This is too good to leave out. Plenty of people study creative arts with the sciences. Good science here and excellent video. Worth watching till the end.
New Look
I’m trying out a new, minimalist look.
The Pages sidebar is now on the left behind a slider and the keyword search is at the end of the first batch of posts.
Moles – Percentage Purity and Yield
The yield is the amount of product you obtain from a reaction. Suppose we own a factory that makes fertilizers. We will want the highest yield possible, for the lowest cost.
If we are making medical drugs then the yield will still be important, but the purity of the product may be even more important. This is because the impurities may harm the people using the drugs.
Finding the percentage yield
The formula for percentage yield is:
Aspirin is made from salicylic acid. 1 mole of salicylic acid gives 1 mole of aspirin. The chemical formula for salicylic acid is C7H6O3 and the chemical formula for aspirin is C9H8O4.
In an experiment, 100.0 g of salicylic acid yielded 121.2 g of aspirin. What was the percent yield?
Solution:
1. Calculate the Mr (RMM = relative molecular mass) of the substances.
- Ar : C = 12, H = 1, O = 16
- So, Mr : salicylic acid = 138, aspirin = 180.
2. Convert grams to moles for salicylic acid
- 138 g of salicylic acid = 1 mole
- So, 100 g = 100 ÷ 138 mole = 0.725 moles
3. Work out the calculated mass of the aspirin.
- 1 mole of salicylic acid gives 1 mole of aspirin
- So, 0.725 moles gives 0.725 moles of aspirin
- 0.725 moles of aspirin = 0.725 × 180 g = 130.5 g
- So, the calculated mass of the reaction is 130.5 g
4. Calculate the percent yield.
- The actual mass obtained is 121.2 g
- So, the percent yield = 121.2 ÷ 130.5 × 100% = 92.9%
Finding Percentage Purity
When we make something in a chemical reaction, and separate it from the final mixture, it will still have small amounts of other substances mixed with it. It will be impure.
The formula for percentage purity is:
Example:
The aspirin from the above experiment was not pure. 121.2 g of solid was obtained, but analysis showed that only 109.2g of it was aspirin. Calculate the percentage purity of the product.
Solution:
Percentage purity = 109.2 ÷ 121.2 × 100% = 90.0%
PhET Science Simulations
Have a look at this site. Here’s one to start you off but there are lots more to try in chemistry and biology as well as physics. the simulations don’t run natively on a Mac, so you’ll need the Java app from the app store to run them. If your security preferences default to native Mac apps in System Preferences, you’ll need to disable this.
| Click to Run |
Khan Academy
Click on the image. This site is making hot news worldwide. What do you want to know? It’s all here as short video tutorials. Maths, Physics, Chemistry – almost anything to any level is either online or being rolled out. All you need to do is type a search word, sit down and watch.
Click here as an example, showing refraction of light in water – or why a drinking straw looks bent.
Redox Reactions
Redox (reduction-oxidation) reactions are a family of reactions that are concerned with the transfer of electrons between reactants. Redox reactions are a matched pair – you don’t have an oxidation reaction without a reduction reaction happening at the same time. Oxidation means loss of electrons while reduction means gain of electrons. Each reaction by itself is called a “half-reaction”, simply because we need two half-reactions to form a whole reaction. We write them out like this.
Mixing magnesium powder and copper oxide together – a displacement reaction.
The copper ion has gained 2 electrons, and is reduced, the Mg atom has lost 2 electrons and is oxidised
The OILRIG rule: Oxidation is loss, reduction is gain of electrons.
Revision Tips: Periodic Table
Here it is. Yeah, OK it’s a shower curtain. Go find a proper one in a book or online. Proton number top left – nucleon numbers not shown because…isotopes.
