Every day of our lives we encounter gases – we use them, we breathe them in, we walk through them without noticing.
Today in Chemistry Creations, we are exploring just some of the gases that affect our lives.
Check out this video from Matt to get us started.
As Matt says, oxygen is just one of the gases in the air around us – in fact it’s not even the most abundant.
The mixture of gases we call “air” is made up of lots of different kinds. Have a look at the pie chart
Earth Atmosphere Composition to learn more.
The gas making up most of the air is called nitrogen, with oxygen being the next most abundant. Small amounts of other gases are also present – including argon (a gas that is pretty much completely unreactive). Also in the mix is carbon dioxide (CO2) – a gas we hear a lot about in the news because of its role in driving climate change. It’s also a very interesting gas in its own right – with lots of uses. We use CO2 to make drinks fizzy – we call them "carbonated". Carbon dioxide is also very useful in its frozen form - below you can see a video of Matt using some frozen carbon dioxide (usually called "dry ice").
You can try making your own lava lamp at home – driven by CO2 – try the instructions
in our Make Your Own Lava Lamp (PDF).
You’ll be making a lava lamp using oil, water, and fizzy tablets.
It turns out it is not just gases that make up the air around us – there are also things like dust and pollen, as well as smoke or soot or forms of pollution. And of course if the amount of water in the air becomes too great, it falls as liquid water – rain!
The air around us also weighs something! We think of air as being easy to move through (we walk through it without noticing). It’s hard to imagine the air having mass – but at sea level every litre of air weighs just over one gramme. By comparison, one litre of water weighs almost one kilogram (a thousand times as much). Air isn’t very heavy, but there sure is a lot of it! Imagine you’re stood in a square that’s one metre on each side – the column of air above your square weighs 10 tonnes (10,000 kg)!
Water and Weather
In Scotland we get a lot of weather – sometimes all sorts of weather in the same hour! And a lot of the world’s weather can be explained by looking at just one chemical - water. Over 70% of the Earth’s surface is liquid water so it’s no surprise it has such a big effect. We normally think of water as the liquid that we drink – but the solid form (ice) and the gas form (steam/water vapour) are important too.
When water changes from one form to another, it can have dramatic effects - check out this video below!
In terms of weather, the water cycle involves the transportation of water around the landscape. It falls as rain on higher ground, finds its way into streams and rivers. The rivers widen as they flow downhill, shaping the landscape as they go, until they eventually meet the sea. But this is a cycle – the water in lakes and seas evaporates, turning from liquid into a gas, and rising into the air as water vapour and travels across the landscape. When there is too much water for the air to hold, it falls as rain again. Of course, in Scotland we contend with frozen water quite lot – ice, sleet, snow, hail – but in the past there was a lot more ice about. Looking back tens of thousands of years, Scotland was covered in ice – with huge rivers of ice called glaciers – that moulded the landscape we see today.
The weather is also influenced by things like temperature, pressure, time of year, wind speed.
There are all sorts of different weather phenomena that depend on water – from light rain showers to dramatic tornados! Tornados are tall but narrow columns of air that spin violently across the land during thunderstorms. They can be destructive but are also very interesting.
Do you want to make your own tornado at home? You can make on using a large bottle and some water – take care though! This one can be messy!
Water is a small molecule – consisting of just three atoms: two hydrogen atoms, and one oxygen – giving it the chemical symbol of H2O. Hydrogen and oxygen are chemical elements – join us tomorrow when we explore the periodic table of elements in our topic "The Chemistry of Fireworks".
The Periodic Table
Today, we are exploring something that is used lots and lots when talking about chemistry – the Periodic Table – or to give it its full name: The Periodic Table of Elements.
You might recognize the shape of the table from the picture below (the design of the table gets put on all sorts of items like lunchboxes, shower curtains, coffee mugs).
But what is it?
The Periodic Table is a summary of all the chemical elements that make up the world – the building blocks of chemistry.
For more of an introduction check out this video from Matt, featuring the huge Periodic Table we have in the Dundee Science Centre Café.
There are lots of ways you can use the Periodic Table – it contains a lot of information – in fact some ways of showing the table have lots of detail included, like this version here.
