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Atomic Foundations of Matter

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Chapter Notes

Overview
This chapter builds on the structure of the atom to explain how and why atoms combine to form molecules and compounds. It introduces two fundamental chemical laws—the Law of Conservation of Mass and the Law of Constant Proportions—which form the basis of Dalton's Atomic Theory. The central idea is that atoms are indivisible, rearrange during chemical reactions without being created or destroyed, and combine in fixed ratios to form stable substances with distinct properties. A student must walk away understanding that matter is conserved, compounds have fixed compositions, and chemical bonds (covalent and ionic) arise from atoms sharing or transferring electrons to achieve stability.
Key Concepts (15)
Law of Conservation of Mass
Matter can neither be created nor destroyed in a chemical reaction.
e.g. When 4.0 g of calcium carbonate reacts with 2.92 g of hydrochloric acid in a closed container, the total mass of products (1.76 g CO₂ + 0.72 g H₂O + 4.44 g CaCl₂) equals 6.92 g, the same as the total mass of reactants.
Law of Constant Proportions
In any compound formed by two or more elements, the elements combine in a fixed ratio by mass, irrespective of its source.
e.g. Purified water from any source always contains hydrogen and oxygen in a mass ratio of 1:8.
Dalton's Atomic Theory
A theory proposing that all matter is made of tiny indivisible particles called atoms, which cannot be created or destroyed in a chemical reaction; atoms of a given element are identical in mass and properties; atoms of different elements have different masses; atoms combine in simple whole-number ratios to form compounds; and the relative number and kinds of atoms are constant in a given compound.
e.g. When magnesium burns in air, magnesium atoms combine with oxygen atoms to form magnesium oxide, and the atoms themselves are not destroyed.
Molecule
An electrically neutral entity consisting of more than one atom that is capable of independent existence and shows all the properties of that substance.
e.g. A hydrogen molecule (H₂) consists of two hydrogen atoms and exists independently.
Chemical bond
The force that holds atoms together.
e.g. In a water molecule, the shared pairs of electrons between hydrogen and oxygen atoms form covalent bonds that hold the atoms together.
Covalent bond
A type of chemical bond formed by the sharing of electrons between atoms.
e.g. In a hydrogen molecule (H₂), each hydrogen atom shares one electron with the other, forming a single covalent bond (H–H).
Single bond
A covalent bond formed when two atoms share one electron each, depicted by a single line between the symbols of the atoms.
e.g. The bond in H–H or Cl–Cl is a single bond.
Double bond
A covalent bond formed when two atoms share two pairs of electrons, depicted by two lines between the symbols of the atoms.
e.g. In an oxygen molecule (O₂), two oxygen atoms share two electrons each, forming a double bond (O=O).
Ionic bond
The electrostatic force of attraction between oppositely charged ions that holds them together.
e.g. In sodium chloride (NaCl), the Na⁺ cation and Cl⁻ anion are held together by ionic bonds.
Cation
A positively charged ion formed when an atom loses one or more electrons.
e.g. A sodium atom (Na) loses one electron to become a sodium cation (Na⁺).
Anion
A negatively charged ion formed when an atom gains one or more electrons.
e.g. A chlorine atom (Cl) gains one electron to become a chloride anion (Cl⁻).
Polyatomic ion
An ion formed by the combination of atoms of two or more elements.
e.g. The ammonium ion (NH₄⁺) and the sulfate ion (SO₄²⁻) are polyatomic ions.
Molecular mass
The total mass of a molecule, calculated by adding the atomic masses of all the atoms constituting it.
e.g. Molecular mass of water (H₂O) = (1 u × 2) + (16 u × 1) = 18 u.
Formula unit mass
The sum of the atomic masses of all the atoms present in a formula unit (the simplest whole number ratio of ions in an ionic compound).
e.g. Formula unit mass of sodium oxide (Na₂O) = (23 u × 2) + (16 u × 1) = 62 u.
Valency
The combining capacity of an element, equal to the number of electrons an atom can lose, gain, or share to achieve a stable electronic configuration.
e.g. Sodium has a valency of 1 (it loses one electron), and oxygen has a valency of 2 (it gains two electrons).
Formulas & Equations (12)
Mass of reactants = Mass of products
