Main Difference – Colloidal vs Crystalline Precipitate
Precipitation is the formation of an insoluble solid mass in a liquid solution; this insoluble solid mass is called the precipitate. A precipitate is formed when two soluble ionic compounds are mixed. Soluble ionic compounds can break into their ions in the solution. Then these ions can react with each other to form a precipitate or stay as a solubilized ion in that solution. The chemical species that cause this precipitation is called precipitants. In addition, precipitates can form when the temperature of the solution is lowered. The low temperature reduces the solubility of salts, causing them to precipitate in the solution. The formed precipitate may stay as a suspension in the solution if there isn’t sufficient gravity. But later on, the precipitate particles will sediment to the bottom of the container unless disturbed. There are two types of suspensions as colloidal suspensions and crystalline suspension based on the particle size in the suspension. Colloidal precipitates are formed in colloidal suspensions while crystalline precipitates are formed in crystalline suspensions. The main difference between colloidal precipitate and crystalline precipitate is that colloidal precipitates do not form easily and are difficult to be obtained via filtering whereas crystalline precipitates are easily formed and are easily obtained via filtering.
Key Areas Covered
1. What is Colloidal Precipitate
– Definition, Explanation
2. What is Crystalline Precipitate
– Definition, Explanation
3. What is the Difference Between Colloidal and Crystalline Precipitate
– Comparison of Key Differences
Key Terms: Crystalline Suspension, Crystalline Precipitate, Colloidal Precipitate, Colloidal Suspension, Ionic Compound, Precipitants, Precipitation, Relative Supersaturation, Solubility
What is Colloidal Precipitate
Colloidal precipitates are solid masses formed in colloidal suspensions. A colloidal suspension is composed of particles having diameters ranging from 10-7 to 10-4 cm. These particles are invisible to the naked human eye.
Since the effect of gravity on these particles is very little, they do not tend to settle down at the bottom of the container. Since these particles are very tiny, it is difficult to obtain them via filtering. But by adding a suitable coagulating agent, we can form large particles or a precipitate that is easy to filter. Colloidal suspensions often look like clear solutions due to the scattering of visible radiation.
Figure 1: Particles Settle down at the Bottom to Form a Colloidal Precipitate
Brownian motion is the reason why colloidal particles do not spontaneously precipitate. Brownian motion is the random movement of particles in a fluid due to their collisions with other atoms or molecules.
The precipitation or the coagulation of colloidal particles can be enhanced by heating, stirring, or by adding an electrolyte to the suspension. Colloidal particles having electrical charges on their surface can be precipitated using a method of adsorption of ions.
What is Crystalline Precipitate
Crystalline precipitates are solid masses formed in a crystalline suspension. A crystalline suspension is composed of particles having large diameters about a tenth of a millimeter or greater. The effect of gravity on these large particles is considerably higher than that of colloidal particles.
Hence, the particles of the crystalline suspensions tend to settle down spontaneously and are easily filtered. These precipitates are easily purified. The particle size of a precipitate is influenced by precipitate solubility, temperature, reactant concentration and the rate in which the reactants are mixed. The net effect of these variables is called relative supersaturation.
Relative Supersaturation = (Q-S)/S
Q is the concentration of the solute and S is its equilibrium solubility. The particle size of crystalline precipitates can be improved by minimizing Q (using dilute solutions), maximizing S (adjusting pH or by precipitating from a hot solution) or from both methods. Digestion improves the purity and filterability of the precipitate.
Difference Between Colloidal and Crystalline Precipitate
Definition
Colloidal Precipitate: Colloidal precipitates are solid masses formed in colloidal suspensions.
Crystalline Precipitate: Crystalline precipitates are solid masses formed in a crystalline suspension.
Particle Size
Colloidal Precipitate: Particles in colloidal suspensions have diameters ranging from 10-7 to 10-4 cm. Thus, a precipitate is not formed easily.
Crystalline Precipitate: Particles in crystalline suspensions have diameters about a tenth of a millimeter or greater. Thus, a precipitate can be formed easily.
Effect of Gravity
Colloidal Precipitate: The effect of gravity on colloidal particles is less; thus these particles do not easily settle down.
Crystalline Precipitate: The effect of gravity on crystalline particles is considerably higher than colloidal particles; thus these particles spontaneously settle down.
Filtration
Colloidal Precipitate: Colloidal precipitates cannot be filtered easily.
Crystalline Precipitate: Crystalline precipitates can be easily filtered.
Conclusion
Precipitation is a very important phenomenon since the formed precipitate is visible. The formation of a precipitate can indicate the presence of a chemical reaction. The main difference between colloidal precipitate and crystalline precipitate is that colloidal precipitates do not form easily and are difficult to be obtained via filtering whereas crystalline precipitates are easily formed and are easily obtained via filtering.
References:
1. “11.7: Colloidal Suspensions.” Chemistry LibreTexts, Libretexts, 21 July 2016, Available here.
2. “Precipitation (Chemistry).” Wikipedia, Wikimedia Foundation, 12 Dec. 2017, Available here.
Image Courtesy:
1. “Silver chloride by Danny S. – 001” By Danny S. – Own work (CC BY-SA 3.0) via Commons Wikimedia
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