Things that can be included in a copy of the Periodic Table:
Name and Symbol: The most important things to include are the name or the element or its symbol.
Take oxygen for example – the symbol is O (which makes a lot of sense) - or for iron the symbol is Fe.
The symbols make it easier to talk about combining elements into compounds.
Group Number: this is a way of labelling the columns of the table – columns of elements tend to have similar properties. The left-most column, called Group 1 is made up mostly of reactive metals (with the exception of hydrogen). The right-most column, sometimes called Group 0, is made up of the “Noble Gases” – gases like helium or neon that react with almost no other chemicals.
Period Number: this is a way of labelling the rows (or “Periods”) of the table.
Atomic Number: the element’s atoms have a particular structure, made up of a nucleus at the centre and electrons orbiting around it. The nucleus is made up of two kinds of particle: protons and neutrons – the atomic number tells you how many protons are in the nucleus – for hydrogen it is 1, for carbon it’s 12, and for uranium it’s 92. The atomic number actually forms the definition of an element – a pure substance made of atoms with a fixed atomic number.
Atomic Mass: this is another property of the atomic structure. It’s the total number of protons and neutrons together in the nucleus – and can vary from atom to atom of an element. For example chlorine atoms all have 17 protons in the nucleus, but there can be either 18 or 20 neutrons (and very rarely 19). On average the atomic mass comes out as 35.45, which is somewhere in between! The heaviest natural element is uranium with an average atomic mass 238.03 (Note: the mass of the electrons hardly counts at all to the mass of the atom – an electron is over 1,800 times lighter than a proton or neutron, which have masses roughly the same).
And more! Some tables include things like the melting point and boiling point of the element, or are colour coded to show which elements are solids, liquids, or gases at room temperature.
But chemistry doesn’t stop at elements! You can combine elements in many different way to make what we call “compounds”. For example:
Water: has a chemical formula of H20 – meaning there are two hydrogen atoms and one oxygen atom in each molecule of water.
Carbon Dioxide: has a chemical formula of CO2 – meaning there is one carbon atom and two oxygen atoms in each molecule of carbon dioxide.
Table Salt: has a chemical formula of NaCl – meaning that for every atom of sodium there is one atom of chlorine.
But say you don’t know the chemical formula of your compound? How do you figure it out?
Well, one starting point is to look for tell-tale colours when your compound burns – as Matt demonstrates in this video.
Come back tomorrow where we’ll be exploring two very important types of chemicals – acids and alkalis!
Acids and Alkalis
There are lots of ways to classify chemicals. One very useful way is to decide whether or not you have an acid.
Particularly in films and TV, acids often shown as the dangerous, deadly, corrosive liquids that bubble green and
eat their way through wood, metal, and plastic alike. But the word "acid" covers a whole variety of chemicals
including some that we eat and drink.
Some common acids include:
Vinegar (contains acetic acid).
Lemon Juice (contains citric acid).
Vitamin C (aka “ascorbic acid”).
Fizzy drinks (contain carbonic acid).
Stomach acid (contains hydrochloric acid).
Car battery acid (sulphuric acid).
You can tell if you have an acid, by using an indicator. Acid indicators change colour when acid is present.
There are lots of different kinds of indicators, and you can even make your own at home – try these instructions
for making your own indicator from cabbage water!
More on Acids
So what do these different acids have in common? If you write out the chemical formulas of some acids (see Wednesday’s topic The Periodic Table) – they look like:
HCl (hydrochloric acid)
H2SO4 (sulphuric acid)
H2CO3 (carbonic acid)
The formulas are all different, but they all include atoms of hydrogen. An acid is capable of donating hydrogen when it
reacts – particularly when dissolved in water. Acids dissolved in water have a sour taste (in fact the latin word "acidus"
means sour) - although only some are safe to consume.
So if acids are donating some of their hydrogen atoms, what are they donating it to?
Acids will react with their chemical opposites – the alkalis – chemicals dissolved in water that will
accept the hydrogen donated by an acid.
Baking soda (aka “sodium bicarbonate” aka “bicarbonate of soda”).
Indigestion tablets (can contain magnesium hydroxide or calcium carbonate).
