Matter is defined as anything that has mass and takes up space. It is constantly undergoing transformation, shifting and rearranging itself into new configurations. Understanding these transformations is fundamental to science. Matter’s changes fall into two broad categories: those where the substance remains chemically the same and those that create entirely new substances. This article explores these ways matter changes and the principles that govern these transformations.
Transformations Where Identity Stays the Same
Physical changes alter the form, size, or state of a substance, but the chemical composition remains unchanged. The atoms and molecules are not broken apart or reformed into a new kind of matter. A common example is changing the state of matter, often called a phase change.
When ice melts into liquid water or liquid water boils into steam, the substance is still water (\(\text{H}_2\text{O}\)). The change involves moving the molecules further apart or closer together, not altering the chemical structure. Melting, freezing, evaporation, condensation, and the direct change from solid to gas, known as sublimation, are all examples of phase changes.
Beyond phase changes, other transformations where the identity is conserved include crushing, cutting, or bending a material. Tearing a piece of paper changes its shape and size, but the paper itself remains cellulose. Dissolving sugar in water is another physical change, where the sugar molecules disperse throughout the water. This type of change is often reversible using simple physical means, such as evaporating the water to recover the solid sugar.
Transformations That Create New Substances
Chemical changes result in the formation of entirely new substances with properties different from the starting materials. This transformation involves a chemical reaction where atomic bonds are broken and rearranged to create new molecular structures. The starting substances are called reactants, and the newly formed substances are called products.
A common example is combustion, such as burning wood, which combines organic compounds with oxygen to form ash, carbon dioxide gas, and water vapor. Another familiar chemical change is the formation of rust, which occurs when iron reacts with oxygen and water to form iron oxide. Cooking is also an example, where heat applied to an egg or dough causes proteins and other molecules to alter their structure permanently.
Several observable signs indicate that a chemical reaction has taken place, signaling the formation of a new substance. These signs include:
- The production of light or heat
- A sudden change in color
- The formation of a gas (bubbles or fizzing)
- The creation of a solid substance called a precipitate
Unlike physical changes, chemical transformations are difficult or impossible to reverse using basic physical methods; for example, burned wood cannot be turned back into a log.
The Energy Required to Drive Change
Every transformation of matter, whether physical or chemical, involves an exchange of energy necessary to drive the change. Energy must be either absorbed from the surroundings or released into them for a process to occur. This explains why changes rarely happen spontaneously without energy input.
Processes that absorb thermal energy from the environment are called endothermic changes. Examples include melting ice, which must absorb heat to break the solid structure, and photosynthesis, where plants absorb light energy to create glucose. The surroundings often feel cooler because the system is drawing heat inward.
Conversely, processes that release thermal energy into the surroundings are known as exothermic changes. The warmth from a burning candle or the heat produced by an explosion are results of exothermic chemical reactions. Physical changes can also be exothermic, such as when water freezes into ice, releasing the energy required to lock the molecules into a solid structure.
The Law of Conservation of Mass
A fundamental principle governing all transformations is the Law of Conservation of Mass. This law states that matter is neither created nor destroyed during any physical or chemical change. The total mass of the substances present before a transformation must equal the total mass of the substances present afterward.
In a chemical reaction, atoms are merely rearranged into new molecules, meaning the total count and type of atoms remain constant from reactants to products. For instance, when a log burns, the mass of the resulting ash, smoke, and gases is equivalent to the original mass of the log plus the mass of the oxygen consumed. The law confirms that all transformations matter undergoes are simply changes in form, ensuring the total quantity of matter in a closed system remains constant.