Kinetic Theory and Phase Changes: Crash Course Physics #21

Summary

On November 14, 1971, NASA's Mariner 9 probe became the first spacecraft to orbit Mars, tasked with mapping the Martian surface. The mission faced a unique challenge in choosing a reference point for altitude, as Mars has no sea level. They chose the triple point of water, where liquid water can exist. The video explores the kinetic theory of gases, phase changes, and how these concepts relate to Mars exploration, including the critical and triple points, helping to identify locations where liquid water might have existed and where life could potentially survive.

Takeaways

• 😀 Mariner 9, launched by NASA in 1971, became the first spacecraft to orbit another planet: Mars.
• 😀 The Mariner 9 mission used the triple point of water (6.105 millibars) as a reference point for Martian altitude mapping.
• 😀 The triple point of water is the pressure and temperature at which water can exist as solid, liquid, and gas at the same time.
• 😀 The reference point chosen for mapping Mars is significant because it marks the pressure at which liquid water could exist—key in the search for life on Mars.
• 😀 The kinetic theory of gases explains how temperature is related to the average kinetic energy of gas molecules.
• 😀 Temperature increases the kinetic energy of gas molecules, which is mathematically tied to their average velocity and speed.
• 😀 The root mean square (RMS) speed of gas molecules is a key value used to describe their typical velocity in a given environment.
• 😀 The RMS speed is calculated using the equation √(3 * k * T / m), where k is Boltzmann’s constant, T is temperature, and m is the molecule's mass.
• 😀 At high pressures or low temperatures, gases deviate from ideal behavior, with molecular interactions starting to affect the gas's volume and properties.
• 😀 Sublimation occurs when a substance transitions directly from solid to gas, and deposition is when it goes from gas to solid—both of which occur below a substance’s triple point.

Q & A

What was the primary mission of NASA's Mariner 9 probe?

The primary mission of NASA's Mariner 9 probe was to map the Martian surface and take photographs to better understand Mars.

Why did NASA engineers have to find a new reference point for measuring altitude on Mars?

NASA engineers had to find a new reference point because Mars does not have seas, which is typically used as a baseline for altitude measurements on Earth.

What pressure did NASA choose as a reference point for Martian altitude?

NASA chose the pressure at which liquid water can exist, specifically 6.105 millibars, as the reference point for Martian altitude.

Why can't liquid water exist at very low pressures?

Liquid water cannot exist at very low pressures because molecules don’t have enough energy to stay in the liquid phase, causing them to either be in a solid or gas state instead.

What is the kinetic theory of gases?

The kinetic theory of gases explains that gas molecules are in constant motion and that the average kinetic energy of the molecules is proportional to the temperature of the gas.

How does the temperature of a gas affect its kinetic energy?

As the temperature of a gas increases, the kinetic energy of its molecules also increases, leading to higher speeds and more energetic collisions.

What is the difference between the average speed of gas molecules and the root mean square speed?

The root mean square speed (v_RMS) is the square root of the average squared velocity of gas molecules, which is different from the average speed and represents a more accurate measure of how fast molecules are moving on average.

How is the typical speed of gas molecules calculated?

The typical speed of gas molecules can be calculated using the formula: square root of (3 × k × temperature) / mass, where k is the Boltzmann constant.

What is a phase diagram and how is it useful in understanding substances like water?

A phase diagram shows the states (solid, liquid, gas) of a substance under different temperatures and pressures. It helps to identify the conditions under which a substance will change phases, such as the triple point or critical point.

Why is the triple point of water important for defining the Kelvin scale?

The triple point of water is used to define the Kelvin temperature scale because it is a reproducible and consistent point where water exists as a solid, liquid, and gas at the same time, and its temperature is set to 273.16 Kelvin by definition.

What is sublimation and when does it occur?

Sublimation is the process in which a substance transitions directly from a solid to a gas without passing through the liquid phase. It occurs when the pressure and temperature conditions are below the substance's triple point.

How did Mariner 9 use the triple point pressure to map Mars?

Mariner 9 used the triple point pressure of water (6.105 millibars) as a reference to map out regions of Mars where liquid water could potentially exist, helping scientists identify areas where life could have survived.

What does the critical point of a substance represent?

The critical point represents the maximum temperature and pressure at which a substance can exist as a liquid. Above this point, the substance cannot condense into a liquid, no matter the pressure.