Reads as: The total mass of reactants equals the total mass of products.
Variables: No variables; it is a statement of the Law of Conservation of Mass.
Use when: Verifying whether a chemical reaction obeys the Law of Conservation of Mass.
⚠ Students forget to account for gases that escape in an open system, leading them to think mass is not conserved.
Vinegar + Baking soda (Sodium hydrogencarbonate) → Carbon dioxide + Other substances
Reads as: Vinegar plus baking soda yields carbon dioxide plus other substances.
Variables: No variables; it is a word equation.
Use when: Describing the reaction used in Activity 9.2 to demonstrate conservation of mass.
⚠ Students write the products incorrectly or forget that carbon dioxide is a gas that can escape.
Sodium sulfate + Barium chloride → Barium sulfate + Sodium chloride
Reads as: Sodium sulfate plus barium chloride yields barium sulfate plus sodium chloride.
Variables: No variables; it is a word equation.
Use when: Describing the reaction used in Activity 9.3 to verify the Law of Conservation of Mass.
⚠ Students think this reaction produces a gas; it does not — it forms a precipitate.
Carbon + Oxygen → Carbon dioxide
Reads as: Carbon plus oxygen yields carbon dioxide.
Variables: No variables; it is a word equation.
Use when: Solving problems like Example 9.2 where given masses of reactants are used to find the mass of product.
⚠ Students forget to use the fixed ratio by mass (12 g C : 32 g O₂ : 44 g CO₂).
Zinc + Hydrochloric acid (dilute) → Zinc chloride + Hydrogen
Reads as: Zinc plus dilute hydrochloric acid yields zinc chloride plus hydrogen.
Variables: No variables; it is a word equation.
Use when: Designing an experiment to test the Law of Conservation of Mass (Think as a Scientist box).
⚠ Students forget that hydrogen gas escapes, so the experiment must be done in a closed system.
H₂O
Reads as: H two O
Variables: H = hydrogen atom; O = oxygen atom; subscript 2 indicates two hydrogen atoms.
Use when: Writing the chemical formula of water.
⚠ Students write HO or H₂O₂ instead of H₂O.
NaCl
Reads as: N a C l
Variables: Na = sodium; Cl = chlorine.
Use when: Writing the chemical formula of sodium chloride.
⚠ Students write Na₂Cl or NaCl₂ because they forget the charges balance as 1:1.
Mg(OH)₂
Reads as: M g O H twice
Variables: Mg = magnesium; OH = hydroxide polyatomic ion; subscript 2 indicates two hydroxide ions.
Use when: Writing the chemical formula of magnesium hydroxide.
⚠ Students forget the brackets and write MgOH₂, which is incorrect.
CaCO₃
Reads as: C a C O three
Variables: Ca = calcium; CO₃ = carbonate polyatomic ion.
Use when: Writing the chemical formula of calcium carbonate.
⚠ Students write Ca₂(CO₃)₂ instead of simplifying to CaCO₃.
Al₂(SO₄)₃
Reads as: A l two S O four three
Variables: Al = aluminium; SO₄ = sulfate polyatomic ion; subscripts indicate 2 aluminium and 3 sulfate ions.
Use when: Writing the chemical formula of aluminium sulfate.
⚠ Students forget the brackets and write Al₂SO₄₃, which is meaningless.
Molecular mass of H₂O = (1 u × 2) + (16 u × 1) = 18 u
Reads as: Molecular mass of water equals 1 u times 2 plus 16 u times 1 equals 18 u.
Variables: u = atomic mass unit; 1 u = mass of one hydrogen atom; 16 u = mass of one oxygen atom.
Use when: Calculating the molecular mass of a covalent compound.
⚠ Students forget to multiply the atomic mass by the number of atoms of each element.
Formula unit mass of Na₂O = (23 u × 2) + (16 u × 1) = 62 u
Reads as: Formula unit mass of sodium oxide equals 23 u times 2 plus 16 u times 1 equals 62 u.
Variables: u = atomic mass unit; 23 u = atomic mass of sodium; 16 u = atomic mass of oxygen.
Use when: Calculating the formula unit mass of an ionic compound.
⚠ Students treat it as molecular mass and forget that ionic compounds do not have molecules.
How It Works
Verification of Law of Conservation of Mass using a chemical reaction (Activity 9.2)
  1. Place a clean, dry 100 mL conical flask and a medium-sized balloon on a weighing balance and tare to zero.
  2. Pour about 20 mL of vinegar into the conical flask.
  3. Put about 2 g of baking soda into the balloon.
  4. Fix the balloon to the mouth of the conical flask without allowing the baking soda to mix with the vinegar, and record the initial mass.
  5. Lift the balloon to allow the baking soda to fall into the vinegar and observe the reaction (brisk effervescence inflates the balloon).
  6. Record the final mass of the conical flask and balloon together.
  7. Compare the initial and final readings — they are the same, showing mass is conserved.
