Explain the difference between a physical change and a chemical change.

Understanding the concept of a physical change is crucial when studying the various transformations that matter can undergo. A physical change alters the physical attributes of a substance, such as its shape, state, or volume, without changing its fundamental chemical identity. Imagine ice melting into water, a paper being folded, or sugar dissolving in tea. In each of these actions, the substances involved retain their chemical characteristics, meaning that no new substances are created. Moreover, physical changes are often reversible, exemplified by water that can be refrozen into ice.
Physical changes can be influenced by temperature and pressure. Changing these conditions can cause substances to transition between solid, liquid, and gas phases. A critical aspect of physical changes is that they involve energy changes that do not alter the substance chemically, like when ice absorbs heat to become liquid water without the water molecules themselves changing.

A chemical change, in contrast to a physical change, results in the formation of one or more new substances with new chemical properties. This type of change is fundamental to the study of chemistry as it concerns the transformation in the composition of matter. Take, for instance, the rusting of iron when it combines with oxygen to form a totally new substance: iron oxide. Here, we see the original substance, iron, loses its identity as it reacts chemically and changes into another compound.
During a chemical change, the bonds between atoms are broken and reformed, leading to new molecular structures. These processes can release or absorb energy, often in the form of light or heat. Unlike physical changes, chemical changes are usually not easily reversible because they involve changing the substances at a molecular level.

Chemical changes come with a set of characteristics that allow us to identify them.

• Production of a Gas: When a new gas is emitted during a reaction, like bubbles during vinegar and baking soda reacting, it signals a chemical change.
• Change in Color: A sudden change in color, such as the browning of an apple when exposed to air, reveals a chemical alteration in the substance.
• Temperature Change: Exothermic or endothermic reactions involve temperature changes without external heat being added or removed.
• Precipitate Formation: The appearance of a solid in a solution after mixing two liquids is indicative of a chemical change.
• Irreversibility: Chemical changes often produce results that cannot simply be undone, highlighting a substantial alteration of the matter.

Each of these indicators points to the rearrangement of the molecular structure within a substance, thereby altering its chemical identity.

The properties of physical changes allow us to distinguish them from their chemical counterparts.

• Reversibility: Physical changes can often be reversed, such as water freezing and melting.
• No New Substances: The change affects form or appearance, but the substance's chemical makeup remains unaltered.
• Energy Changes: These changes can involve energy transfers such as absorbing or releasing heat, but without chemical transformation.
• Phase Changes: Moving from one state of matter to another, such as solid to liquid or liquid to gas, is a classic example of a physical change.

Understanding these features helps to categorize processes and phenomena in the physical world appropriately. For students grappling with differentiating between physical and chemical transformations, remembering these characteristics provides a solid foundation for their exploration of material science.