2. Introduction to Equilibrium

So now, will go back to the characteristics of some of the Chemicals. So, we are talking about physical and chemical characteristics of the chemicals now. So, we are looking at the physical properties. From a fate and transport point of view, when we have already setup the problem, what could be what is our area of interest what is it that you are interested? We are interested in chemicals entering water, we are interested in chemicals entering air, chemicals entering plants and animals, soil, sediment and all that.

In this section, we explore the physical and chemical properties of chemicals that are critical for understanding their behavior in the environment. Specifically, the focus is on how chemicals interact with water, air, soil, and living organisms. When analyzing environmental pollution or contamination, it becomes essential to consider which substances can enter the water, air, and even the soil, as well as their potential effects on biodiversity and health. A foundational understanding of these properties sets the stage for predicting how chemicals move through and affect different environmental compartments.

Think of it as being aware of how a stain on a shirt behaves. The stain can spread differently depending on the material of the shirt (water, air, etc.). In this analogy, just as different types of fabric react differently to water (some absorb it, others repel it), various chemicals behave distinctly when introduced to different environments, informing decisions on how to tackle contamination.

So, there are some properties of chemicals that are of interest to in this context, ok. So, which are these properties that are affected. So, straight away can you what is one of, what is what are the properties that could be of interest? If I gave if I give a name of chemical and if I ask, is it going to be health hazard in water, what is your first first question or response? Students: solubility. Professor: What is the? Student: solubility. Professor: what is the solubility? So aqueous solubility is usually referred see we are going to be in this class, we are going to be using absolute units. We are we are going to be using units, no. We will try to refrain from any other we will use absolute units like this milligram per litre or gram per litre or some such things.

Solubility and vapor pressure are two fundamental properties of chemicals that determine their behavior in the environment. Aqueous solubility refers to how well a chemical dissolves in water, typically measured in units like milligrams per liter. This is crucial for assessing whether a chemical poses a health hazard when it enters water sources. Vapor pressure measures how easily a chemical transitions from liquid to gas, with units often expressed in millimeters of mercury (mmHg) or Pascals. These properties are interconnected: high solubility usually indicates that a chemical will stay in the water, while high vapor pressure suggests that it may evaporate into the air.

Imagine sugar dissolving in a cup of water. The more sugar you add, the sweeter the water becomes up to a point (its solubility). If you add too much sugar, it stops dissolving, just like how certain chemicals stop dissolving in water at their solubility limit. Now, consider how some smells, like perfume, waft through the air. That’s similar to how chemicals with high vapor pressures can easily escape into the atmosphere.

So solubility, we are talking about solubility; dissolving, you take a pure substance and put it into water it will dissolve. We are trying to find out what is this aqueous solubility. What is this solubility this number how do you find out? Student: concentration, Professor: Ah, concentration when? Student: till saturation. Professor: till saturation, how do you find out saturation?

Achieving saturation is key to understanding solubility. When you dissolve a chemical in water, the dissolution process continues until no additional chemical can dissolve; this state is called saturation. At saturation, any added chemical will not dissolve and will remain undissolved. This balance between the dissolved molecules and the undissolved substance represents an equilibrium state. At equilibrium, the rate of dissolution equals the rate of crystallization back into the solid state, resulting in a constant concentration of solute in the solution.

Think of making a lemonade with sugar. You keep stirring sugar into the water until it stops dissolving—at that point, no matter how much more sugar you add, it will just sit at the bottom. This is like reaching the saturation point where the solution cannot hold any more sugar, and you're effectively in a state of equilibrium where the sugar molecules are balanced between dissolved in the water and undissolved.

Equilibrium is defined between two states, between two particular states. So, what is this equilibrium between? ... Solubility, equilibrium of pure substance of A between its pure form and water is solubility. So, equilibrium of pure substance of A between its pure form and air is saturated vapor pressure.

In the context of environmental chemistry, equilibrium involves understanding the balance between two phases: for example, a solid and a liquid or a liquid and a gas. The solubility of a substance in water represents the equilibrium between the dissolved particles in water (the aqueous phase) and the undissolved particles (the solid phase). Similarly, the concept of saturated vapor pressure describes the equilibrium between a liquid and its vapor above it. Both types of equilibrium are influenced by temperature and pressure, and they are fundamental to predicting how chemicals distribute themselves in the environment.

Consider a jelly in a bowl of water. The jelly represents a substance that can be either dissolved or undissolved in the water. When the jelly is stirred, it may dissolve (representing solubility) until a point is reached where it no longer dissolves, while some remains undissolved (the solid phase). If we look at a covered bowl with water, the water will eventually reach a certain vapor level above it—this is similar to how vapor pressure reflects the relationship between water and its gaseous form.

So if I have a mixture with A plus water and there is air, how much of A will go to the air if I leave it long enough to go to equilibrium? ... Specific name? ... This is called Vapour Liquid Equilibrium or VLE.

The partition coefficient describes how a chemical is distributed between two phases, such as air and water. It becomes important when understanding the behavior of chemicals that exist in both liquid and vapor forms. Vapor Liquid Equilibrium (VLE) refers to the state where the liquid and vapor phases of a substance are in equilibrium. This concept helps us determine how much of a liquid will evaporate into the air when it is in contact with it, providing insight into the potential for air pollution and the behavior of volatile organic compounds in the environment.

Imagine a balloon filled with air that has a small opening. Over time, some of the air will escape. The amount of air that leaves the balloon can be thought of as being in equilibrium with the air outside. Similarly, when a chemical in water vaporizes, it follows a principle of VLE, where the amount transitioning from liquid to gas and vice versa is balanced.

Henry’s Law which says that there is this linear relationship is very linear. On the other hand, it is called as Raoult’s law on the higher end.

Henry's Law provides a simple model to understand how gases dissolve in liquids. It states that the amount of gas that dissolves in a liquid is directly proportional to its partial pressure above the liquid. This is particularly important in environmental chemistry when dealing with pollutants in water bodies and air. It allows us to predict how much of a controlled substance will escape from water into the atmosphere. Raoult's Law, on the other hand, deals with the vapor pressure of solutions, providing another viewpoint on phase equilibrium.

Think of carbonated beverages. When you open a soda can, the sudden release of pressure allows carbon dioxide gas (the gas responsible for fizz) to escape from the liquid which is governed by Henry's Law. As the pressure of the gas decreases, more gas bubbles escape until a new equilibrium is reached. This concept underscores how gas behaves when subjected to changes in pressure and concentration.

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