Problem 75 - 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 white chemical compound has a mass of \(1.456 \mathrm{~g}\). When placed in water containing a dye, it causes the red color of the dye to fade to colorless. (b) A sample of lithium metal, with a mass of \(0.6 \mathrm{~g}\), was placed in water. The metal reacted with the water to produce the compound lithium hydroxide and the element hydrogen.

Step-by-Step Solution

Answer

(a) Quantitative physical; Qualitative chemical. (b) Quantitative physical; Qualitative chemical.

Step 1: Analyze Statement (a)

In statement (a), "A white chemical compound has a mass of \(1.456 \mathrm{~g}\)" is quantitative because it provides a numeric measurement. The mass relates to a physical property because it describes the substance's measurable characteristic without changing its chemical identity. "It causes the red color of the dye to fade to colorless" is qualitative because it describes a change observed with senses (color change) but provides no numeric value. It relates to a chemical property because it describes a change in identity resulting from a chemical reaction.

Step 2: Analyze Statement (b)

In statement (b), "A sample of lithium metal, with a mass of \(0.6 \mathrm{~g}\)," is quantitative because it provides a numeric measurement. The mass relates to a physical property as the mass can be measured without altering the chemical makeup of lithium. "The metal reacted with the water to produce the compound lithium hydroxide and the element hydrogen" is qualitative because it describes what happens without numeric data, and it relates to a chemical property because it involves a chemical reaction, indicating a change in substance.

Key Concepts

Quantitative Analysis

Quantitative analysis involves the measurement of specific numerical values associated with a substance or material. In chemistry, quantitative data is crucial because it provides exact information about the masses, volumes, or concentrations of substances. For instance, when you know a compound's mass to be precisely \(1.456 \mathrm{~g}\), you're dealing with quantitative data.
You can measure:

• Mass (e.g., \(1.456 \mathrm{~g}\))
• Volume
• Concentration

These measurements allow scientists to calculate and predict various outcomes within experimental setups, making quantitative analysis an essential tool in scientific investigation.

Qualitative Analysis

Qualitative analysis, unlike quantitative analysis, does not involve numerical measurements. Instead, it focuses on descriptions and characteristics determined by the senses. Observing changes such as color, texture, form, and smell fall under qualitative analysis. For instance, "the red color of the dye fading to colorless" is a qualitative observation.
Qualitative analysis helps identify substances and their properties, which are not immediately measurable in numerical terms:

• Color change (e.g., red to colorless)
• Formation of gas
• Observing physical changes like dissolving or precipitating

This type of analysis is valuable in identifying substances and understanding their interactions and transformations.

Physical Property

Physical properties are characteristics of a substance that can be observed or measured without altering the chemical identity of the substance. For example, the mass of a substance, its melting point, boiling point, and density are all physical properties. These properties can help in identifying substances, determining purity, and predicting how it might behave under different physical conditions.
Key physical properties include:

• Mass
• Volume
• Density
• State (solid, liquid, gas)

When a compound's mass is given, as in "a compound has a mass of \(0.6 \mathrm{~g}\)", it refers to a physical property because the mass measurement does not involve changing the substance's chemical make-up.

Chemical Reaction

Chemical reactions involve transformations where one or more substances change into new substances. These changes indicate chemical properties and often involve changes in energy, color, temperature, and the formation of new compounds or elements. For example, when lithium reacts with water to form lithium hydroxide and hydrogen, it demonstrates a chemical reaction.
Key indicators of a chemical reaction include:

• Change in color
• Formation of precipitate
• Gas evolution
• Temperature change

Chemical reactions are essential for understanding how different substances interact and transform, which can aid in developing new materials, processes, and products.