Problem 58 - Chemistry: Principles and Reactions

Question

The following data refer to the compound water. Classify each as a chemical or a physical property. (a) It is a colorless liquid at \(25^{\circ} \mathrm{C}\) and \(1 \mathrm{~atm}\). (b) It reacts with sodium to form hydrogen gas as one of the products. (c) Its melting point is \(0^{\circ} \mathrm{C}\). (d) It is insoluble in carbon tetrachloride.

Step-by-Step Solution

Answer

Question: Classify the following properties of water as either chemical or physical properties: (a) Colorless liquid at \(25^{\circ} \mathrm{C}\) and \(1 \mathrm{~atm}\). (b) Reacts with sodium to form hydrogen gas as one of the products. (c) Melting point is \(0^{\circ} \mathrm{C}\). (d) Insoluble in carbon tetrachloride. Answer: (a) Physical, (b) Chemical, (c) Physical, (d) Physical.

Step 1: (Step 1: Analyze Property (a))

To determine if this property is chemical or physical, consider whether it requires a chemical reaction to be observed. The color and state of water at \(25^{\circ} \mathrm{C}\) and \(1 \mathrm{~atm}\) can be observed without any chemical change, so this is a physical property.

Step 2: (Step 2: Analyze Property (b))

The reaction of water with sodium to form hydrogen gas as one of the products requires a chemical reaction to occur. Therefore, this property is a chemical property.

Step 3: (Step 3: Analyze Property (c))

The melting point of a substance is a property that can be determined without causing a chemical change, as only the state of the substance changes from solid to liquid. Hence, the melting point of water is a physical property.

Step 4: (Step 4: Analyze Property (d))

Solubility is a property that is determined by observing if a substance dissolves in another substance. In this case, we are looking at whether water is soluble in carbon tetrachloride, which can be determined without causing any chemical change to water. Thus, this is a physical property. In conclusion, the properties of water can be classified as follows: (a) Physical property - colorless liquid at \(25^{\circ} \mathrm{C}\) and \(1 \mathrm{~atm}\). (b) Chemical property - reacts with sodium to form hydrogen gas as one of the products. (c) Physical property - melting point is \(0^{\circ} \mathrm{C}\). (d) Physical property - insoluble in carbon tetrachloride.

Key Concepts

Physical Properties

Physical properties of water are aspects that can be observed or measured without altering the chemical structure of water itself. A simple example is water being a colorless liquid at standard room temperature and pressure, which is 25°C and 1 atm. This observation can be directly made with our senses and doesn't involve inducing any chemical reaction. Another physical property is the state of water, which under typical conditions is a liquid but varies depending on the temperature and pressure. To understand physical properties:

• They are detectable without changing the material's chemical identity.
• They include states like liquid, solid, or gas.
• They also cover boiling points, color, and odor.

You can think of these properties as the observable characteristics that make up the everyday experience of water.

Chemical Properties

Chemical properties of water come into focus when a chemical reaction occurs, involving changes in the chemical structure. Like when water reacts with sodium, producing hydrogen gas and sodium hydroxide. This reaction exemplifies a significant change because new substances are formed, indicating a chemical property. Some important characteristics of chemical properties include:

• They involve the transformation of substances into different ones.
• They are recognizable during a chemical reaction.
• They often involve the release or absorption of energy.

Remember, these properties tell us about the potential chemical changes water can undergo, revealing important insights into its reactivity and interaction with other chemicals.

Solubility

Solubility refers to the capability of a substance to dissolve in a solvent. Water's solubility determines how it interacts with other substances when mixing them. In the exercise provided, water is described as insoluble in carbon tetrachloride, a type of halogenated hydrocarbon, meaning it does not mix well at all. Solubility's key features include:

• It determines how substances interact at the molecular level.
• It describes how much of a substance can be dissolved until saturation.
• It can change with temperature and pressure.

Understanding solubility can help explain why some mixtures work as solutions and others don't. Water's effectiveness as a solvent makes it vitally important in biological, chemical, and industrial processes.

Melting Point

The melting point of a substance is the temperature at which it changes its state from solid to a liquid. For water, this occurs at 0°C under normal atmospheric pressure. The melting point is a critical physical property that does not involve breaking or reforming chemical bonds. It reflects the energy required to loosen the molecular structure of ice into liquid water. Here’s what to remember about melting points:

• It is an indication of the purity and the energy stability within the material's structure.
• It involves a physical change, not a chemical one.
• It is sensitive to environmental conditions like pressure and presence of solids.

The melting point of water is a fundamental concept, especially in understanding processes like phase changes, and it plays a crucial role in the environmental thermodynamics and climate studies.