Question

Classify each change as physical or chemical. (a) A balloon filled with hydrogen gas explodes upon contact with a spark. (b) The liquid propane in a barbecue evaporates away because someone left the valve open. (c) The liquid propane in a barbecue ignites upon contact with a spark. (d) Copper metal turns green on exposure to air and water.

Short answer

(a) Chemical change, (b) Physical change, (c) Chemical change, (d) Chemical change.

Step-by-step solution

Understand Physical and Chemical Changes

Recognize that a physical change is a change in the state or properties of matter without any accompanying change in its chemical composition. A chemical change (chemical reaction) involves making or breaking bonds between atoms, resulting in the formation of different substances with new properties.

Analyzing Each Scenario Individually

(a) Examine if the balloon explosion involves a new substance being formed. (b) Determine whether the evaporation of liquid propane is a change in state or a chemical composition. (c) Check if the ignition of propane causes the formation of new substances. (d) Investigate if copper changing color involves new substances being formed due to a reaction with elements in the air and water.

Classifying Each Change

(a) The hydrogen in the balloon reacts violently upon contact with a spark, forming water, thus it's a chemical change. (b) The evaporation of propane is a transition from liquid to gas without forming a new substance, so it's a physical change. (c) Igniting propane leads to combustion, creating carbon dioxide and water, which makes it a chemical change. (d) Copper turning green is due to the formation of copper carbonate as a result of its reaction with air and water, this is a chemical change.

Key concepts

Chemical Change

Do substances transform into entirely new materials during certain processes? This is a hallmark of a chemical change, one of the most fascinating occurrences in the field of chemistry. Imagine baking a cake; once you've mixed and baked your ingredients, you cannot retrieve the original flour, eggs, or sugar. This symbolizes a chemical change, where substances react and create new compounds with new properties.

In the example of the hydrogen balloon from a textbook exercise, upon contact with a spark, the hydrogen reacts to form water (\( H_2O \)), which is an iconic chemical change. Similarly, when copper metal turns green due to exposure to air and water, it forms copper carbonate, indicating a chemical change. These changes are irreversible under ordinary conditions, showcasing the permanency that often accompanies chemical transformations.

Physical Change

When discussing changes in matter, it's imperative to differentiate between chemical and physical changes. Physical changes occur when the form of matter is altered without a chemical reaction, such as changing the state, shape or size, while maintaining the substance's underlying identity. Melting ice to water or breaking a glass are simple examples of physical changes.

Pertaining to our textbook scenario, the evaporation of propane in a barbecue is an exemplary physical change. The propane merely shifts from liquid to gas form, without altering its molecular structure. This type of change is typically reversible, simply cooling the vapor could return it to its liquid state, underlining the non-permanent nature of physical changes.

Chemical Reactions

Chemical reactions are events that involve the transformation of one set of chemical substances into another. They are the processes underlying chemical changes, where the reactants (the starting materials) go through a chemical reaction to become products (new substances). These reactions can manifest in various ways such as color changes, temperature changes, gas production, or precipitate formation.

When examining our textbook exercise, the ignition of propane is a quintessential chemical reaction, where propane combines with oxygen to produce carbon dioxide and water, releasing energy in the form of heat and light. Chemical reactions are governed by the laws of conservation of mass and energy, ensuring that no atoms are lost or gained, just rearranged.

States of Matter

Matter exists fundamentally in different forms, known as the states of matter. Traditionally, we categorize these states as solid, liquid, and gas, but there are also others like plasma and Bose-Einstein condensates. Each state has distinctive characteristics based on the arrangement and movement of its particles.

A solid has a fixed shape and volume due to its particles being closely packed in a regular pattern. Liquids have a definite volume but take the shape of their container because their particles are close but can move past one another. Gases, on the other hand, do not have a fixed shape or volume and will expand to fill any container as their particles move fast and freely. The conversion between these states, such as the evaporation or condensation of water, is a central concept in understanding physical changes in matter.