Question

Write a balanced chemical equation to describe how magnesium ions are removed in water treatment by the addition of slaked lime, \(\mathrm{Ca}(\mathrm{OH})_{2}\).

Short answer

The balanced chemical equation for the removal of magnesium ions in water treatment through the addition of slaked lime is: \(Mg^{2+} + Ca(OH)_2 \rightarrow Mg(OH)_2 + Ca^{2+}\).

Step-by-step solution

Identify the reactants and products

Magnesium ions (Mg²⁺) are the species we want to remove from the water. In the presence of slaked lime [Ca(OH)₂], a precipitation reaction occurs, which forms solid magnesium hydroxide (Mg(OH)₂) that can be separated from the water. The reactants in this reaction are magnesium ions (Mg²⁺) and slaked lime [Ca(OH)₂], while the product is magnesium hydroxide (Mg(OH)₂). The remaining species, calcium ions (Ca²⁺), will stay in solution.

Write the unbalanced chemical equation

Now that we have identified the reactants and products, we can write an unbalanced chemical equation representing this reaction: Mg²⁺ + Ca(OH)₂ ⟶ Mg(OH)₂ + Ca²⁺

Balance the chemical equation

In order to balance the chemical equation, we need to make sure that the number of atoms for each element in the reactants is equal to the number of atoms for each element in the products. In this case, the equation is already balanced, since there is one Mg atom, one Ca atom, and two O and two H atoms on both sides of the equation. The balanced chemical equation is: Mg²⁺ + Ca(OH)₂ ⟶ Mg(OH)₂ + Ca²⁺

Key concepts

Water Treatment

Water treatment is a vital process that guarantees the safety and quality of water for human consumption and industrial use. The goal is to remove unwanted substances such as dirt, chemicals, and biological contaminants from the water supply. Among the many substances that may need to be removed are magnesium ions, which in high concentrations can cause the water to be 'hard'. This hardness can lead to problems such as scale formation in pipes and boilers and interfere with soap's ability to lather.

The treatment process includes several stages, such as sedimentation, filtration, and disinfection, but we will focus on the specific part where chemical reactions are used to remove ions. Specifically, we're interested in how slaked lime is employed to eliminate magnesium ions through a chemical reaction known as the precipitation reaction, resulting in a substance which can then be physically separated from the water.

Slaked Lime

Chemical Composition and Properties

Slaked lime, or calcium hydroxide, is a highly versatile chemical with the formula \(\mathrm{Ca}(\mathrm{OH})_{2}\). It is an alkaline compound that is obtained by adding water to calcium oxide (quicklime). Slaked lime is soluble in water and can also serve as a neutralizing agent due to its basic nature.

Role in Water Treatment

In water treatment, slaked lime serves primarily as a reagent to precipitate out magnesium ions. Upon mixing with water containing magnesium ions, it reacts to form magnesium hydroxide, which is a solid. This transformation plays an essential role in improving water quality and is a brief example of how chemicals can be used to alter the physical properties of contaminants to facilitate their removal.

Magnesium Hydroxide

Magnesium hydroxide is represented by the chemical formula \(\mathrm{Mg}(\mathrm{OH})_{2}\) and is known for its use as an antacid and mild laxative in medicinal applications. However, in the context of water treatment, it emerges from the interaction between magnesium ions in water and slaked lime.

What's crucial about magnesium hydroxide in this scenario is that it is insoluble in water. This means it forms a solid precipitate, which can be removed by filtration or settling, thereby reducing the magnesium ion concentration in the water. This occurrence is particularly beneficial in treating 'hard' water to make it more suitable for everyday uses, protecting plumbing systems, and improving soap efficacy.

Precipitation Reaction

A precipitation reaction in chemistry involves the formation of an insoluble solid from the reaction of two soluble substances. When the product of the reaction is a solid that separates from the solution, it is known as a precipitate.

In the context of water treatment, when slaked lime reacts with water containing magnesium ions (\(\mathrm{Mg}^{2+}\)), magnesium hydroxide precipitates out of the solution. This reaction can be summarized by the balanced chemical equation: \[\mathrm{Mg}^{2+} + \mathrm{Ca}(\mathrm{OH})_{2} \longrightarrow \mathrm{Mg}(\mathrm{OH})_{2} + \mathrm{Ca}^{2+}\]Here, magnesium hydroxide (\(\mathrm{Mg}(\mathrm{OH})_{2}\)), the precipitate, can be separated easily from water, removing the unwanted magnesium ions and thereby 'softening' the water.