Density, Thermal Insulators, Metals

Density

Density is an important concept and we used it to sort and identify polymers in our last session. This time we explored the idea further. First, Alison placed a can of Coke and a can of Diet Coke in water. The Coke sat lower in the water than the Diet Coke, showing that Coke is more dense, ie has more mass per unit volume. Each cubic centimetre of Coke weighs more than Diet Coke.

We discussed how to find densities of other objects. Measuring the mass is easy if you have a good balance, but for density we also need the volume - the amount of space the object takes up. With a regular object like a cube, it’s easy to calculate volume by measuring the sides and multiplying height x width x length, but what about irregular objects like the lumps of Mystery Metal that Angie had found? We tried to calculate the density by placing samples in a displacement vessel and measuring how much water was displaced. This could work well but we found that, if the sample was very small, the water in the displacement vessel just “bulged” and didn’t overflow. This means the surface tension on the water was able to absorb the extra volume. So, we ideally would need to know what range of equipment was accurate for.

Properties of metals 

We had twelve rectangular pieces of sheet metal, each labelled with a letter. We passed them round and tried to work out what each mystery metal was, and how we might investigate that without damaging the sample. Using magnets, observing the colour, and comparing the masses and feeling how easily they conducted heat, we were able to deduce most of them - but there were some tricky ones, eg non-magnetic steel. Our samples included tin, copper, brass, steel, iron, zinc, lead, bronze.

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Thermal Insulators

Thermal insulators are materials which don’t allow heat to pass through them easily, whereas thermal conductors do.  We wanted to investigate which was the best insulator :- bubble wrap, woollen cloth, or aluminium cooking foil. Each group was provided with three identical empty cans and the insulation materials, hot water, thermometers, and sticky tape. Groups designed their own method of testing and chose how to write up their results. For example, one group produced bar charts showing how much the temperature changed over 30 seconds, while another took several measurements over 10 minutes and charted the rate of cooling. We discussed how our tests might be biased or inaccurate, eg : 

• was it easier to coat the can in one insulating material compared to the others?
• Did we have to remove insulation to take temperature readings, and if so, did heat escape while we did it?

Our findings were consistent overall, that bubble wrap was the best insulator, followed by wool, and then cooking foil last of all.  This was a surprise for some people because aluminium foil is used in cooking, but it’s used for a different purpose - usually to help distribute heat evenly over the surface of food, to stop it burning.

For a recap on thermal insulators, and a quick quiz, see BBC Bitesize on Heat .

Polymers and Hydrogels

Hydrogels are polymers which can absorb large quantities of water.  They are alsol known as “smart” polymers, because they can change shape quickly in response to changes in their environment.  Examples include hair gel, water-retaining gel, eg in disposable nappies or plant pots, and soft contact lenses. Alison demonstrated hair gel’s excellent qualities on Tiny.  One source explains:  

“Hydrogels are networks of long polymer chains that attract and store large volumes of water. It is not uncommon for 90% of a hydrogel to be water. As water molecules are attracted and stick to the polymer chain network, this causes it to swell forming a gel.”

Hair Gel Collapse Experiment

We added salt to hair gel and found that the gel shrank and just a little liquid was left.

Salt replaces water in the polymer structure of the gel, because it’s more strongly attracted to the polymer than water is.  This is why you should choose your styling products carefully if you want to look cool in the sea.

RSC : Experiments with hydrogels 

University of Warwick : Hair Gel collapse experiment

Polymers : Disposable Nappies

We looked at a type of hydrogel, found in disposable nappies, and tested which of several brands of nappy was best value for money in terms of absorbency.

Method: We had Boots own brand, which cost 18.5p per nappy, Pampers, which cost 26p each, and Naty “natural” nappies, which were 28.2p.  We pulled the nappies apart and extracted the polymer granules, then added distilled water and measured how much water each collection of granules could absorb.

Results: Boots own brand absorbed 650ml ( so the cost of nappy divided by the liquid absorbed gives us 0.028p per ml), Pampers absorbed 800ml (0.0325p per ml) and Naty absorbed 680ml (0.041p per ml), although one group had an unexpected result of 900ml.

