Question

State whether each of the following is a physical or a chemical change: (a) mercury cools to a silvery solid. (b) natural gas burns with a blue flame. (c) ammonia and copper solution turns deep blue. (d) sugar crystals dissolve in water.

Short answer

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

Step-by-step solution

Define Physical and Chemical Changes

A physical change is when a substance changes its state or form but not its chemical composition. Examples include melting, freezing, and dissolving. A chemical change involves a transformation that changes the substance's chemical composition, often indicated by color change, gas production, or energy release.

Analyze Example (a)

When mercury cools to become a silvery solid, it is changing from a liquid to a solid state without altering its chemical identity. This is a physical change.

Analyze Example (b)

Natural gas burning with a blue flame combines the gas with oxygen, producing carbon dioxide and water. This alters the chemical composition and releases energy, indicating a chemical change.

Analyze Example (c)

The reaction between ammonia and a copper solution forming a deep blue solution involves a change in chemical composition due to the formation of a new complex. This is a chemical change.

Analyze Example (d)

Dissolving sugar crystals in water involves the sugar mixing with water at a molecular level without any change in its chemical composition. Therefore, this is a physical change.

Key concepts

Physical Change

A physical change involves a transformation in the state or appearance of a substance, without affecting its chemical composition. This means that while the form or state of the substance might change, the substance itself remains chemically the same. Common examples of physical changes include:

• Melting ice to water.
• Freezing water to ice.
• Breaking a glass.
• Dissolving sugar in water.

These changes are generally reversible. When mercury cools down to form a silvery solid, it changes from a liquid to a solid, maintaining the same chemical identity. Understanding physical changes is crucial because they help identify processes where substances remain the same chemically, even if they look different.

Chemical Change

A chemical change results in the formation of one or more new substances with new chemical properties. During this process, the original substance’s chemical composition is altered. Signs of a chemical change can include:

• Change in color.
• Production of gas (bubbles).
• Release or absorption of energy (heat or light).
• Formation of a precipitate.

For instance, when natural gas burns, it reacts with oxygen to produce carbon dioxide and water while releasing energy in the form of a blue flame, highlighting a chemical transformation. Chemical changes are typically not reversible through simple physical means, and understanding these changes is essential in chemistry as it helps to identify reactions that alter the essence of substances.

State of Matter

The state of matter refers to the distinct forms that different phases of matter take on. The primary states of matter are solid, liquid, and gas:

• Solid: Fixed shape and volume.
• Liquid: Fixed volume but takes the shape of its container.
• Gas: Neither fixed shape nor volume, expands to fill its container.

Changes between these states are physical changes, often involving temperature. For example, water turning to ice involves removing heat, changing from liquid to solid while the chemical composition remains unchanged. Understanding states of matter helps in grasping how substances interact in different conditions and is vital in both physical and chemical processes.

Chemical Composition

Chemical composition refers to the identity and relative number of elements that make up any given substance. Determining the chemical composition provides insight into what substances are involved in making a particular compound or mixture. When a chemical change occurs, the chemical composition of the original material changes, producing new substances. For example, when ammonia and a copper solution combine, they create a deep blue complex, signifying a change in chemical structure, a hallmark of a chemical reaction. Recognizing changes in chemical composition is key, as it helps in identifying new substances formed and understanding the underlying reactions taking place.