💡 In an open system, the gas escapes and mass appears to change; the balloon traps the gas so the system is closed and mass is conserved.
Formation of a covalent bond by sharing of electrons
  1. Write the electronic configuration of each atom involved.
  2. Determine how many electrons each atom needs to achieve a stable octet (or duplet for hydrogen).
  3. Each atom contributes one or more valence electrons to form shared pairs.
  4. The shared pair of electrons attracts both nuclei, forming a covalent bond.
  5. Represent the bond with a single line (single bond), two lines (double bond), or three lines (triple bond).
💡 In covalent bonding, atoms share electrons; no electrons are lost or gained, so no ions are formed.
Formation of an ionic bond by transfer of electrons
  1. Identify the metal (which has few valence electrons) and the non-metal (which has many valence electrons).
  2. The metal atom loses one or more valence electrons to become a positively charged cation.
  3. The non-metal atom gains those electrons to become a negatively charged anion.
  4. The oppositely charged ions are held together by electrostatic force of attraction, forming an ionic bond.
  5. The ions arrange in a regular, repeating three-dimensional pattern called a crystal lattice.
💡 Ionic compounds do not form discrete molecules; they exist as formula units in a crystal lattice.
Writing chemical formulae of covalent compounds using the criss-cross method
  1. Write the symbols of the constituent elements.
  2. Write the valencies of each element above or below the symbol.
  3. Criss-cross the valencies: the valency of the first element becomes the subscript of the second, and vice versa.
  4. If the valency is 1 after criss-crossing, it is not written.
  5. Write the resulting formula.
💡 For covalent compounds, the formula represents the actual number of atoms in a molecule.
Writing chemical formulae of ionic compounds using the criss-cross method
  1. Write the symbol of the cation first, followed by the symbol of the anion.
  2. Write the charges under the symbols (not as superscripts).
  3. Criss-cross the numbers of the charges (ignoring the sign) to become subscripts.
  4. If the subscripts have a common factor, divide by that factor to get the simplest ratio.
  5. If a polyatomic ion appears more than once, enclose it in brackets and write the subscript outside.
💡 The overall charge of an ionic compound must be zero; the criss-cross method ensures charge balance.
Naming covalent compounds using prefixes
  1. Write the name of the first element (no prefix if only one atom, except for CO where mono- is used for carbon).
  2. Write the name of the second element with an -ide ending.
  3. Add a prefix (mono-, di-, tri-, tetra-, penta-, hexa-, etc.) before the second element to indicate the number of atoms.
  4. If the prefix ends with 'o' or 'a' and the element starts with a vowel, drop the last vowel (e.g., monoxide, pentoxide).
  5. If the prefix ends with 'i', keep it (e.g., dioxide, trioxide).
💡 Common names like water (H₂O) and ammonia (NH₃) are exceptions and are used instead of systematic names.
Exam Traps — Don't Make These Mistakes
⚠ Mass appears to change in an open system
✗ The Law of Conservation of Mass is violated because the mass after the reaction is less than before.
→ In an open system, gaseous products escape, so the measured mass decreases. The law still holds — the total mass of all products (including the escaped gas) equals the total mass of reactants.
✓ The Law of Conservation of Mass is obeyed. The apparent loss of mass is due to the escape of carbon dioxide gas. In a closed system, the total mass remains unchanged.
⚠ Confusing molecular mass with formula unit mass
✗ The molecular mass of sodium chloride (NaCl) is 58.5 u.
→ Ionic compounds like NaCl do not have molecules; they exist as formula units. The term 'molecular mass' applies only to covalent compounds. For ionic compounds, we use 'formula unit mass'.
✓ The formula unit mass of sodium chloride (NaCl) is 58.5 u.
⚠ Writing incorrect formulae by ignoring charge balance
✗ The formula of magnesium chloride is MgCl.
→ Magnesium has a charge of 2+ (Mg²⁺) and chlorine has a charge of 1– (Cl⁻). To balance charges, two chloride ions are needed for one magnesium ion, giving MgCl₂.
✓ The formula of magnesium chloride is MgCl₂.
⚠ Forgetting brackets for polyatomic ions
✗ The formula of magnesium hydroxide is MgOH₂.
→ MgOH₂ suggests one magnesium, one oxygen, and two hydrogens, which is incorrect. The hydroxide ion (OH⁻) is a single unit. Two hydroxide ions require brackets: Mg(OH)₂.
