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Methods for Separation and Purification of Substances

CSCA Methods for Separation and Purification of Substances study guide organized around the publicly available CSCA syllabus. Practice Chemistry questions on aicsca.com.

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Methods for Separation and Purification of Substances

Core Concepts and Basic Principles

Separation and purification of substances refer to the process of separating the components of a mixture and obtaining pure substances through appropriate operations, based on differences in the physical or chemical properties of the components. This is a fundamental operation in chemical experiments and industrial production. The core principles are: **No Addition** (do not introduce new impurities), **No Reduction** (do not lose the substance to be purified), **Easy Separation** (simple operation, easy to separate), and **Recovery** (if the purified substance undergoes a chemical change, it should be recoverable to its original form). The CSCA exam focuses on testing the ability to select appropriate methods based on the properties of mixtures.

Main Separation and Purification Methods

Based on separation principles, they are mainly divided into physical methods and chemical methods.

#### Physical Methods

Utilize differences in physical properties (e.g., state, solubility, boiling point, particle size, magnetism).

1. **Filtration**: Separates mixtures of **solid and liquid**, or mixtures of **solids** where one is soluble and the other is insoluble.

* **Principle**: Difference in particle size.

* **Key Operations**: "One fit, two lows, three leans." Filter paper fits tightly against the funnel wall; filter paper edge below the funnel rim, liquid level below the filter paper edge; beaker leans against the glass rod, lower end of the glass rod leans against the three-layer filter paper, funnel stem leans against the inner wall of the receiving beaker.

* **Application**: Purification of crude salt (removing insoluble sand).

2. **Evaporation Crystallization**: Separates a solid solute from a solution whose **solubility changes little with temperature**.

* **Principle**: Volatilization of the solvent.

* **Key Operations**: Heat in an evaporating dish, stir constantly with a glass rod to prevent localized overheating and splattering. Stop heating when a large amount of solid appears, and use residual heat to evaporate to dryness.

* **Application**: Obtaining table salt ($\ce{NaCl}$) from seawater.

3. **Crystallization by Cooling (Cooling a Hot Saturated Solution)**: Separates a solid solute whose **solubility decreases significantly with decreasing temperature** from a solution, or purifies a substance containing a small amount of such an impurity.

* **Principle**: Difference in temperature dependence of solubility.

* **Operation**: First prepare a saturated solution at high temperature by heating, then cool it.

* **Application**: Purifying $\ce{KNO3}$ (containing a small amount of $\ce{NaCl}$ impurity).

4. **Distillation**: Separates mixtures of **miscible liquids** with **different boiling points**.

* **Principle**: Difference in boiling points.

* **Apparatus Key Points**: Thermometer bulb positioned at the sidearm opening of the distillation flask; add boiling chips to prevent bumping; condenser water flows in from the bottom and out from the top.

* **Application**: Preparing distilled water, separating ethanol and water.

5. **Liquid-Liquid Separation (Using a Separatory Funnel)**: Separates mixtures of **immiscible liquids**.

* **Principle**: Different densities, forming layers.

* **Apparatus**: Separatory funnel.

* **Operation**: Drain the lower layer from the bottom stopcock, pour the upper layer out from the top opening.

* **Application**: Separating $\ce{CCl4}$ and water.

6. **Extraction**: Uses the difference in solubility of a solute in **two immiscible solvents** to transfer the solute from one solvent to the other.

* **Principle**: Partition law.

* **Apparatus**: Separatory funnel.

* **Extractant Requirements**: Immiscible with the original solvent; the solute must be much more soluble in it than in the original solvent; does not react with the solute.

* **Application**: Extracting iodine from an iodine-water solution using $\ce{CCl4}$ or benzene.

7. **Sublimation**: Separates a solid that **sublimes** from non-subliming solid impurities.

* **Principle**: Certain solids transform directly into gas upon heating and condense back upon cooling.

* **Application**: Separating a mixture of iodine and sand.

8. **Magnetic Attraction**: Separates magnetic substances from non-magnetic ones.

* **Application**: Separating iron powder and copper powder.

#### Chemical Methods

Convert impurities into easily separable substances (e.g., gas, precipitate, water) or the target product through chemical reactions.

* **Principles**: The chosen reagent reacts only with the impurity, not with the substance to be purified; the reaction should not introduce new impurities, or any introduced impurities should be easily removable.

* **Common Types**:

1. **Precipitation Method**: Add a reagent to convert the impurity into a precipitate, then remove by filtration. Example: To remove $\ce{Na2SO4}$ from $\ce{NaCl}$ solution, add an appropriate amount of $\ce{BaCl2}$ solution: $\ce{Na2SO4 + BaCl2 = BaSO4 v + 2NaCl}$.

2. **Gas Evolution Method**: Add a reagent to convert the impurity into a gas that escapes. Example: To remove $\ce{Na2CO3}$ from $\ce{NaCl}$ solution, add an appropriate amount of dilute hydrochloric acid: $\ce{Na2CO3 + 2HCl = 2NaCl + H2O + CO2 ^}$.

3. **Conversion Method**: Convert the impurity into the substance to be purified. Example: To remove $\ce{CO}$ from $\ce{CO2}$, pass the gas over heated $\ce{CuO}$: $\ce{CO + CuO \xrightarrow{\Delta} Cu + CO2}$.

