Classify each of the following changes as physical or chemical: (a) coal burning (b) ice melting (c) mixing chocolate syrup with milk (d) explosion of a firecracker (e) magnetizing of a screwdriver

A chemical change is a process where substances transform into entirely new substances with different chemical properties. This is not merely a change in the physical appearance or shape, but a change at the molecular level. For instance, when coal burns (as mentioned in the exercise), it reacts with oxygen in the air and forms carbon dioxide, water vapor, and other compounds. Such reactions often release or absorb energy in the form of heat or light.

It's important to note that chemical changes are generally irreversible using simple physical means, because new chemical bonds are formed, and original substances lose their characteristic properties. Some signs of a chemical change include change in color, temperature change, formation of precipitates, and the release of gas or odor.

Physical change involves an alteration in the form or physical properties of a substance, without making any change in its chemical composition. For example, the melting of ice is purely a physical change. The ice turns from solid to liquid form when it absorbs heat, but chemically, it remains water (H₂O).

Other typical examples of physical changes are breaking a glass, dissolving sugar in water, or cutting paper. These processes are generally reversible—ice can re-freeze, and water can evaporate into vapor and then condense back into liquid. Physical changes can often be observed as changes in state, size, shape, texture, or phase. Understanding these concepts helps us to classify changes in matter correctly.

The states of matter refer to the distinct forms that different phases of matter take on. The most familiar are solid, liquid, and gas. Solids have a fixed shape and volume, liquids have a fixed volume but can change shape by flowing, and gases have neither fixed shape nor volume, expanding to fill the container they are in.

Melting and Freezing

When a solid turns into a liquid, it is called melting, while the reverse process is freezing.

Evaporation and Condensation

Similarly, evaporation occurs when a liquid becomes a gas, and condensation is when a gas becomes a liquid.

The change from one state to another involves energy. When ice melts, it absorbs heat, and when water freezes, it releases heat. These transitions are examples of physical changes where the molecular structure does not change, but the energy within the system and the arrangement of molecules do.

A chemical reaction is the process that leads to the transformation of one set of chemical substances to another. This involves the breaking of bonds in reactants and the formation of new bonds in products. For example, the explosion of a firecracker is a quick and exothermic chemical reaction where the reactants convert into gas and heat, resulting in sound and light.

Understanding chemical reactions is critical, as they form the basis for a multitude of processes in our daily lives, from the metabolic pathways in our body, to industrial manufacturing, and even the seasonal color change of leaves. Chemical reactions are encompassed by laws of conservation of mass and energy, meaning that in a closed system, matter and energy are neither created nor destroyed, only transformed.