Question

Consider the following equations. For each, decide whether it represents a chemical reaction or a physical change. (a) \(\mathrm{CO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(l) \longrightarrow \mathrm{H}_{2} \mathrm{CO}_{3}(a q)\) (b) \(\mathrm{H}_{2} \mathrm{O}(s) \longrightarrow \mathrm{H}_{2} \mathrm{O}(l)\) (c) \(\mathrm{HOCN}(g) \longrightarrow \mathrm{HCNO}(g)\)

Short answer

(a) Chemical reaction, (b) Physical change, (c) Chemical reaction

Step-by-step solution

Understanding the difference between Chemical reaction and Physical change

A chemical reaction consists in the reorganization of atoms present in the reacting species to form a new substance. In the chemical equations, the reactants (substances before the arrow) get converted to products (substances after the arrow) with different chemical properties. On the other hand, during a physical change, the substance can change its state but the chemical composition remains the same.

Analyzing equation (a) CO2(g) + H2O(l) → H2CO3(aq)

The given equation represents the combination of carbon dioxide (CO2) and water (H2O) to form carbonic acid (H2CO3). Since this process involves reorganization of atoms, and the chemical properties of the product are different from the reactants, this is considered as a chemical reaction.

Analyzing equation (b) H2O(s) → H2O(l)

In the given equation, the solid state of water (H2O) changes to its liquid state. Here, the water is only changing its physical state from solid to liquid, also known as melting, but its molecules do not change. Therefore, this represents a physical change.

Analyzing equation (c) HOCN(g) → HCNO(g)

In the given reaction, isocyanic acid (HOCN) is rearranging into fulminic acid (HCNO). Here, reorganization of atoms is happening and a molecule with different chemical properties is formed. Therefore, this represents a chemical reaction.

Key concepts

Chemical Equations

Let's unravel the mystery behind one of the central concepts in chemistry: the chemical equation. A chemical equation symbolizes the transformation of substances into different substances through a chemical process. Just like a math equation, it has reactants on one side and products on the other, balanced by the iconic arrow that signifies the direction of the change.

For example, when we see \(\mathrm{CO}_{2}(g) + \mathrm{H}_{2}O(l) \longrightarrow \mathrm{H}_{2}CO_{3}(aq)\), it's not just random letters and symbols thrown together. It tells us that carbon dioxide gas reacts with liquid water to form carbonic acid in aqueous solution—a whole new substance with properties distinct from either of the reactants. Changing subscripts, adjusting coefficients, and watching states of matter shift—these all play into the intricate dance of documenting a chemical reaction in the language of chemistry: the equation.

State Change

Imagine a cube of ice melting into a clear puddle of water. This is what we call a state change: a transformation in which a substance changes its physical form, like from solid to liquid, without altering its underlying chemical structure. It's all about energy input and molecules jostling into new arrangements.

When you encounter \(\mathrm{H}_{2}O(s) \longrightarrow \mathrm{H}_{2}O(l)\) in an equation, you're witnessing a textbook example of a state change, specifically the melting of ice. No chemical bonds are broken or formed here; the molecules remain as is, simply getting a little looser and moving into a liquid's flow. State changes are an essential concept for understanding the physical properties of substances and their behaviors under different conditions.

Atomic Reorganization

Diving deeper into the core of chemical changes, atomic reorganization is the hidden engine driving these transformations. It's what happens when atoms are shuffled and reshuffled, forming entirely new molecules with varying arrangements.

Consider the reaction \(\mathrm{HOCN}(g) \longrightarrow \mathrm{HCNO}(g)\). Here, the same atoms from a single molecule rearrange to form a different molecule. This is atomic reorganization at play, an invisible crafting at the molecular level that changes the identity of substances involved. It's a key concept because it distinguishes a chemical change from a mere physical change and it dictates the resulting chemical properties of the material.

Chemical Properties

Now, let's zoom in to the essence of substances: their chemical properties. These properties are intrinsic characteristics that become evident during a chemical reaction. They can tell us how a substance will behave in the presence of others and determine the potential outcomes of a reaction.

Properties like reactivity with acids, tendency to oxidize, or ability to combust are all chemical in nature. When chemical equations indicate the formation of new substances, as seen with \(\mathrm{HOCN}(g)\), they bring about changes to these chemical properties. The ability to detect and predict these changes is vital, as they fundamentally dictate the substance's new identity and how it can be utilized or should be handled.