Question

Does each statement represent a physical change or a chemical change? a) Graphite, a form of elemental carbon, can be turned into diamond, another form of carbon, at very high temperatures and pressures. b) The house across the street has been painted a new color. c) The elements sodium and chlorine come together to make a new substance called sodium chloride.

Short answer

a) Physical change b) Physical change c) Chemical change

Step-by-step solution

Identify Physical Changes

A physical change involves changes in the state or appearance of a substance without altering its composition. These changes are usually reversible. For example, a change in color or form that does not affect the chemical identity is often a physical change.

Identify Chemical Changes

A chemical change involves forming one or more new substances with different compositions and properties. These changes are usually irreversible under normal conditions, and they involve the making or breaking of chemical bonds.

Analyze Statement (a)

Converting graphite into diamond involves rearranging carbon atoms from one structural form to another form of the same element. Both graphite and diamond are forms of elemental carbon, indicating only a change in physical form rather than chemical composition. Thus, it is a physical change.

Analyze Statement (b)

Painting a house involves applying a colored liquid onto a surface, which typically dries and hardens. This process does not change the composition or identity of the house or the paint's components; it only changes the appearance. Therefore, this is considered a physical change.

Analyze Statement (c)

When sodium (Na) and chlorine (Cl) are combined, they undergo a chemical reaction to form sodium chloride (NaCl), a new chemical compound. This involves the creation of a new substance with a different chemical composition and properties, so it represents a chemical change.

Key concepts

Physical Changes

Physical changes occur when the form of a substance is altered but its chemical makeup remains unchanged. You can think of it as the same material taking on a new shape or state.
For instance, when you melt ice into water, it changes from a solid to a liquid; however, it is still H₂O.

• These changes often involve changes in state (like solid to liquid, or liquid to gas).
• They can involve changes in appearance, like size or shape.
• Importantly, they do not result in the production of a new substance.
• Physical changes are generally reversible, meaning the original state can be restored, often by simply reversing the conditions (e.g., refreezing melted ice).

In the exercise:
A example from the exercise is the transformation of graphite to diamond. Both graphite and diamond are forms of carbon, meaning the change only affects the arrangement of the atoms and not the composition. Similarly, painting alters the house’s appearance, but not its chemical makeup.

Chemical Changes

Chemical changes involve fundamentally changing the substance itself. This type of change alters the chemical structure and properties, resulting in the formation of one or more new substances.
In chemical changes:

• The transformation creates different molecular configurations.
• New bonds are formed while old ones are broken, leading to new chemical compositions.
• Such changes are typically not easily reversible.
• Evidence of a chemical change may include color changes, gas production, or temperature change.

During the exercise:
Example (c) illustrates a chemical change when sodium and chlorine form sodium chloride. This process involves a significant alteration in the substances--from separate elements to a chemically bonded compound--creating a completely new material with its own properties.

Substance Composition Change

Substance composition change is a fundamental aspect that separates chemical changes from physical ones. When discussing changes in chemistry, understanding whether the substance’s composition is altered helps determine the type of change.
Key things to note about changes in substance composition:

• In a physical change, the arrangement or form may alter, but the underlying composition remains the same.
• In chemical changes, the structure at the atomic level changes, leading to a different substance altogether.
• Hence, substance composition change is synonymous with chemical change, since the alteration results in a change of the substance's identity.

Remember:
The concept of composition change explains why painting a house is only a physical change. It looks different because of a new layer, but the materials that make up the house are unchanged. Meanwhile, combining elements like sodium and chlorine results in a composition shift, forming a new compound.