Conclusion: If all you’re concerned about is absorbency, the Boots own-brand nappies were best value for money.  However, there might be other factors which would influence your decision, such as whether the nappy fitted well, or environmental factors.

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Things to look at:

Catalyst -a magazine for teenage chemistry students - has a nice article about hydrogels which discusses medical applications.

 RSC experiment on hydrogels - discusses the chemistry behind the hydrogel.

Identifying Polymers

We used density to identify polymers:

Last month we synthesised a polymer, nylon, and this time we investigated some other types of polymer. We talked about what polymers are - long molecules formed when lots of smaller molecules join together, end-to-end.  They occur in nature, eg rubber, DNA, hair and nails are polymers.  We often think first of man-made polymers though, and these are known as “synthetic polymers”, such as nylon, PVC etc.. These can present challenges in waste disposal because they don’t break down, hence the concerns about the environmental hazards of plastics. 

We tried to identify some common polymers by comparing their relative densities.  Each group had six liquids of known densities in test tubes, and a selection of small pieces of polymer - most of which were raided from the recycling bin. We placed a sample of each polymer into each liquid and noted whether it floated or sank.  If it floated, it was less dense than the liquid, and if it sank, it was more dense.  In this way we were able to identify a density range for each sample, in grams per cubic cm (g/cm3).  We looked at a table of common polymers and their densities, and tried to work out which each was.  Finally, we looked at the recycling symbol which had been secretly kept back from each sample, to see if we were right.  

The RSC provided the following table::

Polymer Density range/g cm -3

EPS - expanded polystryrene 0.02 - 0.06

PP - polypropylene 0.89 - 0.91

LDPE - low density polyethylene 0.91 - 0.93

HDPE - high density polyethylene 0.94 - 0.96

PS - polystryrene 1.04 - 1.11

PVC - polyvinyl chloride (PVC) 1.20 - 1.55

PET - polyethylene terephthalate 1.38 - 1.40

When we checked our results, we’d correctly identified many of the polymers, but some were a little outside the ranges stated in the table.  The RSC say: “But note that materials made of polymers may also contain other substances as fillers, plasticisers, stabilisers, etc., which may make the density of a particular sample fall outside the ranges indicated; note the wide range of PVC densities in the table above.”

 It’s important to be able to correctly identify polymers in order to recycle them, and in fact industrial mixed recycling facilities use a process a little like this to sort polymers.  This BBC podcast about carbon, polymers and recycling includes a visit to a recycling plant to find out how it’s done.

Some other links to recap and take you further on this topic:

RSC Identifying Polymers - Classic Chemistry Experiments

The Polymer Party - short video about polymers, good fun.

From DNA to Silly Putty: the diverse world of polymers.   - TED Ed video , including environmental concerns.

BBC Bitesize on Polymers - recap and activities

Fantastic Plastic - from Catalyst, a science magazine for teenagers. “ This Catalyst article looks at the use of polymers in the manufacture of household items. The exciting thing about polymers is that it is possible to make polymers behave in so many different ways by organising their long chain molecules in different ways – polymers are the ultimate designer material. The article looks at their molecular chain and also the future for polymers in manufacturing.”

Robert Krampf : Polymers and Slime - video from the popular science teacher with fun activities.

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Fabrics and Flammability

We investigated the origins and properties of common fabrics.

We looked at:

• Cotton - from the cotton pods to cotton thread and fabric
• Wool - from raw sheeps' wool to felt and woollen cloth
• Silk - from a silk moth's coccoon to silk fabric
• Linen - from flax to fabric
• Man-made fabrics such as nylon

We considered the properties of these fabrics, and tested which were most flammable.

Polymers: Synthesising Nylon

We performed the 'Nylon rope trick' - we synthesised a polymer from two different chemicals.

Angie took two students at a time to do this activity.  We were extremely careful as the chemicals used can be harmful if splashed on the skin or in the eyes.

The membrane of nylon is being drawn up from the interface between the two layers of chemicals. We carefully dripped the less-dense Sebacoyl Chloride Hexane down a glass stirring rod so it would sit on top of the denser 1,6 Hexane Diamine. We wanted to avoid stirring or mixing the layers because where they touch, nylon instantly forms. We wanted to draw up a filament, not stir up a lump!

