Physical and Chemical Changes

Correct Answer

📖 Governing Law / Core Concept

Chemical change refers to a transformation where the chemical properties of the substances involved change fundamentally, resulting in the formation of new substances. The general equation for the souring of milk can be represented by the bacterial fermentation of lactose: where \(\text{C}_{12}\text{H}_{22}\text{O}_{11}\) (lactose) converts into lactic acid (\( \text{C}_3\text{H}_6\text{O}_3\)) and glucose (\(\text{C}_6\text{H}_{12}\text{O}_6\)).

🧪 Step-by-Step Breakdown

- Dissolving sugar in water (Option A) is a physical change; sugar retains its chemical identity. - Melting of ice (Option B) is also a physical change, converting ice (solid) to water (liquid) without altering molecular structure. - Crystallization (Option C) is a physical process where a substance transitions from a solution to a solid form, maintaining its chemical identity. Thus, only Option (d) represents a chemical change resulting in a new product (sour milk).

🔍 Option Analysis

- Option A: Dissolves sugar and retains molecular structure; physical change. - Option B: Melts ice; no new substances formed; physical change. - Option C: Crystallization retains original chemical substances; physical change.

⚡ Mnemonic / Speed-Run

Chemical changes can be remembered as "Rxn = New Substance," indicating a reactant transforms, resulting in a different product.

6. Visual Suggestion

📖 Factual Verification & Reference

Correct Answer

📖 Governing Law / Core Concept

The process described is an example of deposition, where vapours transition directly from the gas phase to the solid state, forming a crust.

🧪 Step-by-Step Breakdown

When sulfur dioxide (\(SO_2\)) vapour escapes from a volcano, it can undergo the following reaction in the atmosphere where it combines with oxygen:

The resulting sulfur trioxide (\(SO_3\)) can further react with water vapour to form sulfuric acid, but upon cooling, the vapour may transition to a solid state, resulting in deposition of crystalline sulfur on the rocks.

However, this acidic solution can dissipate, leading to the solid crust formation as \(SO_2\) directly deposits from vapour to solid.

🔍 Option Analysis

• A: Condensation refers to the transformation from gas to liquid, not solid.
• B: Precipitation is primarily related to the formation of solid particulates from a solution, not from gas vapours.
• D: Evaporation is the process of liquid turning into gas, opposite to the described scenario.

⚡ Mnemonic / Speed-Run

Remember: "Gases deposit, liquids precipitate." This can help differentiate deposition from precipitation in chemistry.

📊 Visual Suggestion

📖 Factual Verification & Reference

Correct Answer

📖 Governing Law / Core Concept

The reaction between sodium hydroxide (NaOH) and hydrochloric acid (HCl) is a neutralization reaction governed by the principles of acid-base chemistry.

🧪 Step-by-Step Breakdown

The balanced chemical equation for the reaction is:

This indicates that NaOH, a strong base, reacts with HCl, a strong acid, to form sodium chloride (NaCl) and water (H₂O). The process involves breaking and forming chemical bonds, confirming it as a chemical change.

🔍 Option Analysis

• A: Sublimation of camphor - Physical change from solid to gas.
• B: Melting of ice - Physical change from solid to liquid.
• C: Heating of iron rod - Physical change involving temperature increase.
• D: Mixing of NaOH and HCl - Chemical change resulting in new products.

⚡ Mnemonic / Speed-Run

Remember: "Acid + Base = Salt + Water" for quick recall of neutralization reactions.

📖 Factual Verification & Reference

Correct Answer

📖 Governing Law / Core Concept

A chemical change involves the formation of one or more new substances with distinct properties. Physical changes alter only the form, not the chemical composition.

🧪 Step-by-Step Breakdown

A brief overview of each option:

• A: Burning of coal - A chemical change where carbon in coal reacts with oxygen to form CO₂ and other substances:
\[ C + O_2 \rightarrow CO_2 \]
• B: Rusting of metal - An oxidation reaction where iron reacts with oxygen and water, forming iron oxide:
\[ 4Fe + 3O_2 + 6H_2O \rightarrow 4Fe(OH)_3 \]
• C: Melting of ice - A physical change where solid ice (H₂O) changes to liquid water without altering its chemical structure:
\[ H_2O(s) \rightarrow H_2O(l) \]
• D: Reaction of acid with base - A chemical reaction producing salt and water:
\[ HA + BOH \rightarrow BA + H_2O \]

🔍 Option Analysis

Options A, B, and D are chemical changes, indicated by the formation of new substances, while Option C represents a physical change, preserving the chemical identity of water.

⚡ Mnemonic / Speed-Run

Remember: Chemical changes Create New Stuff (CCNS). Physical changes Keep Original Substances (PKOS).

6. Visual Suggestion

📖 Factual Verification & Reference

Correct Answer

📖 Governing Law / Core Concept

Physical change is defined as a change where the chemical composition of a substance remains unchanged, while its physical form may alter.

🧪 Step-by-Step Breakdown

The interconversion of states of matter (solid, liquid, gas) is a classic example of physical change:

• Sublimation of dry ice: \( \text{CO}_2 (s) \rightarrow \text{CO}_2 (g) \)
• Melting of ice: \( \text{H}_2O (s) \rightarrow \text{H}_2O (l) \)

In contrast, a chemical change involves the formation or breaking of bonds, resulting in new substances:

• Rusting of iron: \( 4\text{Fe} + 3\text{O}_2 + 6\text{H}_2O \rightarrow 4\text{Fe(OH)}_3 \) (creates rust)
• Burning candle producing wax and CO₂: \( \text{C}_{25}\text{H}_{52} + 38\text{O}_2 \rightarrow 25\text{CO}_2 + 26\text{H}_2\text{O} \)

🔍 Option Analysis

Other options are valid:

• A: Correct, as interconversion of states is a physical process.
• B: Correct, as burning a candle involves both physical (melting wax) and chemical changes.
• D: Correct, rusting of iron is a chemical reaction yielding iron oxide.

⚡ Mnemonic / Speed-Run

Remember: "Physical = Change in Form, Chemical = Change of Substance"

6. Visual Suggestion

📖 Factual Verification & Reference