Groups go DOWN (similar properties), periods go ACROSS (properties change from metal to non-metal)
You should be able to draw an electron configuration up to a proton number of about 20
Magnesium 2,8,2, Group 2 period 2,
Calcium 2,8,8,2, Group 2 period 3
Remember:
Shell 1 2 electrons
Shell 2 8 electrons
Shell 3 8 electrons
Shell 4 18 electrons
Group 1 metals
- From Li to Fr reactivity increases down the group 1 electron in outermost shell. Form 1+ ions
- Monatomic
- Form ionic compounds, white solids that dissolve in water to form colourless solutions
- Highly reactive with water, oxygen and the halogens, kept under paraffin
- Atoms get bigger down the group so electron further away from nucleus
- Good conductors of heat and electricity
- Often very soft can be cut with a knife
- Float on water – low density
- Low mp and bp
(Group II metals (Mg, Ca) have 2 electrons to give away so in general are less reactive than their corresponding group I metal)
Halogens Group VII (7)
- Ionic bonding with metals e.g NaCl, 1– ions formed
- Covalent molecular bonding, diatomic e.g Br2 so low mp and bp increasing down group
- 1 vacancy in outermost shell
- Reactivity decreases down the group because vacancy further away from the nucleus – more difficult to fill it. Cl displaces Br displaces I
- React with metals to form metal salts Example?
- Do I remember the test for a chloride? If not, look it up now.
- Chlorine gas will displace bromine from sodium bromide (Cl more reactive than Br) – green gas turns to brown gas
Noble Gases Group 0 or VIII (8)
- Don’t do much at all. No free electrons in outermost shells
- All monatomic
- Uses: He balloons (very light), Ar in filament lamps (because very unreactive – prevents contamination of the filament), Ne in signs (glows bright green),Xe in flash photography (stroboscopes).
- He/Ne used in lasers (glows red)
Transition Metals – identify where they are in the PT
- frequently colourful compounds (salts) – used in stained glass windows
- high mp and bp.
- high density – none float like Na on water
- hard, tough and strong, malleable and ductile
- good conductors of heat and electricity
- crystalline, hence shiny.
- often alloyed to get the best of both worlds (Ti is alloyed for lightness, strength and corrosion resistance), bronze = Cu alloyed with Sn (tin), harder than Cu alone.
- mainly vary in their inner electrons so often quite similar chemically.
- uses for iron (steelmaking), copper (electrical wiring and water pipes) and zinc (anti-corrosion coatings)
- catalysts – metals like palladium, gold and platinum used often to increase the surface area for reactions to occur. Iron catalyses the Haber Process for ammonia manufacture.
- A few to remember – potassium permanganate (Mn in oxidation state VII) KMnO4 is purple, potassium dichromate (the Cr is in oxidation state VI) K2Cr2O7 is orange. Both are strong oxidising agents and on reduction their colours change. permanganate becomes colourless, dichromate turns from orange to green.
Hints and Tips: Getting it Right in Practical Papers
READ ALL the question first. You have plenty of time. Imagine the apparatus in front of you.
Measurement on-site. For lengths, make sure you have a good quality ruler, able to measure accurately to 1mm. You are usually provided with pretty much everything you might need and, if you are, it’s important to use it all. As an example, if you’re measuring the diameter of a marble and they give you six, plus a ruler and two set squares, this is the way to do it.
Decide where a measurement is to be taken (top/middle/bottom) For example, the length of a pendulum is measured from the support to the middle of the lead weight.
First create a table with headers, then write the measurements down to the nearest unit on the measuring instrument (such as 1 degree or 1mm)
The question often asks you to process the measurement in some way (work out an area or volume/divide by a number of oscillations/ find the sine of an angle/find a volume.) Do the algebra or rearrange first. Then put in the numbers. Quote your answer to the number of decimal places in the question and don’t forget units.
There is often data where you have to fill in the units and there will always be a graph to plot (e.g, a cooling curve) Use sensible and easy-to-plot scales for the graphs and remember that the axes have units. Draw the LOBF through as many data points as possible. Look down the points to see whether it’s a straight line or not. If it is, use a ruler. If not, draw freehand a smooth curve and, if you’re using error bars the line MUST go through them all, both vertical and horizotal Use a sharp HB pencil
The graph tells you something. Think about what the data is telling you – they will either ask you to work out a gradient or an area. Gradients – big is beautiful and keep it simple – look out for division by easy numbers. Both gradient and area under will have units.
You are sometimes asked to read meters. Be careful about accuracy and sensitivity. We used to use these big demonstration meters in school but now mostly they are all digital. You should know how to read both, however.
Precautions to ensure accuracy. When reading meters, volumes from measuring cylinders, lengths and so on, avoid parallax errors. Repeat measurements and average as time allows (3 is good) and zero all meters are the usual ones to write down when asked to comment.
