Many carbonate minerals are insoluble in water and appear in water pipes as 'scale.'
b. Why is scale formation typically only a problem in hot water pipes?

Question

Many carbonate minerals are insoluble in water and appear in water pipes as 'scale.'

b. Why is scale formation typically only a problem in hot water pipes?

Accepted Answer

Welcome back everyone. Magnesium hydroxide is insoluble in water and is one of the solids responsible for the formation of sludge in boilers. Why is this a problem in boilers? We're told that magnesium hydroxide is formed from the hydrolysis of magnesium chloride. We have choice A which states that magnesium chloride is less soluble in hot water than in cold water forming more sludge or choice B which states that magnesium hydroxide is less soluble in hot water than in cold water forming more sludge. And then choice C which states hot water in boilers does not affect the solubility of magnesium hydroxide. So let's begin by showing our dissolution of magnesium hydroxide. M G O H sub two as our chemical formula. So recognize that magnesium is a metal, whereas hydroxide is a poly atomic ion and us together this will form an ionic compound, meaning that we have a salt which is going to be insoluble and will undergo therefore a partial dissociation in equilibrium with the falling ions. Will we reform our magnesium two plus C and two moles of our hydroxide anion in order to have a balanced equation equation. So now with this dissolution outlined. Let's next show the hydrolysis of magnesium chloride which forms magnesium hydroxide. And so beginning with magnesium chloride, we have its formula as M G C L. Sub two, we have magnesium chloride which dissolves in water. So water is the second reactant and we will also have an equilibrium in which for our products, we have a double replacement. And so we form magnesium hydroxide as our first product as an insoluble precipitate salt. And then our second product is going to be hydrogen chloride. Now, let's recall that our conditions is that we have a boiler. So we have boiler conditions and this will therefore correspond to a high temperature as well as a high amount of warm water or hot water. And so we can understand that instead of hydrogen or hydrochloric acid, we're gonna be forming hydrogen chloride gas. Now, what we want to recall is that Henry's law tells us that the concentration of our dissolved gas expressed by C is equal to our Henry's law constant expressed by K H which is temperature dependent and specific to our gas as well as this is multiplied by our partial pressure of our gas. So again, c here tells us the concentration of our dissolved gas. K H is our Henry's law constant. And as we stated, it's temperature dependent. And then we have our variable P, which again is the partial pressure of our gas. And so with this in mind, we can see a direct relationship between the concentration of our dissolved gas and its partial pressure. And with this understanding, we can also interpret that based on Henry's law constant being temperature dependent, we can say that with an increased temperature that will correspond to an increase in our Henry's law constant for our dissolved gas. And so therefore, the kinetic energy of our gas is going to increase, corresponding to a decrease in the intermolecular forces holding our gas together in the solution. And that will ultimately correspond to a and let's just make some room here that will ultimately correspond to a decrease in the solubility of our gas. And we're using S as shorthand for solubility. So, with this outlined, because we have high temperature conditions in our boiler and hot water, we would understand that our hydrochloric gas is going to be evolving as vapor. And with that, we would be losing one of our products technically. And so from our solid precipitate here, magnesium hydroxide, we would ultimately have more of this precipitate form as equilibrium moves further to the right. So we draw a longer arrow to the right to reestablish itself due to the fact that we're losing H C L from its evolution as vapor. And so because of this, we have more solid particles of magnesium hydroxide precipitate. And with the inclusion of our hot water making up our boiler, we would ultimately have the formation of more sludge. And so we can outline, ultimately, that due to the fact that boilers contain hot water and at high temperatures, this will directly correlate to a decrease in the solubility of our gas. And therefore, we would have more of our precipitate magnesium hydroxide. And so, based on what we've outlined, we can already rule out choice c which states that hot water in boilers does not affect the solubility of magnesium hydroxide. We can see that it indeed does affect its solubility, creating more of our solid magnesium hydroxide as a precipitate. Whereas based on our dissolution of magnesium hydroxide in its partial dissociation into its ions, which would be, we would understand in colder water or cooler water, we at least have somewhat of a partial solubility of magnesium hydroxide when it's an equilibrium with its ions. But in a boiler through the hydrolysis of magnesium chloride, we will not observe a partial solubility of magnesium hydroxide. It's just going to precipitate further as a solid as our second product H C L evolves. And so thus, we would also rule out choice A which focuses on magnesium chloride being less soluble in hot water. When ultimately, we would agree with Choice B after ruling out choice A which states that it's magnesium hydroxide that is less soluble in hot water than in cold water forming more sludge. And so Choice B would be our final answer to explain our issue in boilers with the formation of sludge and magnesium hydroxide in solubility. I hope that this made sense and let us know if you have any questions.

Many carbonate minerals are insoluble in water and appear in water pipes as 'scale.'
b. Why is scale formation typically only a problem in hot water pipes?

Verified video answer for a similar problem:

Key Concepts

Solubility of Carbonate Minerals

Temperature Effects on Scale Formation

Water Chemistry and pH Levels

Many carbonate minerals are insoluble in water and appear in water pipes as 'scale.'b. Why is scale formation typically only a problem in hot water pipes?

Verified video answer for a similar problem:

Key Concepts

Solubility of Carbonate Minerals

Temperature Effects on Scale Formation

Water Chemistry and pH Levels

Many carbonate minerals are insoluble in water and appear in water pipes as 'scale.'
b. Why is scale formation typically only a problem in hot water pipes?