Problem 15 - Chemistry The Science in Context

Question

Indicate whether each of the following properties is a physical or a chemical property of the element sodium: a. Its density is greater than that of kerosene and less than that of water. b. It has a lower melting point than that of most metals. c. It is an excellent conductor of heat and clectricity. d. It is soft and can be easily cut with a knife. e. Freshly cut sodium is shiny, but it rapidly tarnishes in contact with air. I. It reacts very vigorously with water to form hydrogen gas \(\left(\mathrm{H}_{2}\right)\) and sodium hydroxide \((\mathrm{NaOH})\).

Step-by-Step Solution

Answer

Question: Classify the given properties of sodium as either physical or chemical. a) Density b) Melting Point c) Conductor of heat and electricity d) Softness e) Tarnishing f) Reaction with water Answer: Physical Properties: a) Density, b) Melting Point, c) Conductor of heat and electricity, d) Softness Chemical Properties: e) Tarnishing, f) Reaction with water

Step 1: Property a: Density

Property a states that the density of sodium is between that of kerosene and water. Density is a physical property because it describes the mass of a substance relative to its volume without undergoing any chemical change.

Step 2: Property b: Melting Point

Property b states that sodium has a lower melting point than most metals. The melting point is a physical property because it reflects the temperature at which a solid substance changes to a liquid without any chemical change taking place.

Step 3: Property c: Heat and Electricity Conductor

Property c states that sodium is an excellent conductor of heat and electricity. This is also a physical property, as it describes how well the substance conducts energy without undergoing any chemical alterations.

Step 4: Property d: Softness

Property d states that sodium is soft and can be easily cut with a knife. Softness is a physical property because it describes the resistance to deformation, which can be observed without changing the substance's identity.

Step 5: Property e: Tarnishing

Property e states that freshly cut sodium is shiny but tarnishes rapidly when in contact with air. Tarnishing is a chemical property because it involves the reaction of sodium with oxygen in the air to form a new compound: sodium oxide.

Step 6: Property f: Reaction with Water

Property f states that sodium reacts very vigorously with water to form hydrogen gas (\(\mathrm{H}_{2}\)) and sodium hydroxide (\(\mathrm{NaOH}\)). This is a chemical property because it involves the reaction of sodium with water to form new substances.

Key Concepts

Physical Property

Physical properties are characteristics of a substance that can be observed or measured without altering its identity. For sodium, these include attributes like density, melting point, and conductivity. Examining physical properties is like taking a closer look at the substance's inherent traits that are evident to our senses or can be determined through simple measurement.

• Density
• Melting Point
• Conductivity
• Softness

These properties provide useful information about how sodium can be used practically, without needing to change its basic composition.

Chemical Property

Chemical properties describe a substance's ability to participate in chemical reactions that involve its transformation into different substances. For sodium, one of the notable chemical properties is its reactivity with water.
When sodium reacts, it changes into new compounds, which is a hallmark of a chemical property. For example, when it tarnishes, it forms sodium oxide by reacting with oxygen in the air, and when it reacts with water, it produces sodium hydroxide and hydrogen gas.

Density

Density is a physical property that defines the mass per unit volume of a substance. For sodium, its density is less than water but more than kerosene. This means it will sink in kerosene and float on water.
Knowing the density of sodium helps understand its buoyancy and how it will interact with other substances in scenarios where these density differences come into play, like separating sodium from mixtures or designing storage solutions.

Melting Point

The melting point of a substance is the temperature at which it changes from a solid to a liquid. Sodium has a lower melting point than most metals, a fact that classifies it as a physical property. This property makes sodium easy to work with in processes requiring low-temperature thermal management.
The low melting point paves the way for its use in various industrial applications where moderate heat is utilized, ensuring sodium remains in its solid form until needed.

Conductivity

Conductivity refers to a material's ability to conduct electricity or heat. For sodium, it is an excellent conductor, making it an ideal element in electrical wiring and thermal exchange systems.

• Electric Conductivity: Sodium's ability to allow electrons to flow through easily makes it valuable in creating conductive materials.
• Thermal Conductivity: This enables sodium to efficiently transfer heat, aiding its use in cooling systems and various heat exchange applications.

These conductive properties are essential traits that capitalize on sodium's efficiency as a conductor, helping to improve designs in the fields of electronics and engineering.