Measuring Rates of Reaction
The rate of a reaction may be measured by following the loss of a reactant, or the formation of a product. Three of the reactions which may be studied to show how the rate can be changed are shown.
They are:
PARTICLE SIZE: The reaction between calcium carbonate and dilute hydrochloric acid.
TEMPERATURE : The reaction between sodium thiosulphate solution and hydrochloric acid.
CATALYST: The decomposition of hydrogen peroxide solution.
The reaction between calcium carbonate and dilute hydrochloric acid.
Hydrochloric acid + calcium carbonate → calcium chloride + carbon dioxide + water.
HCl(aq) + CaCO3(s) → CaCl2(aq) + CO2(g) + H2O(l)
The rate of this reaction can be measured by following the rate at which carbon dioxide is formed. This can be done by conducting the reaction in an open flask on an electric balance (weighing machine). As the carbon dioxide escapes to the air, the mass of the flask will decrease. You can take a reading from the balance every 30 seconds, then plot a graph of mass loss against time.
The gradient of the plot (the steepness of the slope) shows the rate of the reaction (how fast it is going).
A solid in a solution can only react when moving liquid particles collide with the solid surface. The bigger the area of the solid surface, the more particles can collide with it per second, and the faster the reaction rate is.
You can increase the surface area of a solid by breaking it up into smaller pieces.A powder has the largest surface area and will have the fastest reaction rate.
This is why catalysts are often used as powders. The reaction rate is faster (the slope is steeper)
for the reaction with small marble chips (greater surface area).
Note that the final loss of mass is the same for both reactions.This is because the same mass of calcium carbonate (marble chips) will give the same mass of carbon dioxide whether the chips are large or small. The smaller chips will just do it more quickly.
The reaction between sodium thiosulphate solution and dilute hydrochloric acid.
Hydrochloric acid + sodium thiosulphate → sodium chloride + sulphur dioxide + sulphur + water.
HCl(aq) + Na2S2O3(aq) → NaCl(aq) + SO2(g) + S(s) + H2O(l)
The solid sulphur (S(s)) formed in this reaction makes the colourless solution go a cloudy, yellow colour.
The reaction is usually carried out in a flask placed on a piece of white paper which has a black cross on it. At the beginning of the reaction, the cross can be seen easily. As the flask becomes more and more cloudy the cross gets harder to see.
You can measure the time from the start of the reaction until the cross can no longer be seen. This is a way of measuring the rate of formation of sulphur. Increasing the temperature by 10oC halves the time it takes for the cross to disappear
The decomposition of hydrogen peroxide solution.
hydrogen peroxide → oxygen + water.
2H2O2(aq) → O2(g) + 2H2O(l)
The reaction is carried out in a closed flask which has a gas syringe connected to the top of it.
The reaction is started by adding a catalyst to the hydrogen peroxide. Nothing much happens until we add a catalyst – the introduction agency. The catalyst here is manganese (IV) oxide. The volume of oxygen in the syringe increases as the reaction proceeds. The volume of oxygen can be noted every 30 seconds and a graph of volume against time can be plotted. The gradient of the plot (the steepness of the slope) shows how fast the reaction is going.
Summary
We can measure the rate of reaction by following the change in some property of the reacting mixture over time.
We can show the changing rate of a reaction by plotting a graph of amount of reactant remaining or amount of product formed against time.
At any moment during the reaction: the steeper the slope of the graph, the faster the reaction at that point.
The reaction finishes where the line levels off.
Collision theory states that particles must collide before they can react, and that only collisions with sufficient energy (greater than the activation energy) will result in a reaction. Activation energy is the minimum amount of energy needed for a reaction to take place. Think of it as an entrance fee to a party. Increasing temperature increases the heat energy available in the system. This allows for more successful reactions to take place, however activation energy is unchanged by temperature.
Follow the link below to prepare for a short test next lesson.
Rates of Reaction TEST YOUR UNDERSTANDING
2010 in review – we done good
The stats helper monkeys at WordPress.com mulled over how this blog did in 2010, and here’s a high level summary of its overall blog health:
The Blog-Health-o-Meter™ reads Wow.
