Problem 21 - Chemistry The Central Science

Question

Label each of the following as either a physical process or a chemical process: (a) rusting of a metal can, (b) boiling a cup of water, (c) pulverizing an aspirin, (d) digesting a candy bar, (e) exploding of nitroglyerin.

Step-by-Step Solution

Answer

(a) Rusting of a metal can: chemical process (b) Boiling a cup of water: physical process (c) Pulverizing an aspirin: physical process (d) Digesting a candy bar: chemical process (e) Exploding of nitroglycerin: chemical process

Step 1: (a) Classifying the rusting of a metal can

Rusting is a process in which a metal reacts with oxygen and moisture to form a new substance called rust (iron oxide). It is a chemical change because the chemical composition of the metal changes during rusting. So, the rusting of a metal can is a chemical process.

Step 2: (b) Classifying the boiling of a cup of water

Boiling water involves the change of state from liquid to gas (vapor). There is no change in the chemical composition of water during boiling; H2O remains H2O in both liquid and vapor forms. So, boiling a cup of water is a physical process.

Step 3: (c) Classifying the pulverizing of an aspirin

Pulverizing an aspirin involves breaking the aspirin into smaller pieces or powder, which is a change in its physical form. There is no change in the chemical composition of aspirin during pulverization. So, pulverizing an aspirin is a physical process.

Step 4: (d) Classifying the digesting of a candy bar

Digesting a candy bar involves various chemical reactions occurring in the body to break down the candy bar into simpler substances like glucose and amino acids, which can be further utilized by the body. This process involves a change in the chemical composition of the candy bar constituents. So, digesting a candy bar is a chemical process.

Step 5: (e) Classifying the exploding of nitroglycerin

The exploding of nitroglycerin comprises the rapid conversion of nitroglycerin into other substances, such as nitrogen, carbon dioxide, and water vapor, through a chemical reaction. The chemical composition of nitroglycerin changes during the explosion. So, the exploding of nitroglycerin is a chemical process.

Key Concepts

Chemical Change

Chemical changes are fascinating transformations where substances convert into new ones. This means the composition of the original material is altered to form something entirely different. Unlike physical changes, chemical changes usually involve a reaction, breaking and forming of bonds, and they are often irreversible.
For example:

• Rusting of a metal can: Oxygen and moisture react with metal, forming rust, which is a new substance.
• Digesting a candy bar: Chemical processes in our bodies break down the candy into nutrients.
• Exploding nitroglycerin: It involves rapid decomposition into gases like nitrogen and carbon dioxide.

Notice that these examples involve formation of new substances and are accompanied by new properties, such as color change in rusting or release of gases during explosions.

Physical Change

Physical changes are processes that affect the form of a chemical substance but not its chemical composition. These changes are mostly reversible and do not result in new substances. Instead, they may alter the shape, state, or appearance of the material.
Consider the following:

• Boiling water: It changes from liquid to gas, but remains H\(_2\)O.
• Pulverizing aspirin: The pill becomes powder, yet its composition is unchanged.

These examples highlight how the substance itself remains unchanged at a molecular level, with only the physical state or form being altered.

States of Matter

The states of matter refer to the distinct physical forms that different phases of matter take on. The main states are solid, liquid, and gas, each having distinct characteristics and behaviors.

• Solids: Have a fixed shape and volume due to tightly packed particles.
• Liquids: Have a fixed volume but can change shape, adapting to their container due to less tightly packed particles.
• Gases: Have neither fixed shape nor volume. They expand to fill their container due to particles being spread out.

The boiling of water is a perfect example that illustrates the transition between the states of matter, changing from a liquid to a gas without altering the chemical structure of H\(_2\)O.

Chemical Reactions

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions involve making and breaking of bonds, resulting in the formation of new products with different properties from the reactants.
Examples of chemical reactions include:

• Combustion, such as the explosion of nitroglycerin, where rapid oxidation occurs.
• Decomposition, like the breakdown during digestion, where complex molecules are split into simpler ones.
• Oxidation, seen in rusting, where metals react with oxygen to form oxides.

During these processes, bonds between atoms are either formed or broken, resulting in new chemical substances with distinctive properties. Chemical reactions often involve energy changes, such as the release or absorption of heat.