Typical Example Problems

**Example 1**: To remove $\ce{Ca^{2+}}$, $\ce{Mg^{2+}}$, $\ce{SO4^{2-}}$, and sand from crude salt to obtain pure $\ce{NaCl}$ crystals, a student designed the following experimental procedure:

Crude salt $\xrightarrow{\text{Dissolve}}$ Solution $\xrightarrow{\text{①}}$ $\xrightarrow{\text{②}}$ $\xrightarrow{\text{③}}$ Filtrate $\xrightarrow{\text{④}}$ $\ce{NaCl}$ solid.

(1) The names of operations ①, ②, ③, and ④ are ______, ______, ______, and ______, respectively.

(2) Reagents a, b, and c can be chosen as ______, ______, and ______ (fill in chemical formulas). The purpose of adding an excess of reagent b is ______.

(3) After adding reagent c, the components of the precipitate obtained are ______.

**Solution**:

(1) Operation ① involves adding reagents to form precipitates, which is **chemical treatment** (or "adding reagents for reaction"); Operation ② separates solid and liquid, which is **filtration**; Operation ③ involves adding a reagent to remove excess $\ce{Ba^{2+}}$ and $\ce{Ca^{2+}}$, which is **chemical treatment**; Operation ④ obtains $\ce{NaCl}$ solid from the filtrate, which is **evaporation crystallization**.

(2) The typical order for impurity removal is: first add $\ce{BaCl2}$ to remove $\ce{SO4^{2-}}$, then add $\ce{NaOH}$ to remove $\ce{Mg^{2+}}$, and finally add $\ce{Na2CO3}$ to remove $\ce{Ca^{2+}}$ and excess $\ce{Ba^{2+}}$. Therefore, reagent a is $\ce{BaCl2}$, reagent b is $\ce{NaOH}$, and reagent c is $\ce{Na2CO3}$. The purpose of adding an excess of reagent b ($\ce{NaOH}$) is **to ensure complete precipitation of $\ce{Mg^{2+}}$**.

(3) After adding $\ce{Na2CO3}$, it reacts with $\ce{Ca^{2+}}$ to form $\ce{CaCO3}$ precipitate and with excess $\ce{Ba^{2+}}$ to form $\ce{BaCO3}$ precipitate. The previously formed $\ce{Mg(OH)2}$ precipitate also remains in the system. Therefore, the precipitate components are: $\ce{BaSO4}$, $\ce{Mg(OH)2}$, $\ce{CaCO3}$, $\ce{BaCO3}$.

**Example 2**: Which of the following methods for separating or purifying substances is correct? ( )

A. Using filtration to remove $\ce{FeCl3}$ solution from $\ce{Fe(OH)3}$ colloid

B. Using distillation to separate a mixture of ethanol (b.p. 78.5°C) and acetic acid (b.p. 118°C)

C. Using dissolution, filtration, and evaporation to purify $\ce{Na2CO3}$ containing a small amount of $\ce{BaSO4}$

D. Using heating to remove $\ce{NH4Cl}$ from solid $\ce{NaCl}$

**Solution**:

A is incorrect. Colloidal particles can pass through filter paper, so filtration cannot separate a colloid from a solution. **Dialysis** should be used.

B is correct. Ethanol and acetic acid are miscible but have a significant boiling point difference (about 40°C), so distillation can be used for separation.

C is ambiguous but often considered acceptable. $\ce{Na2CO3}$ is soluble, $\ce{BaSO4}$ is insoluble. Dissolving, filtering (removing $\ce{BaSO4}$), and evaporating the filtrate yields $\ce{Na2CO3}$. If the goal is to purify $\ce{Na2CO3}$ (where $\ce{Na2CO3}$ is the main substance with $\ce{BaSO4}$ as impurity), this method works. However, the phrasing could be interpreted differently.

D is clearly correct. $\ce{NH4Cl}$ decomposes upon heating: $\ce{NH4Cl \xrightarrow{\Delta} NH3 ^ + HCl ^}$, and volatilizes away, while $\ce{NaCl}$ is stable.

**Answer: D** (B is also technically correct, but for a single-choice question, D is the classic and unambiguous answer).

Precautions and Common Mistakes

1. **Incorrect Method Selection**: Failing to identify the key property difference between mixture components. For example, using filtration instead of liquid-liquid separation for oil and water; using filtration instead of magnetic attraction or acid dissolution (where Fe reacts) for separating Fe and Cu powder.

2. **Confusing Operational Details**: For example, incorrect thermometer placement in distillation; confusing the outlets for upper and lower layers in liquid-liquid separation; not following "one fit, two lows, three leans" in filtration, leading to slow or failed filtration.

3. **Introducing New Impurities from Excess Reagents**: For example, using excess $\ce{BaCl2}$ to remove $\ce{SO4^{2-}}$ introduces excess $\ce{Ba^{2+}}$ as a new impurity, which must be removed by $\ce{Na2CO3}$ in a subsequent step.

4. **Incorrect Order of Impurity Removal**: For example, in crude salt purification, if $\ce{Na2CO3}$ is added before $\ce{BaCl2}$, the excess $\ce{Ba^{2+}}$ cannot be removed. Generally, $\ce{Na2CO3}$ must be added after $\ce{BaCl2}$ to remove excess $\ce{Ba^{2+}}$.

5. **Ignoring Substance Specificity**: For example, heating can remove $\ce{NaHCO3}$ from $\ce{Na2CO3}$ ($2\ce{NaHCO3} \xrightarrow{\Delta} \ce{Na2CO3} + \ce{H2O} + \ce{CO2 ^}$), but cannot be used to remove $\ce{NaHCO3}$ from $\ce{NaCl}$ because it would introduce the new impurity $\ce{Na2CO3}$.

The key to mastering separation and purification methods lies in a deep understanding of the property differences between substances and the flexible application of physical and chemical means.