Which of these is an example of a physical change in chemistry?

A physical change in chemistry refers to a process where the form of matter is altered, but its chemical identity remains the same. During a physical change, the molecules undergo a change in state or are rearranged, but the structure of the molecules—the atoms they are made of and the way they are bonded together—does not change. This means that a physical change is usually reversible.

Here are some common examples of physical changes:

1. Melting of ice: When ice melts, it changes from a solid to a liquid. The molecular structure of water (H2O) remains the same, but the physical state changes due to the increase in temperature.

2. Boiling of water: Similar to melting, boiling water is a physical change where liquid water is transformed into water vapor (steam) when heated to its boiling point.

3. Dissolving sugar in water: When sugar dissolves in water, it spreads out within the water, but the chemical structure of the sugar molecules does not change. The sugar can be recovered by evaporating the water.

4. Crushing a can: Deforming a can by crushing it changes its shape but does not alter the chemical composition of the metal it is made from.

5. Breaking a glass: When a glass object is broken, the material remains the same chemically, but its shape and size are altered.

6. Sublimation of dry ice: Dry ice (solid carbon dioxide) sublimates, meaning it changes directly from a solid to a gas without passing through a liquid phase. The CO2 molecules are not chemically altered during this process.

7. Freezing water: When water freezes, it changes from a liquid to a solid (ice). The arrangement of water molecules becomes more structured, but the molecules themselves do not change.

Among these examples, let's select the melting of ice as a detailed example of a physical change:

Step-by-Step Process of Melting Ice:

1. Starting State: Begin with ice at a temperature below 0°C (32°F), which is the freezing point of water. The ice is in a solid state, with water molecules arranged in a crystalline lattice structure.

2. Energy Absorption: As the ice absorbs heat from its surroundings, the temperature of the ice increases. The energy from the heat causes the molecules within the ice to vibrate more vigorously.

3. Breaking Intermolecular Bonds: As the temperature reaches 0°C (32°F), the added energy is sufficient to break some of the hydrogen bonds that hold the water molecules in place within the solid structure.

4. Phase Transition: With enough hydrogen bonds broken, the ice begins to transition into liquid water. The molecules are still H2O, but they are no longer held in a rigid structure and can move freely past one another.

5. End State: Once all the ice has absorbed enough energy to break the intermolecular bonds, the solid has completely turned into a liquid. The water is now at 0°C (32°F) and in a liquid state, but it is still chemically identical to the ice it was before melting.

Throughout this process, the chemical identity of the substance (water, H2O) does not change, which is why melting ice is classified as a physical change.