What is Gay-Lussac's Law? | Simple Explanation for Class 7

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What is Gay-Lussac's Law (pressure-temperature relationship)?

Grade Level:

Class 7

Space Technology, EVs, Climate Change, Biotechnology, HealthTech, Robotics, Chemistry, Physics

Definition

What is it?

Gay-Lussac's Law states that for a fixed amount of gas in a sealed container, the pressure of the gas is directly proportional to its absolute temperature. This means if you increase the temperature, the pressure also increases, and vice-versa, as long as the volume stays the same.

Simple Example

Quick Example

Imagine a pressure cooker (Patila) used to cook dal. When you heat the cooker, the steam inside gets hotter (temperature increases). Because the steam cannot escape and the volume is fixed, the pressure inside the cooker builds up, causing the whistle to blow. This is Gay-Lussac's Law in action!

Worked Example

Step-by-Step

Let's say a gas in a sealed container has a pressure of 2 atm at a temperature of 300 K. If we increase the temperature to 450 K, what will be the new pressure?

1. **Identify knowns:**
* Initial Pressure (P1) = 2 atm
* Initial Temperature (T1) = 300 K
* Final Temperature (T2) = 450 K
* We need to find Final Pressure (P2).

---2. **Recall Gay-Lussac's Law formula:** P1/T1 = P2/T2

---3. **Substitute the known values into the formula:** 2 atm / 300 K = P2 / 450 K

---4. **Rearrange the formula to solve for P2:** P2 = (2 atm / 300 K) * 450 K

---5. **Calculate:** P2 = (0.00666... atm/K) * 450 K

---6. **Final Answer:** P2 = 3 atm.

The new pressure will be 3 atm.

Why It Matters

Understanding Gay-Lussac's Law is crucial for engineers designing anything from car engines to rockets, and even for space technology. It helps in developing safe pressure vessels and understanding how gases behave in extreme conditions, leading to innovations in EVs and HealthTech.

Common Mistakes

MISTAKE: Forgetting to use absolute temperature (Kelvin) in calculations. | CORRECTION: Always convert Celsius or Fahrenheit temperatures to Kelvin before using them in Gay-Lussac's Law calculations. Kelvin = Celsius + 273.15.

MISTAKE: Confusing direct proportionality with inverse proportionality. | CORRECTION: Remember, pressure and temperature are *directly* proportional. If one goes up, the other goes up. If one goes down, the other goes down.

MISTAKE: Applying the law when volume is changing. | CORRECTION: Gay-Lussac's Law only applies when the volume of the gas is kept constant (fixed container). If volume changes, you need a different gas law.

Practice Questions

Try It Yourself

QUESTION: A gas in a cylinder has a pressure of 1.5 atm at 27 degrees Celsius. If the temperature is raised to 127 degrees Celsius, what is the new pressure? (Remember to convert Celsius to Kelvin!) | ANSWER: 2 atm

QUESTION: A sealed aerosol can is initially at 25 degrees Celsius and has a pressure of 300 kPa. If the can is accidentally left in direct sunlight and heats up to 50 degrees Celsius, what will be the new pressure inside? | ANSWER: Approximately 325.2 kPa

QUESTION: If a car tire is filled to 2.2 atm pressure at a cool temperature of 17 degrees Celsius, and later, after a long drive, the pressure is measured at 2.4 atm, what is the new temperature of the air inside the tire in degrees Celsius? | ANSWER: Approximately 42.9 degrees Celsius

MCQ

Quick Quiz

What happens to the pressure of a gas in a sealed, rigid container if its temperature is increased?

The pressure decreases

The pressure increases

The pressure remains the same

The volume decreases

The Correct Answer Is:

According to Gay-Lussac's Law, pressure and temperature are directly proportional. So, if temperature increases, pressure also increases, assuming volume is constant. Options A and C are incorrect as pressure changes, and option D is incorrect because the container is rigid (volume is constant).

Real World Connection

In the Real World

This law is critical in how car tires behave. When you drive, the tires heat up due to friction, which increases the temperature of the air inside. According to Gay-Lussac's Law, this increased temperature leads to an increase in tire pressure, which is why it's important to check tire pressure when the tires are cold.

Key Vocabulary

Key Terms

PRESSURE: The force exerted by gas particles on the walls of their container | TEMPERATURE: A measure of the average kinetic energy of the gas particles | ABSOLUTE TEMPERATURE: Temperature measured on the Kelvin scale, where 0 K is absolute zero | DIRECTLY PROPORTIONAL: When two quantities increase or decrease together at a constant ratio | KELVIN: The SI unit for absolute temperature, where 0 K = -273.15 degrees Celsius

What's Next

What to Learn Next

Great job understanding Gay-Lussac's Law! Next, you can explore Boyle's Law (pressure-volume relationship) and Charles's Law (volume-temperature relationship). These laws, together, form the foundation for the Ideal Gas Law, which helps us understand gas behavior even better.