Crunchy numbers
The average container ship can carry about 4,500 containers. This blog was viewed about 23,000 times in 2010. If each view were a shipping container, your blog would have filled about 5 fully loaded ships.
In 2010, there were 34 new posts, growing the total archive of this blog to 144 posts. There were 110 pictures uploaded, taking up a total of 8mb. That’s about 2 pictures per week.
The busiest day of the year was November 22nd with 319 views. The most popular post that day was Fractional Distillation *updated*.
Where did they come from?
The top referring sites in 2010 were 74.125.67.100, healthfitnesstherapy.com, statistics.bestproceed.com, alphainventions.com, and mekonik.wordpress.com.
Some visitors came searching, mostly for fractional distillation, fractional distillation of crude oil, diffraction, bauxite, and parrot.
Attractions in 2010
These are the posts and pages that got the most views in 2010.
1
Fractional Distillation *updated* September 2008
2
Aluminium from Bauxite *updated* November 2008
4 comments
3
Diffraction – wave spreading around an edge March 2009
4
Centre of Mass (or Gravity) October 2008
5
Measuring Radioactivity – The Geiger-Muller Tube January 2010
Renewable and Non-renewable Fuels
All life on earth gets its energy from the sun. Plants and animals can store energy and some of this energy remains with them when they die. It is the remains of these ancient animals and plants that make up fossil fuels.
Energy resources
Fossil fuels are non-renewable energy resources and will one day run out and we can’t replace them. Burning fossil fuels generates polluting greenhouse gases and we mustn’t continue to rely on them to make the energy we need.
Renewable or infinite energy resources are sources of energy that can be used again and again.
Some resources can be thought of as both renewable and non-renewable.
- Wood can be used for fuel and is renewable if trees are replanted.
- Biomass, which is material from living things, can be renewable if plants – like sugar cane – are replanted.
Over the last 200 years most of our energy has come from non-renewable sources such as oil and coal.
Non-renewable energy resources
| Type of fuel | Where it is from | ||
| Coal (fossil fuel) |
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| Oil (fossil fuel) |
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| Natural gas (fossil fuel) |
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| Nuclear |
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| Biomass |
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| Wood |
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How long will fossil fuels last?
If we all continue to burn fuels “like there’s no tomorrow”, oil and gas reserves may run out within our lifetimes. Coal is expected to last a little bit longer.
Estimated length of time left for fossil fuels
| Fossil fuel | Time left |
| Oil | 50 years |
| Natural gas | 70 years |
| Coal | 250 years |
The Rock Cycle
What goes around, comes around.
Rocks are constantly being recycled. To recycle means to take something old and change it into something new. So, some old rocks that have been around for more than four billion years are being changed into different, newer rocks. Of course, that doesn’t happen overnight. It takes millions of years. To better understand how this happens, let’s take a journey through the rock cycle.
We begin with a volcano.
There are lots of active volcanoes on the Earth – in times past there were probably a lot more than there are now.
We start in the mantle, in a place where the Earth’s crust is quite thin. Red hot magma is being pushed up towards the crust, which breaks open, and we have a volcano. 
Some of this magma creeps into the cracks of the volcano, cools and forms rocks such as coarse grained granite (right) while, the rest is forced out of the top. When the magma spews out of the volcano, it is called lava. The lava cools and forms igneous rocks such as basalt (left). (Ignis is Latin for ‘fire’)
Some of the igneous rock rolls very slowly down the mountains formed by the volcanoes, helped down by rainwater and eventually ends up in the ocean. As they roll, bits and pieces of the igneous rocks are broken down and form sediments. Layer after layer of sediments are pressed down and cemented together forming sedimentary rocks. This is Jurassic sandstone, a sedimentary rock on its way to the sea, from Utah, in the USA. Look at the layering which indicates a geological event.
Some of the sedimentary rocks on the very bottom get hot because of the pressure. This heat and pressure changes the rock, interacting with water and minerals to form metamorphic rock. When the metamorphic rock is buried deeper, it gets hotter and melts. Once again, it becomes magma and may eventually be pushed up and out of a volcano.
Then, guess what. The whole process starts all over again. Have a look at this animation
Metamorphic Rocks
Metamorphic rocks are rocks that have changed.
