Question

A cold soft drink is kept on the balance. When the cap is open, then the weight (A) Increases (B) Decreases (C) First increase then decreases (D) Remains same

Short answer

When we open the cap of a cold soft drink, the pressure is released, causing the gas bubbles to escape from the soft drink. This results in the overall weight of the soft drink to decrease due to the loss of the gas bubbles' contribution to the weight. Hence, the correct option is (B) Decreases.

Step-by-step solution

Understand the components of the soft drink

A soft drink contains the liquid and gas bubbles within it. Gas bubbles contribute to the weight too, as they are trapped inside the closed container under pressure.

Opening the cap

When we open the cap, we release the pressure. As a result, the gas bubbles begin to escape the soft drink due to the pressure difference between the inside and outside of the container.

Evaluating the weight

As gas bubbles escape, their contribution to the overall weight is lost. Since the liquid itself is not affected by opening the cap, the overall weight of the soft drink will decrease due to the escaping gas.

Choosing the correct option

Based on the analysis, when we open the cap, the weight of the soft drink decreases. Therefore, the correct answer is: (B) Decreases

Key concepts

Pressure

Pressure is a fundamental concept in physics, describing the force exerted over a unit area. In the context of a soft drink, pressure plays a vital role. The beverage itself is stored under pressure. This is typically carbon dioxide gas dissolved under pressure in the liquid, creating what we know as carbonation. When the cap of a soft drink is sealed, the pressure inside the container is higher than outside. This is due to the gas being unable to escape, pushing against the walls of the container and the liquid.

When you open the cap, you allow the pressure to equalize with the outside environment. This is why you often hear a fizzing sound. It is the sound of gas escaping as the pressure is released. This release leads to a decrease in the overall pressure inside the container, affecting the soft drink's properties, including its weight. Understanding how pressure works in this scenario is crucial to grasping other associated concepts like weight measurement and gas laws.

Weight Measurement

Weight can be defined as the force of gravity acting on an object. In the soft drink scenario, weight is the combined effect of the gravity on both the liquid and the gas within the container. Before opening, the weight displayed on the scale includes the weight of the liquid, the container, and the gas trapped inside under pressure.

When you open the soft drink, some of the gas that contributed to the weight escapes into the air. This escape occurs because the pressure holding the gas is released (as explained earlier). Consequently, the weight measured on the balance decreases.

It's fascinating to note that although the gaseous part of the drink has mass and contributes to the overall weight, it is not visually apparent. Hence, weight measurement in this situation provides an important insight into how gases, though invisible, can significantly contribute to the weight of a system.

Gas Laws

Gas laws are essential for understanding the behavior of gases and their relationship with pressure, volume, and temperature. Two main gas laws relevant to the soft drink scenario are Boyle's Law and the Ideal Gas Law.

Boyle’s Law states that the pressure of a gas is inversely proportional to its volume when temperature is held constant. When the soft drink is sealed, gases inside are compressed under high pressure; reducing the volume increases pressure, keeping the gas dissolved in the liquid.

The Ideal Gas Law, represented as \( PV = nRT \), combines various principles, including Boyle's Law, and shows the relationship between pressure (\( P \)), volume (\( V \)), the number of moles of gas (\( n \)), the gas constant (\( R \)), and temperature (\( T \)). Opening the cap causes a change in these parameters, specifically a drop in pressure and a subsequent increase in volume as gas escapes.

By applying these gas laws, we can predict how the gas will behave when the bottle is opened, leading to a decrease in pressure and consequently a decrease in the measurable weight of the bottle as the gas escapes.