Problem 76 - Chemistry The Molecular Science

Question

Classify the information in each of these statements as quantitative or qualitative and as relating to a physical or chemical property. (a) A liter of water, colored with a purple dye, was passed through a charcoal filter. The charcoal adsorbed the dye, and colorless water came through. Later, the purple dye was removed from the charcoal and retained its color. (b) When a white powder dissolved in a test tube of water, the test tube felt cold. Hydrochloric acid was then added, and a white solid formed.

Step-by-Step Solution

Answer

(a) Qualitative and physical property; (b) Qualitative and chemical property.

Step 1: Analyze Statement (a)

In Statement (a), we need to classify the information as quantitative or qualitative. There is no numerical data or measurement given in the statement; it describes the process and outcome (color change and dye retention). Therefore, the information is qualitative. Next, we determine if it relates to a physical or chemical property. The dye passing through the filter and the color retention are related to physical changes and properties. So, this describes a physical property.

Step 2: Analyze Statement (b)

In Statement (b), start by classifying the information as quantitative or qualitative. The statement describes the dissolving of a powder and the feeling of coldness in the test tube; it does not provide numerical measurements, so it's qualitative. Now, classify it as a physical or chemical property. The dissolving process that causes a decrease in temperature is related to chemical change (endothermic reaction), and the formation of a white solid from hydrochloric acid indicates a chemical change. Thus, this describes a chemical property.

Key Concepts

Qualitative and Quantitative Analysis

Qualitative and quantitative analysis are methods used to study substances and physical or chemical phenomena. Qualitative analysis provides descriptive data that cannot be measured with numbers. It involves characteristics like color, texture, smell, or observations of processes or reactions. In the context of chemistry, it might describe how a substance behaves or looks during an experiment, as seen in both statements from the exercise. On the other hand, quantitative analysis deals with numerical measurements and data. This would involve measuring volume, temperature, or other quantifiable attributes. In the FIRST statement (a), the description of the color change and the retention of purple dye are qualitative, since they involve observations without specific numbers. Similarly, in the SECOND statement (b), the observation of the cold test tube upon dissolution is qualitative because there are no specific temperature measurements.

Physical Changes

A physical change is when a substance alters its form or appearance without a change in composition. These changes are usually reversible. Examples include:

• Changing states, like melting or freezing
• Changing shape or size by cutting or breaking
• Mixing substances without a reaction, like dye in water

The first statement (a) in the exercise illustrates a physical change. When the purple dye is filtered, only its presence and later its absence in water is noted. The charcoal filtration process does not inherently alter the chemical nature of the dye or the water; each component retains its original identity, emphasizing that this is purely a physical change.

Chemical Changes

Chemical changes involve substances undergoing a transformation that changes their chemical identity. This results in the formation of new chemical substances. Characteristics of chemical changes often include:

• Color change not due to mere mixing
• Formation of a precipitate
• Release or absorption of energy, often in the form of heat or light

In statement (b) of the exercise, several indicators of chemical changes are observed. When the white powder dissolves and the test tube feels cooler, it implies a chemical reaction (an endothermic reaction, which will be explained later). Later, the addition of hydrochloric acid creates a new white precipitate, which indicates that the original substances have chemically altered to form new compounds. This confirms the occurrence of chemical changes.

Endothermic Reactions

Endothermic reactions are a specific type of chemical reaction in which energy is absorbed from the surrounding environment. This typically results in a decrease in the temperature of the surroundings. Features of endothermic reactions include:

• Absorption of heat or energy
• Cooling of the surroundings

In statement (b) of the exercise, the test tube feels cold when the white powder dissolves in water. This signal an endothermic reaction. Energy is absorbed from the surrounding environment to facilitate the chemical process, causing the observed drop in temperature. Endothermic reactions are a vital aspect of many natural and industrial chemical processes.