You can tell if you have an alkali by using an indicator too! Most indicators will be a different colour
for an acid than for an alkali – and a different colour again if you have neither!
Alkalis are usually soapy to the touch, and many shouldn’t be handled without gloves because they are corrosive.
Their chemical formula usually contains atoms from the first two columns of the Periodic Table –
Group 1 and Group 2, for example: sodium, potassium, magnesium, calcium (in fact these two columns
are often called the “alkali metals” (Group 1) and the “alkaline earth metals” (Group 2) for this reason).
Many alkalis also have an “OH” in their chemical formula (hydrogen and oxygen together making “hydroxide”).
NaOH (sodium hydroxide)
Ca(OH)2 (calcium hydroxide)
The way that acids and alkalis react together is very important.
Acids donate hydrogen, and alkalis accept it. This means they react – the alkali is said to "neutralise" the acid.
In most cases the end result is salty water. For example, hydrochloric acid and sodium hydroxide will react to make sodium chloride and water:
The hydrogen donated by the acid reacts with the hydroxide from the alkali, making H20 or water. The sodium and the chlorine together make sodium chloride, or table salt.
The acid and alkali react to make something neutral – the neutral state will show up with an indicator too – even your cabbage water!
The pH Scale
Acids, alkalis, and neutral chemicals all sit on a scale call the pH scale.
Acids have a pH less than 7, alkalis have a pH of more than 7 and perfectly neutral chemicals have a pH of exactly 7.
pH stands for “potential of hydrogen” – acids have the potential to donate hydrogen and alkalis have the potential to accept, but some more so than others.
Pure water has a pH of 7 – but by dissolving things in water you can change the pH. For instance, seawater has been
gradually more acidic over the decades because human activity is creating more carbon dioxide in the atmosphere.
More CO2 in the air means more dissolving in the oceans, creating carbonic acid (the same acid in fizzy drinks).
Many forms of ocean life have shells based on carbon carbonate – and so just as fizzy drinks can damage our
teeth, this “ocean acidification” is very harmful to life in the oceans.
It turns out that understanding acids and
alkalis is incredibly important!
Working in a Lab
For the final day of Chemistry Creations, we’re thinking about how chemists figure out answers to their questions – usually “what are these chemicals up to?”
To get us started, have a look at this video from Matt, where he takes a look at a technique called fluorescence".
Lots of techniques in chemistry involve shining light on a sample – in particular shining light of different colours to see which colours are absorbed and which are not – a technique called “spectroscopy”. You can put a number to the colour of light your using – called the wavelength – red light has a longer wavelength and violet light has a shorter wavelength – with the other colours of the rainbow having wavelengths in the middle.
In the video above, Matt uses ultra-violet light – the wavelength there is so short that human eyes can’t see it –
not until the sample absorbs it and releases light of a longer wavelength – this is fluorescence.
Instead of light, you can do other things to your chemicals. There is a process called “mass spectrometry” – where molecules
are smashed apart as they are fired through electric and magnetic fields. The heavier fragments take a more curved path and
so hit different parts of the detector to the lighter fragments. This way you can figure out the mass of the atoms in your sample.
Take for example sodium chloride – table salt – by looking at the Periodic Table (see Wednesday’s page) you can find the sodium
atoms have atomic weight 29 units and that chlorine is 35.45 units. The sodium atoms will leave one trace on the detector but the
chlorine will leave two – one trace for the heavier chlorine-37 and one for the lighter chlorine-35. As well as telling you the
elements present, mass spectrometry can tell you the “isotopes” present (chlorine has two common isotopes because of the
two different numbers of neutrons possible in the nucleus) – this is very useful when for example putting a date to a sample in geology or archaeology.
A lot of the time though, chemistry is not about pure substances – but about mixtures. There are lots of techniques for
separating out mixtures into their different parts. One very useful method is “chromatography” – for separating pigments
in inks and dyes. You can try it for yourself! You just need some coloured pens, a pot of water, and some kitchen roll.
Follow the instructions carefully and at the end you will have separated out your pens’ inks into their different components.
You might be surprised how many different colours go into the mixture of inks to make “black” ink.
Women In Chemistry
Learn more about the women we have already featured who work in chemistry.