✓ The formula of magnesium hydroxide is Mg(OH)₂.
⚠ Misapplying the Law of Constant Proportions to mixtures
✗ A mixture of iron and sulfur always contains iron and sulfur in a fixed ratio by mass.
→ The Law of Constant Proportions applies only to compounds, not mixtures. Mixtures can have variable composition. For example, iron and sulfur can be mixed in any ratio.
✓ The Law of Constant Proportions applies to compounds. A compound like iron sulfide always contains iron and sulfur in a fixed ratio by mass, but a mixture of iron and sulfur can have any ratio.
⚠ Thinking atoms are created or destroyed in a chemical reaction
✗ When wood burns, the atoms of wood are destroyed and new atoms of ash and smoke are created.
→ According to Dalton's Atomic Theory and the Law of Conservation of Mass, atoms are neither created nor destroyed in a chemical reaction. They simply rearrange to form new substances.
✓ In a chemical reaction, atoms are rearranged. The total number of atoms of each element remains the same before and after the reaction.
Compare & Contrast
Formation mechanism
Covalent bond
Ionic bond
A covalent bond is formed by the sharing of electrons between atoms, while an ionic bond is formed by the transfer of electrons from one atom to another, resulting in oppositely charged ions held by electrostatic attraction.
Type of substance
Molecular mass
Formula unit mass
Molecular mass is used for covalent compounds (which exist as discrete molecules), while formula unit mass is used for ionic compounds (which exist as a crystal lattice of ions, not molecules).
What it describes
Law of Conservation of Mass
Law of Constant Proportions
The Law of Conservation of Mass states that mass is neither created nor destroyed in a chemical reaction (total mass of reactants = total mass of products). The Law of Constant Proportions states that elements in a compound always combine in a fixed ratio by mass, regardless of the source.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 Very Short Answer (1 mark), 1 Short Answer (3 marks) on laws or bonding, 1 Long Answer (5 marks) on writing formulae and calculating mass, total approximately 10-12 marks.
Most asked: Law of Conservation of Mass — numerical verification and explanation · Law of Constant Proportions — definition and numerical problems · Dalton's Atomic Theory — postulates and their significance · Writing chemical formulae using the criss-cross method · Distinguishing between covalent and ionic bonds · Calculation of molecular mass and formula unit mass · Naming covalent compounds using prefixes
Don't skip: Numerical problems based on Law of Conservation of Mass (e.g., Example 9.1 and 9.2 type); Writing chemical formulae of ionic compounds with polyatomic ions; Calculating molecular mass or formula unit mass; Explaining the formation of covalent and ionic bonds with diagrams
One-Line Revision (read aloud before the exam)
  1. The Law of Conservation of Mass states that matter can neither be created nor destroyed in a chemical reaction.
  2. The Law of Constant Proportions states that elements in a compound always combine in a fixed ratio by mass.
  3. Dalton's Atomic Theory proposed that atoms are indivisible and rearrange during chemical reactions.
  4. A molecule is an electrically neutral entity of more than one atom that can exist independently.
  5. A covalent bond is formed by the sharing of electrons between atoms.
  6. An ionic bond is formed by the transfer of electrons, creating oppositely charged ions.
  7. A single bond involves one shared pair of electrons (e.g., H–H).
  8. A double bond involves two shared pairs of electrons (e.g., O=O).
  9. Cations are positively charged ions formed by losing electrons; anions are negatively charged ions formed by gaining electrons.
  10. The criss-cross method is used to write chemical formulae by crossing over valencies or charges.
  11. Molecular mass is the sum of atomic masses of all atoms in a molecule (for covalent compounds).
  12. Formula unit mass is the sum of atomic masses of all atoms in a formula unit (for ionic compounds).
  13. Ionic compounds conduct electricity when dissolved in water because ions become free to move.
  14. Covalent compounds generally have low melting and boiling points and do not conduct electricity.
  15. Polyatomic ions are groups of atoms that carry a net charge, e.g., NH₄⁺, SO₄²⁻.
  16. In naming covalent compounds, prefixes like di-, tri-, tetra- indicate the number of atoms.