The word comes from the Greek “meta” and “morph” which means to change form. Metamorphic rocks were originally igneous or sedimentary, but due to movement of the earth’s crust, were changed.
If you squeeze your hands together very hard, you will feel heat and pressure.
When the earth’s crust moves, it causes rocks to get squeezed so hard that the heat causes the rock to change.
Marble is an example of a sedimentary rock that has been changed into a metamorphic rock.
- Metamorphic rocks are the least common of the 3 kinds of rocks.
Metamorphic rocks are igneous or sedimentary rocks that have been transformed by great heat or pressure. - Foliated (leaf-like) metamorphic rocks have layers, or banding.
- Slate is transformed shale. It splits into smooth slabs and roof tiles are made from it.
- Schist is the most common metamorphic rock. Mica is the most common mineral.
- Gneiss has a streaky look because of alternating layers of minerals.
Sedimentary Rocks
When mountains are first formed, they are tall and jagged like the Rockies. Over time, millions of years, mountains become old.
When mountains get old, they are rounded and much lower. What happens in the meantime is that lots of rock gets worn away due to erosion. In a rain, freeze/thaw cycle, wind and running water cause the big mountains to crumble a little bit at a time. Eventually most of the broken bits of the rock end up in the streams & rivers that flow down from the mountains. These little bits of rock and sand are called sediments. When the water slows down enough, these sediments settle to the bottom of the lake or oceans they run into. Over many years, layers of different rock bits settle at the bottom of lakes and oceans. Think of each layer as a page in a book. One piece of paper is not heavy. But a stack of telephone books is very heavy and would squash anything that was underneath. Over time the layers of sand and mud at the bottom of lakes and oceans turned into rocks. These are called sedimentary rocks. Some examples of sedimentary rocks are sandstone and shale. Sedimentary rocks often have fossils in them.
Plants and animals that have died get covered up by new layers of sediment and are turned into stone. Most of the fossils we find are of plants and animals that lived in the sea. They just settled to the bottom. Other plants and animals died in swamps, marshes or at the edge of lakes. They were covered with sediments when the lake got bigger. When large amounts of plants are deposited in sedimentary rocks, then they turn into carbon. This gives us our fossil fuels, coal, oil, natural gas and petroleum.
Igneous Rocks
Towering nearly 400m above the tropical stillness of the Sunda Strait in Indonesia, one of the most terrifying volcanoes the world has ever known has begun to stir once more. 126 years since Krakatoa first showed signs of an imminent eruption, stunning pictures released in July 2009 prove that the remnant of this once-enormous volcano is bubbling, boiling and brimming over. Last time, the bang when it erupted was heard 4,000km away.This time there are thousands of people living under its shadow…
When volcanoes erupt and the liquid rock comes up to the earth’s surface, then new igneous rock is made. Igneous means made from fire or heat – from the Latin ‘ignis=fire‘. When the rock is liquid and inside the earth, it is called magma. When the magma gets hard inside the crust, it turns into granite.
Most mountains are made of granite. It cools very slowly and is very hard.
When the magma gets up to the surface and flows out, like what happens when a volcano erupts, then the liquid is called lava. Lava flows down the sides of the volcano. When it cools and turns hard it is called obsidian, lava rock , basalt or pumice – depending on what it looks like.
- Igneous rocks form when molten lava (magma) cools and turn to solid rock. The magma comes from the Earth’s core which is molten rock .
- Obsidian is nature’s glass. It forms when lava cools quickly on the surface. It is glassy and smooth.
- Pumice is full of air pockets that were trapped when the lava cooled when it frothed out on to the surface. It is the only rock that floats.
There are 5 kinds of igneous rocks, depending on the mix of minerals in the rocks.
- Granite contains quartz, feldspar & mica
- Diorite contains feldspar & one or more dark mineral. Feldspar is dominant.
- Gabbro contains feldspar & one or more dark mineral. The dark minerals are dominant.
- Periodotite contains iron and is black or dark.
- Pegmatite is a coarse-grained granite with large crystals of quartz, feldspar and mica.
Here’s a few pictures…
Mica is particularly interesting.
Mined from the earth in thin sheets, this mineral is extremely finely ground for use in cosmetics such as eyeshadow, mineral makeup, powder, lipstick, and sometimes nail polish.
