Chemical and Physical Changes

In this practical, we did loads of heating using the bunsen burner, which is really fun. I like bunsen burners. You get to see both chemical and physical changes of a substance.

What are the differences between a chemical change and a physical change?
The difference is that there is a change in energy (energy taken in or given out) in a chemical change, but not in a physical change. Also, new substances are formed and the process is irreversible. However, no new substances are formed when there is a physical change and that you can still obtain the original state of the substance using other physical means.

Firstly, we heated two different substances: sodium chloride and copper (II) carbonate. The change in the sodium chloride was really really really slow. For my pair, we could not see the change because there wasn't sufficient time for us to see what happens. By right, we should see a clear liquid while melting and a while solid after it is left to cool.

For the copper (II) carbonate, the change was really fast from a green solid to a black solid.

Sodium chloride on the left, copper (II) carbonate on the right.

Heating up the sodium chloride, but nothing happened :(

Copper (II) carbonate after the chemical change.

Next, we heated up a magnesium ribbon. Before the change, it was a silvery-gray metallic strip but after heating, it became white powdery ash. While heating, it actually gave off a really bright white light! So cool! Here's a video of the change, sit back relax and enjoy! :)

Ready for heating! 

Yup that's it for this cool practical! I really enjoying heating all those stuff 8D.

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.

Chromatography and Distillation

As seen on the title, this practical is about chromatography and distillation. 

(This is a loooooooong post)

CHROMATOGRAPHY

A technique used to separate soluble mixtures of substances with different solubilities into individual components. It is used to find out the components of a mixture and they can be identified by comparing their chromatogram with those of known substances. It also determines the purity of a substance. (When there is only one spot observed on the chromatogram, there is no separation so the substance is pure) It can work with very small amounts. 

There are several forms of chromatography. They are: paper chromatography, thin-layer chromatography, gas chromatography and high-pressure chromatography. The one in our practical is paper chromatography. They all have a stationary phase and a mobile phase. 

In paper chromatography, the stationary phase is a very uniform absorbent paper and the mobile phase is a suitable liquid solvent, or a mixture of solvents. It works with the mobile phase travelling up the stationary phase, carrying the mixture with it. The components of the mixture will separate on the stationary phase depending on how strongly they are absorbed by the stationary phase and how much they dissolve in the mobile phase. 

What we did:

During this practical, we did our own paper chromatography experiment to separate the dyes in green ink. It's an individual task actually because the process is very simple and manageable to do alone.

Firstly, I drew a line 1.5cm from the bottom of the chromatography paper slip (the stationary phase) using a pencil. Using a pen will affect the results of the chromatography as it is basically ink and ink contains dyes, and it will mix with the ink that is supposed to be separated. 

Then, I used a capillary tube to transfer a few drops of green ink onto the center of the line. Next, I put distilled water into a boiling tube (around 1.5cm from the bottom). I inserted the chromatography paper into the boiling tube, using a clothes peg to secure it. The drop of ink is not supposed to touch the water or else it will dissolve in the water instead of travelling up the chromatography paper. So all there was left to do was to wait for the results. In the meantime, we learnt about distillation (I'll touch on that later!)

Fast forward.

TA-DA!

Waiting, waiting...

I removed the chromatography paper from the boiling tube and used my pencil to mark the spots of the dyes and the solvent front (where the water has travelled to). But, I made a mistake. As we did not have time on our hands, I hurriedly removed it without waiting for the solvent front to be within 1cm-2cm of the clothes peg. 

Actually, I find that it will not affect the result very much as the position of the dyes are relative to the solvent front. And then after marking, the position of the solvent front and dyes kept moving up! :O

See what I mean?

Okay, so other than comparing the chromatograms, there is a more systemic way to go about finding out the components in a mixture, which is the calculation of R_f values. R_f stands for retention factor. It is basically taking the distance moved by the substances divided by the distance moved by the solvent. 

Taken from Google Images

R_f values should be expressed in decimals. They should never exceed the value of 1, as substances will never travel beyond that of the solvent. 

R_f values can be affected by the type of solvent, type of paper used and the temperature. Different papers have different absorption levels and different solvents have different solubilities. As temperature increases, the solubility increases.

In the case of my chromatogram, the blue spot travelled the same distance (8.3cm) as the water. Therefore, the R_f value is 1. The yellow spot travelled 7.8cm, so the R_f value is 0.94. 

Conclusion:

The green ink is made up of blue and yellow dye. 

(I wish my chromatogram was more colourful, lol) 

SIMPLE DISTILLATION

Is used to obtain a solute from its solvent. 

Below is a set up of simple distillation.  The thermometer measures the temperature of the vapour to identify the distillate by its boiling point. In the round-bottomed flask where the mixture is, there are boiling chips (or boiling stones) to make boiling smooth. Without boiling chips, boiling will be very vigorous and the apparatus may spoil. Water enters the condenser from the bottom instead of the top to ensure that most of the water vapour is condensed and collected as liquid. If cold water enters from the top, it will not fill the condenser. 

Taken from Google Images

We did not try to do distillation on our own as the setup involves many apparatus. So we watched Mr Foo demonstrate the distillation of Coca-Cola. It's supposed to be smelly, but why didn't I smell a single thing?! It was pretty interesting seeing coke getting separated into pure water and sugar with caffeine.

Other than simple distillation, there is fractional distillation to separate two miscible liquids of different boiling points. There is a fractionating column in the setup. The liquid with the lowest boiling point will be distilled first.

Both distillation methods are similar as they both require heat and involve boiling and condensation. 

Reflection:

I've really learnt a lot in this practical. For chromatography, I learnt that the most soluble substance will travel the furthest on the paper. I think I should work on my time management in practicals and I will be more patient in the future for better accuracy.

Theory Lesson: Separation Techniques Introduction

So we are gonna learn a new topic, about using physical methods to separate mixtures. Yay! :)

Notes taken in class!
In nature, almost all substances are mixtures.

Mixtures need to be separated into pure substances for
-Characterization
-Identification
-Production

It is impossible to obtain a 100% pure substance.

DETERMINING THE DEGREE OF PURITY
-the melting points and boiling points of elements are unique
-chromatography

PAPER CHROMATOGRAPHY (eg. separates dyes from ink)
-separate a mixture of solutes with different solubility and degree of absorption
-uses a solvent moving over a porous or absorbent medium