Pure Substances and Property Tables | Thermodynamics | (Solved Examples)

Summary

This video explains the fundamentals of pure substances, phases of matter, and the use of property tables in thermodynamics. It covers concepts like phase changes (liquid to vapor), saturation temperature, and pressure, and introduces important terms like compressed liquids, saturated liquids, and superheated vapors. The video also discusses how to use property tables to determine values such as specific volume, internal energy, and enthalpy. It includes practical examples for calculating properties of water and other substances under different conditions, helping viewers understand how to apply thermodynamic principles in real-world scenarios.

Takeaways

• 😀 A pure substance has a fixed chemical composition and can exist as a single element or compound. Example: water.
• 😀 Air is considered a pure substance because it has a uniform chemical composition, despite being a mixture of gases.
• 😀 Phases of matter (solid, liquid, gas) can change with temperature and pressure, and these transitions are important for thermodynamics.
• 😀 A saturated liquid is a liquid about to vaporize, while a saturated vapor is vapor about to condense.
• 😀 Superheated vapor is vapor that is not about to condense, while compressed liquid behaves like a saturated liquid at a given temperature.
• 😀 The boiling temperature of water increases as pressure increases. This relationship is critical for phase change calculations.
• 😀 Saturation temperature and saturation pressure are used to describe phase changes, and these are listed in property tables for pure substances.
• 😀 The property table includes values for specific volume, internal energy, enthalpy, and other thermodynamic properties at different phases.
• 😀 Quality (x) is the ratio of vapor mass to total mass in a saturated mixture, and it can be used to find average volume and energy.
• 😀 To identify whether a substance is in a superheated vapor or compressed liquid state, compare its pressure and temperature to saturation values.
• 😀 Example problems demonstrate how to use property tables to calculate values like enthalpy, pressure, and specific volume for substances in different phases.

Q & A

What is a pure substance, and can it be made up of more than one chemical element?

A pure substance is defined as having a fixed chemical composition. It doesn't need to be a single chemical element. For example, water is a pure substance because it has a uniform chemical composition, even though it's made of hydrogen and oxygen.

Why is air considered a pure substance despite being a mixture?

Air is considered a pure substance because it consists of a mixture of gases that are chemically homogeneous, meaning the chemical composition is uniform throughout, despite being made up of multiple gases.

What happens to water when it is heated in a sealed container at a constant pressure?

When water is heated in a sealed container at constant pressure, it first behaves as a compressed liquid. As the temperature rises, it reaches a point where the liquid is about to vaporize, known as a saturated liquid. Once boiling begins, the temperature remains constant until all water has vaporized.

What is the difference between a saturated liquid and a saturated vapor?

A saturated liquid is a liquid that is about to vaporize, while a saturated vapor is a vapor that is about to condense. These two phases exist in equilibrium at specific temperatures and pressures.

How does pressure affect the boiling temperature of water?

The boiling temperature of water increases with increasing pressure. For example, if the pressure is raised to 600 kPa, the boiling point of water increases to 158.83°C. This is because the boiling point is dependent on the pressure exerted on the substance.

What is a property table, and how is it used in thermodynamics?

A property table lists the thermodynamic properties of pure substances at various temperatures and pressures. These tables provide information on specific volume, internal energy, enthalpy, and other important values that are used to analyze and solve thermodynamics problems.

What does 'quality' mean in the context of a saturated liquid-vapor mixture?

'Quality' refers to the ratio of the mass of vapor to the total mass of the mixture in a saturated liquid-vapor system. It can range from 0 (saturated liquid) to 1 (saturated vapor), helping to quantify the phase state of the mixture.

How do you calculate the average volume of a saturated liquid-vapor mixture?

The average volume of a saturated liquid-vapor mixture can be calculated using the equation: specific volume = saturated liquid volume + quality * (saturated vapor volume - saturated liquid volume). This equation helps find the average specific volume of the mixture.

What is the difference between superheated vapor and compressed liquid?

Superheated vapor is a vapor that is not about to condense, usually found at lower pressures or higher temperatures. Compressed liquid, on the other hand, refers to liquid that is at a temperature below its saturation temperature but still at a pressure higher than the saturation pressure for that temperature.

How do you use property tables to find specific enthalpy and internal energy for a substance like R134a?

To find the specific enthalpy and internal energy for a substance like R134a, you first need to identify the specific temperature or pressure. Using property tables for R134a, you can then locate values for specific volume, internal energy, and enthalpy. If the substance is a mixture, you can calculate the quality and use it to find the corresponding enthalpy or internal energy.