MCQs — 10 Questions with Answers

Q1. What does the Law of Conservation of Mass state?

  • A. Matter can be created and destroyed in chemical reactions
  • B. Total mass remains the same before and after a chemical reaction ✓
  • C. Only physical changes conserve mass, not chemical changes
  • D. Mass increases when new substances are formed

Answer: B — The Law of Conservation of Mass, proposed by Lavoisier, clearly states that total mass before a chemical reaction equals total mass after the reaction.

Q2. In Activity 9.2 set-up 1, why did the final reading on the weighing balance differ from the initial reading?

  • A. The Law of Conservation of Mass does not apply to this reaction
  • B. Carbon dioxide gas escaped from the open system into the atmosphere ✓
  • C. The digital balance was not calibrated correctly
  • D. Vinegar and baking soda were not properly measured

Answer: B — In the open system, carbon dioxide gas produced in the reaction escaped, removing mass from the closed weighing system and causing a mass difference.

Q3. What is the key difference between Activity 9.2 set-up 1 and set-up 2?

  • A. Different amounts of vinegar and baking soda were used
  • B. Set-up 2 used a balloon to trap the gas, creating a closed system ✓
  • C. Set-up 1 used sodium sulfate instead of baking soda
  • D. The weighing balance was different in both set-ups

Answer: B — Set-up 2 attached a balloon to trap carbon dioxide gas within the system, preventing mass loss and demonstrating that mass is actually conserved.

Q4. When salt dissolves in water to form a solution, what happens to the total mass?

  • A. Total mass increases because water molecules expand
  • B. Total mass decreases because some salt particles escape
  • C. Total mass remains the same and equals mass of salt plus mass of water ✓
  • D. Total mass becomes zero because dissolved substances have no weight

Answer: C — Dissolving salt in water is a physical change where no particles escape; therefore, total mass of solution equals mass of salt plus mass of water.

Q5. Which scientist proposed the Law of Conservation of Mass and in what year?

  • A. Johannes Kepler in 1610
  • B. Isaac Newton in 1687
  • C. Antoine Lavoisier in 1789 ✓
  • D. Albert Einstein in 1905

Answer: C — Antoine Lavoisier, known as the Father of Modern Chemistry, proposed the Law of Conservation of Mass in 1789.

Q6. Ramesh observes that when hydrogen gas and oxygen gas combine to form water, the water has completely different properties than either gas. Which concept best explains why properties change despite mass being conserved?

  • A. The Law of Conservation of Mass only applies to physical changes
  • B. New substances are formed with different atomic arrangements and bonding patterns, creating different properties ✓
  • C. Some of the hydrogen and oxygen are lost during the reaction
  • D. Water molecules are smaller and lighter than hydrogen or oxygen molecules

Answer: B — Chemical reactions rearrange atoms and form new bonds, creating new substances with entirely different properties while maintaining the same total mass.

