Why does scratching the bottom of an Erlenmeyer flask with a glass rod induce crystallization?

Question

Why does it make crystal growth larger?

Answer 1

Scratching the bottom of the flask both creates fresh imperfect glass surface and releases small pieces of glass that can seed the crystallization. Oswald theory of crystal growth has two parts -- nucleation and growth phases. Nucleation is spontaneous (in other words, suddenly a large number of molecules come together and form a minimum size crystal) and growth occurs around these small crystals. As the odds of this occuring are small, seeding the solution with a nucleating agent can cause it to happen.

Keep in mind this is a question of kinetics. At the point you do the scratching the solution is already super-saturated and thermodynamically wanting to produce crystallites, but it can be kinetically very, very slow, but by creating the small nucleation particles it speeds up the process. Once these small crystals are formed, the high concentration solution begins to form large particles around these initial crystals. The fewer nucleations that occur, the better for producing large crystals, because the growth rate is usually faster than the nucleation rate (thus the need for scratching the flask).

Answer 2

Scratching the erlenmeyer creates more imperfections in the glass surface. These imperfections are the places where crystal nucleation starts.
Larry &Barbie, you are part correct, part wrong. Any thing in the solution can induce the crystalization. If you add a small crystal to the solution, this crystal will act as a nucleation point and then more cystal will form. A dust particle can act in the same way.

Answer 3

The theory is that scratching the inside of any flask or test tube releases tiny bits of glass into the solution. These then act as nuclei for the formation of crystals. I have heard the same thing happens if you seed the solution with chalk dust.

Answer 4

Crystallization can be induced by scratching or seeding which present imperfect surfaces into the soln. onto which crystals can begin to form. My question is: if vibrations were responsible for the crystallizing effect, why wouldn't you just tap the side of the beaker instead of scratching the inside?

Answer 5

that's to help on crystal growth , you need and solid subject as nuclei to form the crystal and you must scratch the solids formed at the top to goes down inside the solution at the bottom of your flask

Answer 6

to form a neclues for crystalization

Answer 7

because it creates vibrations (sound), which induce diluted molecules to have more shocks with each other. And if the conditions are proper for crystallisation, these shocks increase the number of encounters per unit of time, and therefore crystallisation happens faster.

Answer 8

I don't know think that is true
whatever is in the flask is causing the crystallization
LOL
Leave it up to the chemist to prove me wrong.

Answer 9

Funny! 100!

Answer 10

it just mixes it better so that the crystals will form.