Investigating Diffusion of Gases and Liquids

Diffusion

My definition of diffusion (as learnt in Biology!) is the movement of particles from a region of higher concentration that of a lower concentration until equilibrium is reached.

Diffusion can be explained by an assumption of the Kinetic Particle Theory, which is that particles are always in constant and random motion.

In today's laboratory lesson, we were given a demonstration on diffusion of two highly corrosive substances, hydrochloric acid and ammonia gas. We were not allowed to handle these substances by ourselves. It felt really scary when a cloud of fumes appeared immediately after the lid of the container was lifted.

So these two acids were each soaked with cotton wool and then placed at opposite ends of a tube. After a while, a white cloud was observed near the cotton wool with hydrochloric acid. The particles of the ammonia gas had diffused with the hydrochloric acid, forming the white cloud. The reason the cloud was formed nearer to the hydrochloric acid is that the molecular mass of hydrogen chloride is heavier than that of ammonia gas. Heavier particles move and travel slower than lighter particles.

After that, we did our own experiment which was very simple actually. Firstly, we poured 50cm³ of sand and 50cm³of beans into two separate measuring cylinders. Next, we just mixed them together. The resulting volume of the mixture is not 100cm³ but around 80cm³ (we all had varying values around  there). Based on my prior knowledge, my prediction to the volume of the mixture was correct.

Our experiment! 

This experiment proves that there are actually spaces in between the beans (and really tiny ones between the sand particles). The amount of space between the particles is about 20cm³.

The next experiment we did was to mix the same amount of ethanol with the same amount of water (I didn't take any pictures for this though). Ethanol and water are two miscible liquids - they can mix with each other. We put the two liquids in a tube and Cindy (my lab partner) sealed the tube with her thumb. When she shook the tube to mix them, there was a vacuum created inside the tube, causing her thumb to look all weird like it was being sucked into the tube. The resulting volume was lower than what we expected it to be. The mixture also felt quite warm.

I felt quite surprised that the volume of the mixture was lower than I expected but I was glad I learnt that the volume actually decreases when you mix them together, as they take up the space between particles.

Till next time! :)

Update on my crystals!

I finally got to see the results of my crystallisation experiment. (Refer to my old post). Hurray!

So these are my (and Cindy's) crystals.

TA-DA! 

I think they are not bad and really pretty! Though there are much nicer ones like Min and Min Chi's. But really there aren't any model answers to this. Each crystal is special! And it was a pity we weren't allowed to take them home.

Anyway, I declare our crystallisation experiment a SUCCESS! :)

Sublimation

We learnt about sublimation while waiting waiting for crystals to form. We did not get to do a practical on sublimation but we saw a demonstration of the process.

Sublimation is used to separate a substance that sublimes from one that does not and has a high melting point. For the demonstration, the substance that sublimed was iodine. See the purplish vapour? That's the gaseous state of iodine. My classmates are photobombing over here.

Iodine subliming

I managed to catch sight of the flask being removed from the evaporating dish briefly but I could not take a picture because it was too quick. When the flask was removed, there was a beautiful purple "cloud" from the bottom of the flask. Felt really mysterious. :o 

I find sublimation really cool because you don't usually see something skip the liquid state. And as you can see from the picture, we all forgot to wear our safety goggles. (How could we?!) We must keep that in mind next time!

Crystallisation

Crystallisation is the most common method to purify soluble solids. It is preferred over evaporation to dryness as many substances decompose upon strong heating. Also, when all the water is removed during evaporation, any soluble impurities will be left on the crystals (eg. sugar decomposes to become carbon when it is heated strongly). The shape and size of crystals can be controlled by controlling variables such as cooling rate and evaporation rate.

In crystallisation, water is removed by heating the solution. Heating stops when a hot saturated* solution is formed. The resulting solution is left to cool and the dissolved solid will then be formed as pure crystals. This is because the solubility of the solute decreases as the temperature drops. At a lower temperature, less solute can be dissolved in the solution.

*A solution is saturated when it contains the maximum amount of dissolved solute at a given temperature. (When no more solvent can be dissolved in the solute). To test if a solution is saturated, a glass rod is placed into the solution and if there are tiny crystals formed when the rod is removed, the solution is saturated.

Analogy time! (A short short one)
Imagine the solute is your brain and then the solvent is information. When your brain is filled with too much information that whatever remaining information cannot go in, your brain is saturated. :D

For crystallisation to take place, several conditions must be met:
1. The solid must be soluble in water.
2. The solubility of the substance should change with the changing temperature.
3. Solution must be saturated with the solute.

For our practical, we were tasked to purify copper (II) sulphate crystals and investigate the effect of cooling rate on the size of crystals obtained.

My group is making crystals by slow cooling.

Steps:
1. Heat 20cm³ of water in a beaker and stop when the water boils.
2. Add one spatula of copper (II) sulphate into the hot water and mix until all the copper (II) sulphates dissolves.
3. Repeat step 2 until the solution is saturated.
4. Filter the solution to remove solid impurities.
5. Heat the solution in an evaporating dish.
6. Stop heating when about half the solvent has evaporated.
7. Pour the solution into a boiling tube and allow it to cool slowly. Leave overnight if necessary.

Step 1. Heating the water

Step 2 and 3. Adding copper (II) sulphate into the water

Step 4. Filtering the solution.

Step 5. Heating the solution.

Okay there is another step but I'm still currently stuck at Step 7 because crystals don't take that quick to form so we are supposed to review it next week. I will update my observations when I see the crystals that are formed.

My takeaways from this practical:
Crystals formed by rapid cooling are smaller and more irregular than crystals obtained by slow cooling. To obtain more crystals, one can add more water and apply slow cooling instead of rapid cooling. We should stop heating before all the solvent has evaporated as it prevents the solute from decomposing and soluble impurities will be left behind.