The word mica comes from the Latin word “micare,” meaning to shine or glitter.
There are almost 50 different varieties of mica. They has a general formula
AB2-3(X, Si)4O10(O, F, OH)2
where A can be either potassium, sodium or calcium, B can be either aluminum, lithium, iron, zinc, chromium, vanadium, titanium, manganese and/or magnesium and X is usually aluminium, all the other symbols having their usual meanings.
Have you noticed? Often, transition metals are present. This is what gives the crystals their colours. Finally, here’s some polished malachite. It contains copper, so this gives it its green colour. It’s copper carbonate mostly and is formed by reactions between other minerals and water. It would be more correct to say that malachite is therefore metamorphic – see next post.
Rocks, an Introduction
Rocks are all the same, aren’t they? Well, not quite. Much of the Earth’s crust is covered by water, sand, soil and ice. If you dig deep enough, you will always hit rocks. Sometimes, of course, they are on the surface and people climb them!
- The Crust makes up less than 1% of the Earth’s mass (0.4%)
It is made of oxygen, magnesium aluminum, silicon calcium, sodium potassium, iron.
There are 8 elements that make up 99% of the Earth’s crust.
The continents are about 35 km thick and the ocean floors are about 7-10 km thick, but the oceanic rock is denser. - The Mantle is the solid casing of the Earth and is about 2900 km thick.
It makes up about 70% of the Earth’s mass (68.1%).
It is made up of silicon, oxygen, aluminum and iron. - The Core is mainly made of iron and nickel and makes up about 30% of the Earth’s mass (31.5%).
The Outer Core is 2200 km thick and is liquid and the Inner Core is 1270 km thick and is solid.
A rock is made up of 2 or more minerals.
Think of a chocolate chip cookie as a rock. The cookie is made of flour, butter, sugar & chocolate. The cookie is like a rock and the flour, butter, sugar & chocolate are like minerals.
You need minerals to make rocks, but you don’t need rocks to make minerals. All rocks are made of minerals. A mineral is composed of the same substance throughout. If you were to cut a mineral sample, it would look the same. throughout.
There are about 3000 different minerals in the world.
Minerals are made of chemicals we’ve seen or heard about before and are sorted into 8 groups.
Some common examples have been listed for each.
- Native Elements~ copper, silver, gold, nickel-iron, graphite, diamond
- Sulphides ~ sphalerite, chalcopyrite, galena, pyrite
- Halides ~ halite, fluorite
- Oxides& Hydroxides ~ corundum, hematite
- Nitrates, Carbonates, Borates ~ calcite, dolomite, malachite
- Sulphates, Chromates, Molybdates, Tungstates ~ celestite, barite, gypsum
- Phosphates, Arsenates, Vanadates ~ apatite, turquoise
- Silicates ~ quartz, almandine garnet, topaz, jadeite, talc, biotite mica
Some people make up stories and legends about rocks. These amethyst crystals in the picture are sometimes naturally hollowed out into ‘geodes’ where the ‘little people’ were supposed to live!
The Structure of our Planet
The Earth is a ball of diameter about 12,800km, which means a journey of nearly 40,000km all the way round – quite a long way.It’s a little bit like an eggshell. The atmosphere is an incredibly thin protective layer of gas that stops the sun from frying us or us freezing to death, sitting over a thin crust between 30 and 40km thick. The deepest places we’ve been to on the earth are in South Africa, where mining companies have excavated 3.5 km into the earth to extract gold. No one has seen deeper into the earth than the South African miners because the heat and pressure felt at these depths prevents us from going much deeper.
The crust is in the form of plates – called tectonic plates – which once fitted together. Over time, the plates have actually moved huge distances – South America once fitted nicely into West Africa like pieces of a jigsaw puzzle. When earthquakes happen, the rock layers can jolt over each other and the shockwaves can damage buildings and sometimes kill a lot of people. The recent earthquakes in Haiti and Chile caused many deaths and a great deal of damage.
Underneath the crust is the mantle, some of which near the crust is molten. Sometimes, the molten material has to escape and a volcano erupts. It is a much thicker layer than the crust. Beneath that, there’ a core which might even be solid because the pressure is so very high, which contains a lot of iron and nickel – making the Earth a magnet.