For instructions, see the RSC Nylon Rope Trick demonstration.

Fabrics, Dyes and Mordants

In December we looked at mordants, chemicals which help dyes bind to fabrics.  We prepared some natural dyes from turmeric, cochineal, onion skins, walnut shells and hulls, and beetroot. Our fabric samples were cotton, wool, linen, silk, and acetate. We discussed where these fabrics come from - plant, animal or synthetic. The mordants we compared were Alum (potassium aluminium sulphate), salt water, and a control of plain water. 

The mordant forms a bond with both the fabric and the dye - as Lachlan said, it acts like double-sided sticky tape. The buckets contain fabric samples which have soaked for several days in solutions of two different Mordants - alum (potassium aluminium sulphate) and salt (Sodium chloride). A control set of fabric samples soaked in tap water. We prepared some natural dyes from cochineal larvae, onion skins, beetroot, walnut husks and shells, and turmeric. Our fabrics were silk, cotton, linen, wool and acetate. We heated the fabrics in the dye, then rinsed them and compared the results in terms of depth of colour.

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Historical Paints and Pigments

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We had a wonderful science session - Alison’s plan gave us chemistry, art and history in one session. We saw some practical and historical applications of chemistry: we made some paints which were used in medieval times, and then painted with them. 
First we looked at sources of paints, and some paintings which they were used in, from a book published for the recent National Gallery colour exhibition, which several of our group had visited. We passed round samples of lapis lazuli and malachite, and heard how they were ground up to make pigments, but that lapis lazuli was expensive. Verdigris was used as an alternative source of blue, although its colour was more turquoise. Alison and Angie had prepared beforehand some verdigris by standing a strip of copper in a small pot within a sealed jar containing a small amount of 3m acetic acid. It had only been in place for two days but plenty of  verdigris had developed.

The children prepared their own artificial verdigris, by mixing copper II carbonate with dilute acetic acid. Acetic acid is the main active ingredient of vinegar and, for example , cider vinegar contains about 5% acetic acid and this is what makes it useful for medical purposes. We noted bubbling as a sign that a reaction was occurring, then saw the colour change from green to turquoise as a solution of copper acetate say formed. We let the unreacted copper carbonate settle, then poured off the top liquid and filtered into a beaker. Then we heated this solution in evaporating dishes over burners. We used retort stands with rings and pipe clay triangles. The turquoise residue was scraped out into a well of a spotting tile.

In other wells of the spotting tile, we placed charcoal that we ground in a pestle and mortar, red iron (iii) oxide (red ochre), yellow iron hydroxide, tin oxide, and chromium oxide.  Thus we had as our pigments the following:-

Verdigris - copper acetate - see this article from ’Pigments through the ages’

Charcoal - one of the oldest artists’ materials, known traditionally as ’carbon black’

Iron (III) Oxide (Red Ochre)  is mined as iron ore, and is also used as a pigment. Its chemical formula is Fe2O3

Yellow Iron Hydroxide (Yellow or Gold Ochre)  has the chemical formula FeO(OH) and it is ‘hydrated’, which means it contains water of crystallisation. 

Chromium Oxide is also known as chromia or the pigment viridian, and it was originally prepared from a rare mineral called Eskolaite.  It was probably not in widespread use until the Victorian era when a process was developed for deriving it by chemical means from a more common mineral.

Tin Oxide SnO2 - this was our white pigment and is commonly used by potters.  It is the main ore of tin, and its mineral form is known as cassiterite.

Making Paints

Next, egg tempera was prepared. We separated egg yolk from the whites, washed the intact yolk, then punctured the yolk membrane and squeezed out the yolk. Some yolk was added to each sample of pigment, and they were mixed thoroughly. A little water was added where necessary to get a smooth consistency. 
Alison had printed traditional icon outlines, and the children painted these using our home - made paints. All of our paints were similar to those in use in the medieval period. Finally, when the paintings were dry, they were gilded using imitation gold leaf. 
We had never seen the children so quiet in a science session!