Boiling water is a chemical change or a physical change one has to first define what is boiling and what is the change brought about in the physical or chemical state by boiling. Taking the three parts one by one let us first define boiling.

Boiling of water is physical change because by cooling we can again change the state of vapour to water..

Physical Change:
A physical change is a type of change in which the substance involved undergoes a transformation that does not alter its chemical composition. In other words, the change affects the physical properties of the substance, but not its chemical properties.

Boiling of Water:
Boiling of water is an example of a physical change. When water is heated to its boiling point (100 degrees Celsius or 212 degrees Fahrenheit), it undergoes a change from a liquid state to a gaseous state. This change is purely physical, as the chemical composition of water (H2O) remains the same. The molecules of water gain enough energy to overcome the forces holding them together as a liquid, and they escape into the air as water vapor.

Explanation:
To further understand why boiling of water is a physical change, let's break down the process:

1. Heating: When heat is applied to water, the molecules gain energy, causing them to move faster and collide with each other more frequently.

2. Evaporation: As the temperature of the water reaches its boiling point, the kinetic energy of the molecules increases to the extent that they overcome the intermolecular forces (hydrogen bonding) holding them together as a liquid. The molecules escape into the air, forming water vapor.

3. Physical Properties: The physical properties of water change during boiling. The temperature of the water rises from room temperature to its boiling point. The appearance of the water changes as it transforms from a liquid to a gas. The volume of the water decreases as it evaporates.

4. Chemical Composition: Despite these changes in physical properties, the chemical composition of water remains the same. Water molecules (H2O) consist of two hydrogen atoms bonded to one oxygen atom, and this arrangement does not change during the boiling process.

Other Options:
a)A bud turning into a flower: This is an example of a biological change, as it involves the growth and development of a living organism.
c)Ripening of a tomato: This is also a biological change, as it involves the biochemical processes that occur within the tomato to change its color, texture, and taste.
d)Rusting of iron: This is a chemical change, as it involves the oxidation of iron when it reacts with oxygen in the presence of moisture to form iron oxide.