A chemical reaction is a process where substances transform into new substances with different chemical identities. This transformation involves a rearrangement of atoms, leading to the formation of new products.
Key Indicators of a Chemical Change
One common indicator of a chemical reaction is the formation of a gas, often observed as bubbling or fizzing. For instance, when vinegar is mixed with baking soda, carbon dioxide gas is produced, creating visible bubbles.
Another sign is the formation of a precipitate, which is a solid that forms and separates from a liquid solution. This can appear as cloudiness or solid particles settling at the bottom of a container. For example, mixing a clear solution of lead(II) nitrate with potassium iodide instantly produces a yellow solid precipitate.
A change in color frequently indicates a chemical reaction has occurred. The original substances react to form new substances that absorb or reflect light differently. When iron rusts, its silvery surface changes to a reddish-brown, demonstrating a chemical change.
Changes in temperature also point to a chemical reaction, as energy is either released or absorbed. Reactions that release heat, making the surroundings warmer, are called exothermic reactions, like the burning of wood. Conversely, endothermic reactions absorb heat, causing the surroundings to cool down.
The emission of light can also signal a chemical change. This is often seen in reactions that produce light, such as in glow sticks or during combustion. A change in odor suggests that new molecules with different smells have formed.
Understanding the Underlying Changes
The observable signs of a chemical reaction, such as a color change or gas production, occur because atoms rearrange themselves. During a chemical reaction, the bonds holding atoms together in the initial substances, called reactants, break apart.
Once these bonds are broken, the atoms then form new connections, creating entirely different molecules known as products. The atoms themselves are not created or destroyed; they are simply reorganized into new arrangements.
Distinguishing Chemical from Physical Changes
It is important to differentiate chemical changes from physical changes. A chemical change results in the formation of new substances with different compositions. In contrast, a physical change alters a substance’s form or state but does not change its chemical identity.
For example, melting an ice cube is a physical change because water remains water, only changing from solid to liquid. Similarly, dissolving sugar in water or tearing paper are physical changes; the sugar and paper retain their original chemical makeup. These physical alterations do not involve the breaking and forming of new chemical bonds, which is a hallmark of a chemical reaction.