Q7. Which of the following is NOT correct regarding the vinegar and baking soda reaction?

  • A. Carbon dioxide gas is produced during the reaction
  • B. The initial mass of reactants equals the final mass of all products when trapped in a closed system
  • C. The balloon expands because oxygen gas is produced ✓
  • D. Brisk effervescence occurs when baking soda is added to vinegar

Answer: C — The gas produced is carbon dioxide, not oxygen; the balloon expands due to carbon dioxide gas production during the vinegar-baking soda reaction.

Q8. Why is it necessary to use the tare or reset button before adding substances to a weighing balance?

  • A. To ensure the balance can measure up to 100 grams accurately
  • B. To set the balance reading to zero so only the added substance is measured, not the beaker's weight ✓
  • C. To prevent the digital display from showing negative numbers
  • D. To calibrate the balance for measuring gases

Answer: B — The tare button resets the balance to zero, so the displayed reading reflects only the mass of the substance added, not the container.

Q9. In a chemical reaction carried out in a closed container, if the mass of reactants is 50 grams, what will be the mass of products?

  • A. Less than 50 grams because some matter is converted to energy
  • B. More than 50 grams because new atoms are created
  • C. Exactly 50 grams according to the Law of Conservation of Mass ✓
  • D. It depends on the type of chemical reaction

Answer: C — The Law of Conservation of Mass states that in a closed system, the mass of products must equal the mass of reactants.

Q10. When a piece of paper is torn into smaller pieces, the total mass of all the pieces compared to the original paper would be:

  • A. Less, because tearing creates new air particles
  • B. More, because physical changes always increase mass
  • C. The same, because tearing is a physical change where no matter is created or destroyed ✓
  • D. Unrelated to mass, as physical changes do not involve mass

Answer: C — Tearing paper is a physical change; no matter is created or destroyed, so total mass of pieces equals the original paper's mass.

Flashcards

What is the Law of Conservation of Mass?

Matter can neither be created nor destroyed in a chemical reaction; total mass before reaction equals total mass after reaction.

Who proposed the Law of Conservation of Mass and in which year?

Antoine Lavoisier proposed this law in 1789 and is known as the Father of Modern Chemistry.

In Activity 9.2 set-up 1, why does the final reading not match the initial reading?

Carbon dioxide gas produced during the reaction escapes into the atmosphere, so the closed system loses mass.

In Activity 9.2 set-up 2, why do the initial and final readings match?

The balloon attached to the flask traps the carbon dioxide gas inside the closed system, preventing mass loss.

What is the difference between a physical change and a chemical change in terms of mass?

Both preserve total mass, but physical changes (like dissolving salt) do not produce new substances, while chemical changes produce new substances.

Why does water have different properties than hydrogen and oxygen gases?

Water is a new substance formed by chemical combination; atoms rearrange and bond differently, creating new properties.

What happens to the total mass when salt dissolves in water?

The total mass remains unchanged; mass of solution equals mass of salt plus mass of water.

Why is it important to use a closed system when verifying the Law of Conservation of Mass?

A closed system prevents reactants or products from escaping, ensuring all matter remains within the system for accurate mass measurement.

In the vinegar and baking soda reaction, what is the chemical equation representation?

Vinegar + Sodium hydrogencarbonate → Carbon dioxide + Other substances (demonstrating mass is conserved when gas is contained).

What does the tare or reset button on a digital balance do?

It sets the balance reading to zero, allowing accurate measurement of only the substances placed on the balance.

Important Board Questions

State the Law of Conservation of Mass and name the scientist who proposed it. [2 marks]

Define that matter cannot be created or destroyed in chemical reactions; mention Antoine Lavoisier and the year 1789 as the source.

Explain why the mass reading in Activity 9.2 set-up 1 was different from that in set-up 2, even though the same chemical reaction occurred in both cases. [3 marks]

In set-up 1, carbon dioxide gas escaped from the open system (mass loss observed). In set-up 2, the balloon trapped the gas inside (no mass loss). The closed vs. open system determines whether the Law of Conservation of Mass appears to hold.

