Key Stage 4 - Cold chemistry
Chemistry of the cosmos
Briefing sheet 1: What is interstellar space?
There are hundreds of millions of stars in our galaxy, the Milky Way. But what is between the stars? Just empty
space?
The space between stars is called interstellar space. It is not empty. It is filled with matter, called the interstellar
medium. This matter makes up about 10 % of the mass of our galaxy. The matter originates from stars, which eject
many substances in explosions and winds. Some of these substances react in interstellar space to make new
substances.
Most of the interstellar medium is in the gas state. But about 1% of its mass exists as small solid dust grains.
Interstellar clouds
The interstellar medium is not evenly spread out. Much of it is concentrated into huge regions called interstellar
clouds. Interstellar clouds are denser than the rest of interstellar space. Some interstellar clouds eventually collapse
to form new stars.
In the Milky Way, most interstellar clouds are along the spiral arms of the galaxy. Some are between the arms. A few
are above and below the plane of the Milky Way.
Scientists classify interstellar clouds into two groups – diffuse clouds, and dense clouds.
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Diffuse interstellar clouds
The particles in diffuse clouds are very spread out. There are between 10 and 1000 particles in one cubic centimetre
(cm3). Compare this to the Earth’s atmosphere, which has about 10 billion billion particles/cm3. Diffuse clouds are
cold, with temperatures between 50 K and 100 K (−253 °C and −173 °C). They are much colder than anywhere on
Earth – even Antarctica, with the lowest recorded temperature of 184 K (−89 °C).
Dense interstellar clouds
The particles in dense clouds are closer together than the particles in diffuse clouds. In the core of a dense cloud,
there can be as many as 100 000 particles/cm3.
Dense clouds are even colder than diffuse clouds. In their cores, temperatures of 10 K (−263 °C) are common. The
matter around the core is slightly warmer, with temperatures up to 25 K (−248 °C). These low temperatures are close
to absolute zero, 0 K (−273 °C). The temperatures show that particles in interstellar clouds move very slowly indeed.
Dense interstellar clouds are enormous, with diameters of several light years. Their cores have diameters of up to 1
light year.
Guiding questions
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What is the interstellar medium?
What is an interstellar cloud? What might be formed when it collapses?
Where, in our galaxy, are most interstellar clouds?
Draw a table to compare diffuse and dense interstellar clouds. Include density and temperature data.
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Chemistry of the cosmos
Briefing sheet 2: What particles are in interstellar space?
There are hundreds of millions of stars in our galaxy. But what is between the stars? Just empty space?
The space between stars is filled with matter. The matter is called
interstellar medium. Most of the interstellar medium is concentrated
into huge interstellar clouds. Scientists classify interstellar clouds into
two groups – diffuse clouds and dense clouds.
Most of the interstellar medium is in the gas state. But about 1% of
its mass exists as small solid dust grains. The interstellar medium is a
mixture of substances. Some of these substances are new to science.
They exist only in the conditions of interstellar space.
The mixtures that make up the two types of interstellar clouds are different.
Particles in diffuse interstellar clouds
Most particles in diffuse clouds are simple atoms and molecules. Hydrogen molecules (H2) are most common, then
helium atoms. There are smaller amounts of oxygen atoms, nitrogen molecules, and carbon monoxide molecules.
Scientists have also detected ions in diffuse interstellar clouds. No one has identified all the ions. Many of the ions
seem exotic, as they do not exist naturally on Earth. These include ions with the formulae S+, CH+, C7- and C60+.
Particles in dense interstellar clouds
Most gas particles in dense interstellar clouds are molecules. Amazingly, scientists have detected molecules of at
least 120 substances. The most common are hydrogen and carbon monoxide. Others include water, ammonia and
methanol.
Scientists have also detected molecules of compounds that do not exist naturally on Earth, including those with the
formulae C2H, HNC, and HC11N. There are also strange molecules that include three carbon atoms joined in a ring, for
example C3H and C3H2.
Some scientists report that they have found evidence that amino acids, for example glycine, exist in dense
interstellar clouds. Amino acids are the compounds that join together to make proteins in living things.
Some of the particles in dense interstellar clouds are positively-charged ions. They include H3+, HCO+ and H3O+.
Interstellar dust
Scientists have studied interstellar dust grains carefully. The grains include compounds of silicon, carbon, and oxygen
– rather like soot and sand. In dense interstellar cores, substances such as carbon monoxide, carbon dioxide, and
methanol gather on the dust in their solid states.
Guiding questions
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Is the interstellar medium an element, compound, or mixture?
Name the most common element in the interstellar medium. Draw a dot and cross diagram of its molecule.
Draw dot and cross diagrams for molecules of water, ammonia, and methanol (CH3OH).
