Chapter 4, Problem 121A

(a)

Interpretation Introduction

Interpretation:

Whether boiling of water is a physical change or chemical change needs to be explained.

Concept introduction:

In the physical change, the rearrangement of molecules takes place, but their composition remains the same.

In the chemical change, the rearrangement and the composition of the molecules both change.

Answer to Problem 121A

In the process of boiling of water, water (liquid) gets evaporated (gas) there is only state changes.

Thus, the boiling of water is a physical change.

Explanation of Solution

When the water starts to boiling means there is change in the arrangement of molecules of water. In the liquid state, the water molecule is close to each other but in the gaseous states the intermolecular space is larger and molecules are spread out. But the composition of water in liquid and gaseous state is same.

(b)

Interpretation Introduction

Interpretation:

When a match stick burns it comes under physical or chemical change needs to be explained.

Concept introduction:

A match stick burns, a wooden stick changes into an ash form.

Answer to Problem 121A

The burning of match stick is a chemical change.

Explanation of Solution

When a match stick is burned, a solid stick changes into powdery form and production of some gases takes place. The starting reactant is different from produced product, and some heat and light is produced. When in any changes the energy or light is produced it comes under the category of chemical changes. In these types of changes the composition of molecule totally changes.

(c)

Interpretation Introduction

Interpretation:

The process of dissolving sugar in water is chemical or physical change needs to be determined.

Concept introduction:

When the solid and liquid mix together in a reaction, if at the end of the reaction the composition remain same it is a physical change.

Answer to Problem 121A

The dissolving of sugar in water is a physical change.

Explanation of Solution

When the solid particle of sugar is dissolve in water it is a physical change because in this change no new substances. So it is example of physical change.

Interpretation Introduction

(d)

Interpretation:

Sodium reacts with water is the example of physical or chemical change needs to be explained.

Concept introduction:

If at the end of any reaction a new product is formed it is considered as a chemical change.

Answer to Problem 121A

The reaction of sodium with water is a chemical change.

Explanation of Solution

Sodium is a metal and when a metal is reacted with water it produces hydrogen gas and a hydroxide of sodium is formed. In this reaction, the original substance can’t be recovered. Thus, it is an irreversible reaction which is important point of a chemical change.

Interpretation Introduction

(e)

Interpretation:

When ice cream melts it consider as physical or chemical change needs to be explained.

Concept introduction:

When in any process only physical state changes it consider as a physical change.

Answer to Problem 121A

The melting of ice cream is a physical change.

Explanation of Solution

The solid form of ice cream is changes into liquid form that is no new product is formed, only solid state change to liquid form. It is a temporary change because if temperature of melted ice cream is decreased it again changes into a solid form.