{BaCl}_2(\text{aq}) + \text{Na}_2\text{SO}_4(\text{aq})\)
(c) Both are correct
(d) None of the options
Answer: (a) \(\text{BaCl}_2(\text{aq}) + \text{Na}_2\text{SO}_4(\text{aq}) \longrightarrow \text{BaSO}_4(\text{s}) + 2\text{NaCl}(\text{aq})\)
Reaction (a) has equal numbers of each atom on both sides, making it balanced.
In simple words: Option (a) correctly shows the precipitation reaction with balanced atoms.

Exam Tip: Always verify atom counts on reactant and product sides.

Question 3. Reaction between aquous solution of Barium Chloride and Sodium Sulphate is an example of:
(a) displacement reaction
(b) double displacement reaction
(c) substitutic reaction
(d) combination reaction
Answer: (b) double displacement reaction
The ions \(\text{Ba}^{2+}\) and \(\text{Na}^{+}\) exchange places with each other.
In simple words: Exchange of ions between reactants classifies it as a double displacement reaction.

Exam Tip: Mutual exchange of ions is the hallmark of double displacement reactions.

Question 4. Reaction between aqueous solution of Barium Chloride and Sodium Sulphate is also known as:
(a) Precipitation
(b) Galvanisation
(c) Isomerisation
(d) None of the options
Answer: (a) Precipitation
The reaction produces an insoluble white precipitate of barium sulphate.
In simple words: Because a solid precipitate forms, it is called a precipitation reaction.

Exam Tip: Precipitation reactions are a subclass of double displacement reactions.

Question 5. White precipitate is the reaction of Barium Chloride and Sodium Sulphate is of:
(a) \(\text{BaSO}_4\)
(b) \(\text{NaCl}\)
(c) None of the two
(d) Sodium Borate
Answer: (a) \(\text{BaSO}_4\)
Barium sulphate (\(\text{BaSO}_4\)) is insoluble in water and forms a dense white precipitate.
In simple words: The white solid formed is barium sulphate.

Exam Tip: Remember that \(\text{BaSO}_4\) precipitate is characteristic and insoluble in mineral acids.

Question 6. In experiment to verify law of conservation of mass, what is the colour of precipitate of Barium Sulphate:
(a) buff
(b) white
(c) pink
(d) yellow
Answer: (b) white
Barium sulphate forms as a fine white precipitate.
In simple words: Barium sulphate precipitate is strictly white in colour.

Exam Tip: Note visual color changes carefully as they serve as qualitative test results.

Question 7. The colour of Barium Chloride solution is:
(a) green
(b) yellow
(c) blue
(d) colourless
Answer: (d) colourless
Aqueous barium chloride forms a clear, colourless solution.
In simple words: Barium chloride solution does not have any color.

Exam Tip: Most alkali and alkaline earth metal salt solutions without transition metals are colourless.

Question 8. The colour of Sodium Sulphate solution is:
(a) light blue
(b) pale
(c) grey
(d) colourless
Answer: (d) colourless
Sodium sulphate dissolves completely in water to form a colourless solution.
In simple words: Sodium sulphate solution is clear and colourless.

Exam Tip: Solutions of sodium salts are generally colourless.

Question 9. In the reaction mixture \(\text{NaCl}\) remains in the form :
(a) solid
(b) aqueous solution
(c) semi solid
(d) precipitate
Answer: (b) aqueous solution
Sodium chloride (\(\text{NaCl}\)) is soluble in water and remains dissolved in the aqueous phase.
In simple words: \(\text{NaCl}\) stays dissolved as aqueous ions (\(\text{Na}^{+}\) and \(\text{Cl}^{-}\)).

Exam Tip: Soluble products do not precipitate out; they remain in solution.

Question 10. Which compounds are present in the product mixture is the reaction of Barium Chloride and Sodium Sulphate ?
(a) \(\text{BaSO}_4, \text{NaCl}\)
(b) \(\text{BaCl}_2, \text{NaCl}\)
(c) \(\text{BaSO}_4, \text{Na}_2\text{SO}_4\)
(d) \(\text{BaCl}_2, \text{Na}_2\text{SO}_4\)
Answer: (a) \(\text{BaSO}_4, \text{NaCl}\)
The products of the reaction are barium sulphate precipitate and aqueous sodium chloride.
In simple words: The products formed are \(\text{BaSO}_4\) and \(\text{NaCl}\).

Exam Tip: Identify products directly from the right-hand side of the balanced chemical equation.

Question 11. What happens when a solution of sodium sulphate is added to a solution of barium chloride ?
(a) A yellow precipitate is formed
(b) A white precipitate and a colourless solution is formed
(c) Reaction takes place with evolution of heat but overall colourless solution is formed
(d) No reaction appears to have taken place
Answer: (b) A white precipitate and a colourless solution is formed
Mixing these solutions yields insoluble white \(\text{BaSO}_4\) and colourless \(\text{NaCl}\) solution.
In simple words: White precipitate appears along with a clear solution.

Exam Tip: Observation questions require noting both precipitate and filtrate characteristics.

Question 12. When sodium sulphate solution is added to a solution of barium chloride, a white precipitate is formed in
(a) an hour
(b) within a few seconds
(c) two hours
(d) within a second only on heating
Answer: (b) within a few seconds
Precipitation reactions between ionic solutions occur almost instantaneously.
In simple words: The reaction is extremely fast and happens in seconds.

Exam Tip: Ionic reactions in aqueous solutions are generally instantaneous.

Question 13. Which of the following is a combination reaction ?
(a) Formation of white precipitate from mixing solutions of sodium sulphate and barium chloride
(b) Formation of brown precipitate from heating of lead nitrate
(c) Formation of white powder from heating of magnesium in air
(d) Formation of zinc sulphate when zinc is added to sulphuric acid
Answer: (c) Formation of white powder from heating of magnesium in air
Magnesium combines with oxygen to form a single product (\(\text{MgO}\)), which is a combination reaction.
In simple words: Two reactants combining to form a single substance is a combination reaction.

Exam Tip: Combination reactions have the general form \(A + B \longrightarrow C\).

Question 14. When a solution of sodium sulphate is added to a solution of barium chloride the following combination represent the right conclusion :
(a) a precipitate with a displacement reaction
(b) a clear solution with a double displacement reaction
(c) a precipitate with a double decomposition reaction
(d) a precipitate with a combination reaction
Answer: (c) a precipitate with a double decomposition reaction
Double displacement reactions are also known as double decomposition reactions, resulting in a precipitate.
In simple words: The reaction involves mutual decomposition and exchange, forming a precipitate.

Exam Tip: Double displacement and double decomposition are synonymous terms in many contexts.