Draw a table to compare the names and formulae of some of the substances in diffuse and
dense interstellar clouds.
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Chemistry of the cosmos
Briefing sheet 3: How do reactions happen in interstellar space?
There are hundreds of millions of stars in our galaxy. But what is between the stars? Just empty space?
The space between the stars is filled with matter. The matter is called interstellar medium. The interstellar medium
is a mixture of particles of many substances. Most of the interstellar medium is in the gas state. But about 1% of its
mass exists as small solid dust grains.
The particles in the interstellar medium are very spread out. In some regions, there are only 10 particles per cubic
centimetre (cm3). Compare this to the Earth’s atmosphere, which has about 10 billion billion particles/cm3.
Interstellar space is cold. Temperatures between 10 K (−263 °C) and 100 K (−173 C) are common.
How do the conditions in interstellar space affect gas reactions?
Chemical reactions make new substances in interstellar space, just as they do on Earth.
Particles must collide before they can react. In interstellar space, particles move slowly. A particle might collide with
another particle once a fortnight. In Earth’s atmosphere, particles collide 10 billion times every second. However,
astronomers measure timescales in space in millions of years, so there is plenty of time for collisions.
Only some colliding particles actually react, on Earth and in space. The collisions that lead to reactions are those that
have enough energy to break bonds to get the reaction started. This is the activation energy.
On Earth, many reactions occur between neutral molecules. These
reactions have high activation energies. In interstellar space, many
reacting particles are ions. Other reacting particles in space are free
radicals – atoms or molecules that need just one more electron to
gain a full outer shell. The activation energy of many reactions
involving ions or free radicals is zero. At very low temperatures,
these reactions may actually speed up.
Reactions on dust grains
Some reactions in interstellar space happen on the surface of dust grains. The dust changes the pathway of a
chemical reaction. It acts like a catalyst.
Imagine a hydrogen atom colliding with a piece of dust. It sticks to the surface of the dust. It may diffuse over the
surface of the dust, and find another hydrogen atom. The two atoms join together. They release a little energy. This
energy allows the newly-formed hydrogen molecule to move away from the dust.
Other particles form on the surface of dust grains, for example OH, CH, and NH. Eventually these particles join with
hydrogen atoms to form water (H2O), methane (CH4), and ammonia (NH3).
Guiding questions
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Explain why you might expect reactions in the interstellar medium to be much slower than reactions in Earth’s
atmosphere.
Explain why many gas reactions in interstellar space are not as slow as expected.
Describe how hydrogen atoms react to make molecules on the surface of a dust grain.
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Chemistry of the cosmos
Briefing sheet 4: How do scientists study interstellar chemistry?
There are hundreds of millions of stars in our galaxy. But what is between the stars? Just empty space?
The space between the stars is filled with matter. The matter is called interstellar medium. Scientists have found out
a huge amount about the interstellar medium. They have identified the substances that are in it. They have found
out about its chemical reactions. But how do scientists study chemicals and their reactions many light years from
Earth?
Scientists ask questions. They use creative thought to suggest explanations. They then collect evidence to test the
explanations. Here are some of the ways they collect evidence.
Emission spectroscopy
Many molecules in the interstellar medium emit (give out) electromagnetic radiation. Scientists use radio telescopes
to detect this radiation, and measure its wavelength. Molecules of different substances emit radiation of different
wavelengths. This means that scientists can use wavelength values to identify substances.
Absorption spectroscopy
Stars emit electromagnetic radiation of all wavelengths. The radiation travels from the star in all directions. Some of
the radiation may travel through an interstellar cloud. Scientists observe the radiation that reaches Earth. Some of
the wavelengths are missing. They have been absorbed by particles in the interstellar cloud. Particles of different
substances absorb radiation of different wavelengths. Scientists use instruments to find out which wavelengths are
missing. They use these values to identify substances in the interstellar cloud.
Scientists cannot study every interstellar cloud like this. The technique depends on radiation from a star reaching an
interstellar cloud, and there being nothing in the way.
Laboratory simulations
Scientists also collect evidence by doing experiments. Of course,
they cannot travel to the interstellar medium. So they reproduce
the conditions in the laboratory.
One group of scientists cools reactants to 3 K. At this temperature,
they can control the position and speed of the particles. They
observe them for up to 10 seconds as they react.
In another experiment, Oxford scientists use a Stark decelerator to
slow molecules down. The molecules move so slowly that the
temperature decreases to 0.01 K. This is close to absolute zero.
Guiding questions
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Describe how scientists use emission spectroscopy to identify molecules in the interstellar medium.
Describe how scientists use absorption spectroscopy to identify particles in the interstellar medium.
Explain why some scientists reproduce the conditions of interstellar space in the laboratory.
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