Problem 43 - General Chemistry

Question

Which of the following are physical changes and which are chemical changes? a. melting of sodium chloride b. pulverizing of rock salt c. burning of sulfur d. dissolving of salt in water

Step-by-Step Solution

Answer

a. Physical, b. Physical, c. Chemical, d. Physical

Step 1: Identify the Process

First, review each scenario given in the exercise and identify the type of process occurring, whether it's melting, pulverizing, burning, or dissolving.

Step 2: Define Physical and Chemical Changes

Recall that a physical change involves a change of state or form without changing the chemical identity, while a chemical change results in the formation of a new substance with different properties.

Step 3: Analyze Option (a): Melting of Sodium Chloride

Melting is a physical process where sodium chloride changes from a solid to a liquid, but the chemical identity remains the same since no new substances are formed.

Step 4: Analyze Option (b): Pulverizing of Rock Salt

Pulverizing is a physical change since it involves breaking rock salt into smaller pieces without changing its chemical composition.

Step 5: Analyze Option (c): Burning of Sulfur

Burning is a chemical change because sulfur reacts with oxygen to form sulfur dioxide, a new substance with different chemical properties.

Step 6: Analyze Option (d): Dissolving of Salt in Water

Dissolving is typically a physical change because salt dissociates into its ions in water, but its chemical identity is unchanged and can be recovered by evaporating the water.

Key Concepts

Melting

Melting is a fundamental concept in the study of matter and its changes. It is the process where a substance transitions from a solid to a liquid state. This occurs when a solid absorbs enough thermal energy to overcome the forces holding its particles in a fixed position.

• During melting, there is no change in the chemical identity of the substance.
• The process is reversible; freezing can solidify the liquid back to its original solid state.

For instance, when sodium chloride (table salt) is melted, it merely changes form from solid to liquid without forming a new compound. Given that there aren't any new substances formed, this makes melting a prime example of a physical change.

Pulverizing

Pulverizing is another interesting physical change that is commonly performed. It involves reducing the size of solid particles by crushing or grinding them into smaller pieces.

• Despite the change in size and form, the chemical composition of the substance remains unchanged.
• It is a common technique in various industries such as mining and pharmaceuticals.

When you pulverize rock salt, it is simply broken into finer particles. The essential properties remain the same as the granite's chemical structure does not change at all. Hence, pulverizing is classified as a physical change.

Burning

Burning or combustion is a prime example of a chemical change where a substance reacts with an oxidizer, typically oxygen, to form new compounds.

• This process is usually characterized by the production of heat and light, along with new substances that have different properties from the original material.
• The process is irreversible under normal conditions.

When sulfur burns in the presence of oxygen, it forms sulfur dioxide. This is a clear indication of a chemical change as the sulfur atoms and oxygen atoms combine to form a new molecule, altering the chemical identity of the original substances.

Dissolving

Dissolving is an interesting process that can sometimes be confusing when distinguishing between physical and chemical changes. Typically, dissolving signifies a physical change where the solute integrates into the solvent to form a homogeneous mixture.

• The individual molecules or ions of the solute spread out and mix with the solvent molecules.
• This process is generally reversible, allowing the recovery of the solute.

For example, when salt dissolves in water, it dissociates into sodium and chloride ions. The crucial point is that the ions maintain their chemical identity, and no new substances are formed. Thus, dissolving salt in water is considered a physical change.