Here’s an animation of how the continents moved apart…
Carbon dating
Carbon has several isotopes. Carbon 12 is the stable variety, radioactive Carbon 14 has a half life of just under 5800 years. Any living organism takes in both radioactive and non-radioactive carbon, either through the process of photosynthesis, or by eating plants or eating animals that have eaten plants. When the animal dies, however, uptake of carbon stops. As a result, radioactive carbon atoms are not replaced as they decay, and the amount of this material decreases over time. The rate of decrease is predictable and can be described with some accuracy, increasing our ability to perhaps date the biological events of our planet.
A famous experiment was done in the 1970’s to date the Turin Shroud.
Carbon 14 is produced in the upper atmosphere when cosmic radiation from space interacts with nitrogen gas, converting nitrogen 14 to carbon 14. These carbon 14 atoms combine with oxygen to form carbon dioxide gas, which is absorbed by plants. We don’t actually know for sure if the rate of carbon dioxide formation has stayed constant over time, however, It’s usually quoted as a “small part of less than 1%” The difference casts doubt on the accuracy of the method.
Radioactivity – quick history
Henri Becquerel had already noticed that uranium bearing crystals in a locked drawer could darken wrapped photographic film and such ’emanations’ – as he called them – could turn air into an electrical conductor, it seemed. His crystals needed no energy source, they seemed to provide their own. The Curies, Marie and Pierre, who shared the Nobel Prize with Becquerel in 1903, brought fresh, original minds to the problem. They used an ore of uranium, pitchblende that produced a current 300 times larger than Becquerel’s original and reasoned that the ore must contain a new substance. They named it Polonium, after Marie’s home country of Poland and coined a new word – “radio-active”. The work was hard, gruelling and tedious, but they finally isolated another element, Radium, established the chemical properties of both and set the first standards by which the emission rate from different materials could be measured and compared. Additionally, they realised that the rate of emission decreased over time which could be calculated and predicted. Her greatest achievement however was to realise that radioactivity was a unique atomic property of matter. All this before Rutherford established for all time the essential structure of the atom.
Three types of ’emanation’ were eventually discovered.
Alpha particles are helium nuclei; large, heavy and lose energy quickly. A hand or thin piece of paper stops them. Beta particles are high speed electrons that travel close to the speed of light and can penetrate a hand but not concrete. Gamma rays are emitted with either of the other two because the nucleus is left in an excited state after emission of alpha or beta. A nucleus thus has a radioactive ‘fingerprint’ which is unique to a particular isotope in terms of decay pattern and half life. The activity or the rate of decrease is predictable and can be described with accuracy, vastly increasing our ability to date the biological events of our planet.
Balancing Equations
This sometimes seems hard, but it isn’t really.
Reactants go on the left, products on the right. The number of reactant and product particles has to be the same on both sides, which means that we can’t actually lose anything. The see-saw balances, or mass is conserved in other words – the particles are like money and we have to account for every single atom. Here’s an example.
When heated, aluminium reacts with solid black copper oxide to produce copper metal and aluminium oxide:
Writing the reaction down in symbols, making sure you KNOW them:
Al + CuO→ Al2O3 + Cu
The equation doesn’t balance, until we do this:
2Al + 3CuO→ Al2O3 + 3Cu
To solve this, I started by saying ‘I need 2 Al’s on the left and 3 more oxygens, so 3CuO’s’
A few tricks:
You can only put numbers in front of molecules, never change the formula of the compound itself.
H4O5 No! No! This isn’t water!
Don’t worry if when you start adding up, the numbers turn out to be fractions – you can always double or treble all the numbers at a later stage.
1/3H2O
Balance complicated molecules with lots of different atoms first. Putting numbers in front of these may mess up other molecules, so use the simpler molecules to adjust these major changes.
If you recognise the atoms making up a standard group such as sulphate, nitrate, phosphate, ammonium etc.that survive unscathed throughout the chemical reaction, treat them as an indivisible item to be balanced as a whole. This makes life easier and helps understanding of the chemistry.
Leave molecules representing elements until last. This means that any numbers you put in front of those molecules won’t unbalance any other molecule.
Click here for all the details. Work through all of this, there’s an exercise at the end.
John Vagabond's Physics and Chemistry Blog 
