Problem 33 - Chemistry and Chemical Reactivity

Question

Identify the following as either physical changes or chemical changes. (a) Dry ice (solid \(\mathrm{CO}_{2}\) ) sublimes (converts directly from solid to gaseous \(\mathrm{CO}_{2}\) ). (b) Mercury's density decreases as the temperature increases. (c) Energy is given off as heat when natural gas (mostly methane, \(\mathrm{CH}_{4}\) ) burns. (d) NaCl dissolves in water.

Step-by-Step Solution

Answer

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

Step 1: Understanding Physical and Chemical Changes

A physical change involves altering a substance's physical properties without changing its composition, such as melting or dissolving. A chemical change results in the formation of one or more new substances through a chemical reaction, like burning or rusting.

Step 2: Analyze Dry Ice Sublimation

Dry ice sublimes when it converts from solid carbon dioxide (\(\mathrm{CO}_{2}\)) directly to its gaseous form. This process changes the state of the substance but not its chemical composition. Thus, sublimation of dry ice is a physical change.

Step 3: Evaluate Mercury's Density Change with Temperature

When mercury's density decreases as temperature increases, it means that mercury is simply expanding, affecting its physical attribute (density). However, its chemical structure remains unaltered. Changing density with temperature is a physical change.

Step 4: Examine the Burning of Natural Gas

Burning natural gas (mostly \(\mathrm{CH}_{4}\)) results in new substances - carbon dioxide and water, accompanied by the release of heat. This process involves breaking and forming chemical bonds and thus is a chemical change.

Step 5: Determine the Nature of Dissolving NaCl

When NaCl dissolves in water, it dissociates into sodium and chloride ions. While this process involves interaction with water molecules, it does not transform the chemical structure of NaCl; hence, it is considered a physical change.

Key Concepts

Sublimation

Sublimation is a fascinating phase transition. It happens when a solid changes directly into a gas without passing through the liquid state. Maybe you've seen dry ice, which is solid carbon dioxide ( CO_2 ). When left out at room temperature, it sublimes and turns directly into gaseous CO_2 .
This transition occurs because the molecules in the solid gain enough energy to overcome their rigid structure and escape as gas.
It's important to realize that sublimation, though a dramatic change in state, does not alter the innate composition of the substance itself. The CO_2 molecules remain CO_2 throughout. As such, sublimation is classified as a physical change. Here are some key points about sublimation:

• Direct transition from solid to gas.
• No liquid state involved.
• No change in chemical composition.
• Considered a physical change.

Density and Temperature

The relationship between density and temperature is interesting and straightforward. Density refers to how much mass is contained in a given volume.
As a substance heats up, its particles gain energy and begin moving more vigorously, causing them to take up more space and resulting in an increase in volume. This is why substances like mercury expand when heated.
Even though the substance's mass stays constant, the increase in volume leads to a decrease in density. These changes affect physical properties only, without altering chemical makeup. With mercury:

• Density decreases as temperature increases.
• Mass remains constant as volume increases.
• It's a physical change since the substance's chemical identity remains unchanged.
• Common in liquids and gases.

Burning of Natural Gas

Burning natural gas is an excellent example of a chemical reaction. Natural gas is primarily composed of methane ( CH_4 ), which reacts with oxygen in the air to produce carbon dioxide ( CO_2 ), water ( H_2O ), and energy in the form of heat.
This transformation involves breaking and forming chemical bonds, leading to the creation of new substances which are chemically different from the reactants. Consequently, burning natural gas is a chemical change. Let's outline the key characteristics of this process:

• Methane and oxygen react to form CO_2 and H_2O .
• Heat energy is released during the reaction.
• Involves chemical bonds breaking and forming.
• Represents a chemical change.

Dissolution of NaCl

Dissolving is a common physical change, and NaCl (table salt) dissolving in water showcases this well. When NaCl is added to water, it dissociates into its constituent sodium (Na^+) and chloride (Cl^-) ions.
These ions become surrounded by water molecules, but the NaCl does not transform into any new substances chemically. The dissolution process involves dispersion of the ions in water, preserving their original chemical makeup. Therefore, the dissolution of NaCl is considered a physical change. Important notes on this process include:

• NaCl dissociates into Na^+ and Cl^- ions.
• No formation of new chemical substances occurs.
• The chemical structure of NaCl remains unchanged.
• It is a physical change.