Question

Classify each change as physical or chemical. a. Natural gas burns in a stove. b. The liquid propane in a gas grill evaporates because the valve was left open. c. The liquid propane in a gas grill burns in a flame. d. A bicycle frame rusts on repeated exposure to air and water.

Short answer

a. Chemical change. b. Physical change. c. Chemical change. d. Chemical change.

Step-by-step solution

Define Physical and Chemical Change

Identify the definitions of physical and chemical changes. A physical change alters the form or appearance of matter but does not turn any substance in the matter into a different substance. A chemical change, on the other hand, occurs when one or more substances are changed into entirely new substances with different properties.

Analyze Natural Gas Burning

Determine if natural gas burning in a stove is a physical or chemical change. Burning is a clear indicator of a chemical change, as it results in new substances being formed, such as carbon dioxide and water from the combustion of methane in natural gas.

Evaluate Propane Evaporation

Assess whether the evaporation of liquid propane is a physical or chemical change. Evaporation is a change of state from liquid to gas and does not produce a new substance. Thus, it is considered a physical change.

Assess Propane Burning

Examine the process of propane burning in a flame to determine the type of change. Similar to natural gas burning, the combustion of propane is a chemical change as it entails the transformation of the propane into carbon dioxide and water, among other products.

Determine Rust Formation

Analyze the rusting of a bicycle frame to classify the change. Rusting involves iron reacting with oxygen and water to form iron oxide (rust), which is a new compound with different properties than the original substances. This process is a chemical change.

Key concepts

Combustion

The process of combustion typically involves a substance, often a hydrocarbon, reacting rapidly with oxygen to release energy in the form of heat and light. This exothermic reaction is a classic example of a chemical change because it results in the formation of new substances. For instance, when natural gas (primarily methane, CH4) burns, it reacts with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O), two entirely new compounds. The natural gas is no longer the same at a chemical level after combustion, signaling a fundamental transformation characteristic of a chemical change.

To understand this further, let's consider energy changes. Combustion releases energy, indicating that the reactants (e.g. methane and oxygen) had more stored chemical energy than the products (e.g. carbon dioxide and water). This transition from higher to lower energy states is a hallmark of chemical reactions and is why combustion is used widely as an energy source for heating, cooking, in engines, and more.

Evaporation

Evaporation is the process whereby molecules in a liquid state gain enough energy to enter the gaseous state. It is important to note that during evaporation, no new substances are formed. The substance merely changes from one phase (liquid) to another (gas), which is indicative of a physical change. The molecules in liquid propane, for example, will escape into the air as gas when the valve on a gas grill is left open, but the propane remains chemically the same. It's just in a different state of matter.

Consider a water puddle drying up on a sunny day; the water evaporates due to the heat, but it is still water in the form of water vapor in the air. The ability for a substance to evaporate depends largely on its intermolecular forces and the surrounding conditions like temperature and pressure. These factors affect the rate at which evaporation occurs without altering the chemical composition of the substance.

Chemical Reaction

A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. During a chemical reaction, bonds between atoms break and new ones form, creating new products with properties distinct from those of the reactants. Combustion and rusting are both prime examples of this process. When propane from a grill burns, a series of reactions occur, breaking the bonds in the propane molecules and forming new compounds like carbon dioxide and water. Similarly, the rusting of a bicycle frame involves iron atoms reacting with oxygen in moisture to produce rust, a new compound with a different composition and color.

One important aspect is the conservation of mass, which states that matter can neither be created nor destroyed in a chemical reaction. This means that the total mass of the reactants equals the total mass of the products. However, the properties of the products are significantly different, which is what characterizes a chemical change.

State of Matter

Matter can exist in different states, primarily known as solid, liquid, gas, and plasma. These states differ in the arrangements and movements of particles. In solids, the particles are close together and vibrate in place, which gives solids a definite shape and volume. Liquids have particles that are less tightly packed than in a solid and can move past one another, giving liquids an indefinite shape but a definite volume. Gases, on the other hand, have particles that are much further apart and move freely, resulting in both an indefinite shape and volume.

Changes in state are typically physical changes because they only affect the form of the material and not its chemical identity. For example, the evaporation of water or propane involves a change from liquid to gas without a change in the chemical makeup of the substance. Understanding the state of matter is crucial when analyzing physical changes, such as melting, freezing, condensation, and vaporization.