Question

Which observation is consistent with a chemical reaction occurring? Why? (a) Propane forms a flame and emits heat as it burns. (b) Acetone feels cold as it evaporates from the skin. (c) Bubbling occurs when potassium carbonate and hydrochloric acid solutions are mixed. (d) Heat is felt when a warm object is placed in your hand.

Short answer

(c) Bubbling occurs when potassium carbonate and hydrochloric acid solutions are mixed. This is because the formation of gas bubbles is a common indicator that a chemical reaction is taking place, leading to the production of new substances.

Step-by-step solution

- Understand Chemical Reaction Indicators

A chemical reaction often has several indicators such as a change in temperature, emission of a gas, formation of a precipitate, or a color change. These observed changes suggest that a new substance has been formed due to the re-arrangement of atoms.

- Analyze Each Observation

Examine each observation to determine if it is consistent with known indicators of a chemical reaction. (a) Propane forming a flame and emitting heat suggests a chemical reaction because new substances are produced (carbon dioxide and water) and energy is released. (b) Acetone cooling the skin as it evaporates is a physical change because it involves a phase change without altering the chemical's composition. (c) Bubbles forming when potassium carbonate and hydrochloric acid are mixed indicates a gas is released, suggesting a chemical reaction. (d) Heat being transferred from a warm object to your hand is not a chemical change but the transfer of thermal energy.

- Identify the Observation That Indicates a Chemical Reaction

After analyzing the observations, (a) and (c) indicate chemical reactions. However, the question asks for only one observation, so both observations need to be critically evaluated on which is the more indicative of a chemical reaction. In (a), the combustion of propane is a classic example of a chemical reaction. Observation (c), where bubbling occurs due to a gas being released, explicitly demonstrates a chemical reaction between potassium carbonate and hydrochloric acid forming carbon dioxide. Since both exhibit clear signs of a chemical reaction, for this exercise, one needs to be chosen as the answer. Typically, gas formation, such as in (c), is a more direct observation demonstrating a reaction.

Key concepts

Chemical vs Physical Changes

Understanding the difference between chemical and physical changes is crucial when observing transformations in matter. A physical change simply affects the physical properties of a substance without altering its intrinsic chemical structure. Examples include changes in state (like ice melting to water) or the process of a substance, such as acetone, evaporating from the skin. No new substances are created during physical changes; instead, the existing substance might look or feel different.

On the other hand, a chemical change results in the formation of new chemical substances. When propane burns, for instance, it reacts with oxygen in a combustion reaction to form new substances, specifically carbon dioxide and water, releasing energy in the form of heat and light. Chemical changes are marked by the rearrangement of atoms and often result in a change in color, emission of odor, production of heat, light or sound, or the formation of a precipitate or gas.

Observation of Chemical Reactions

Observing chemical reactions involves looking for evidence that indicates the formation of new substances. When potassium carbonate and hydrochloric acid mix, and we see bubbling, we are actually witnessing the release of carbon dioxide gas. This is a direct sign that a chemical reaction is taking place. Understanding such

Observable Indicators of Chemical Reactions

can include several phenomena:

• Change in color (e.g., silver tarnishing)
• Emission or absorption of heat (e.g., hand warmers)
• Production of light (e.g., glow sticks)
• Formation of a precipitate (e.g., forming a cloudy mixture)
• Release of gas (e.g., effervescence in acid-base reactions)
• Change in odor (e.g., production of hydrogen sulfide with its characteristic rotten egg smell)

Seeing, smelling, or feeling these changes helps us to identify when and how a chemical reaction occurs.

Chemical Reaction Evidence

Evidence of a chemical reaction can be decisive in experiments.

Endothermic vs Exothermic Reactions

They are two classic types of chemical reactions that exhibit evidence through temperature change. Exothermic reactions release heat to the surroundings, as in the burning of propane, which is consistent with a chemical change. In contrast, endothermic reactions absorb heat, feeling cold to the touch; however, a sensation of cold alone (as with evaporating acetone) is not enough to conclude a chemical reaction.

It is vital to differentiate between heat being produced as a result of new substances forming and heat that is merely transferred, which can be confused with a physical change. For example, placing a warm object in your hand does not cause any transformation within the object's chemical structure – it simply transfers heat to your hand, indicating a physical process rather than a chemical reaction.