When hydrogen gas and oxygen gas combine to form water, the properties of water are completely different from either reactant gas. Discuss why the properties change but the Law of Conservation of Mass still applies. Use the concept of chemical bonding in your explanation. [5 marks]

Explain that new chemical bonds form between atoms, creating a new substance (water) with rearranged atoms and different properties. Despite new properties, the total mass remains constant because atoms are only rearranged, not created or destroyed. Show understanding by connecting atomic rearrangement to property change while maintaining mass conservation.

Assertion-Reason Questions for Atomic Foundations of Matter

Q1. Assertion (A):

Water formed from hydrogen and oxygen has different properties than either hydrogen or oxygen.

Reason (R):

In a chemical reaction, atoms rearrange to form new substances with new properties.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true. The assertion is directly stated in the chapter (hydrogen and oxygen are gases, while water is a liquid that neither burns nor supports combustion). The reason correctly explains why properties change—atoms break old bonds and form new ones, creating substances with entirely different properties. This is the fundamental explanation for the assertion.

Q2. Assertion (A):

Dissolving salt in water is a physical change, and the mass remains constant during this process.

Reason (R):

Physical changes always preserve the identity of the substance and conserve mass because no new chemical substances are formed.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true and directly related. Activity 9.1 demonstrates that when salt dissolves in water, the mass of the solution equals the sum of masses of salt and water—the mass is conserved. The reason correctly explains why: no new substances are chemically formed during dissolution, so mass is conserved. The salt and water maintain their chemical identity in the solution (dissolved salt is still salt in ionic form).

Q3. Assertion (A):

The Law of Conservation of Mass applies only to chemical reactions and not to physical changes.

Reason (R):

Physical changes do not involve the formation of new substances.

Answer: D — A is false but R is true — The assertion is false. The Law of Conservation of Mass applies to both physical and chemical changes. Activity 9.1 demonstrates mass conservation in the physical change of salt dissolution. The reason, while true (physical changes do not form new substances), does not support the false assertion because mass is conserved precisely because no mass is added or removed, whether substances are new or not.

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Case-Based Questions (CBQ) for Atomic Foundations of Matter

Case Study:

During a school science fair, students performed an experiment to verify the Law of Conservation of Mass. They used a digital weighing balance to measure 20 mL of sodium sulfate solution (weighing 25 g) in flask A and 20 mL of barium chloride solution (weighing 23 g) in flask B. After mixing the solutions carefully, they obtained a white precipitate. They quickly transferred all of the mixture (precipitate, liquid, and everything else) to flask C and weighed it immediately, obtaining a reading of 48 g.

Q1. What is the significance of obtaining a final mass of 48 g (which equals 25 g + 23 g)?

  • A. The balance was faulty because the mass should have decreased
  • B. It verifies the Law of Conservation of Mass in a chemical reaction ✓
  • C. It proves that barium sulfate is lighter than the original solutions
  • D. It indicates that the reaction was incomplete

Answer: B

Q2. Why is it important that the students transferred all of the mixture, including the precipitate and liquid, to flask C for weighing?

  • A. To make the mixture cooler before weighing
  • B. To ensure all products remained in the system so no mass was lost ✓
  • C. To prevent the precipitate from dissolving again
  • D. To allow the liquid to evaporate before weighing

Answer: B

Q3. What type of change occurred when sodium sulfate and barium chloride solutions were mixed?

  • A. Physical change because a precipitate formed
  • B. Physical change because the solutions mixed
  • C. Chemical change because new substances (precipitate) were formed ✓
  • D. Neither physical nor chemical change

Answer: C

Q4. If the students had poured the mixture into an open container instead of a sealed flask, and some liquid splashed out, what would the final mass reading likely be?

  • A. Exactly 48 g, following the Law of Conservation of Mass
  • B. More than 48 g due to water vapor absorption
  • C. Less than 48 g because some mass escaped in the liquid ✓
  • D. Unpredictable and dependent on external factors

Answer: C

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