Before we look at chemical change, let’s first get familiar with some of the theory behind the practical stuff.
One of the most popular theories about how life formed on Earth is based on experiments by Miller and Urey.
Stanley Miller and Harold Urey carried out some experiments in 1952. The aim was to see if substances now made by living things could be formed in the conditions thought to have existed on the early Earth.
Their theory consisted of containing a mixture of water, ammonia, methane and hydrogen in a sterile flask.
This was then heated to evaporate water to produce water vapour. Electric sparks were passed through the mixture of water vapour and gases, simulating lightning.
After a week, contents were analysed. Amino acids, the building blocks for proteins, were found. Hooray!
In a completely separate theory, a guy called Alfred Wegener came up with the idea of continental drift.
Wegener first put forward his theory in 1915. At this time, scientists believed that mountain ranges were formed as the crust shrank and the early molten rocks in the Earth cooled down.
They stuck with this idea until 50 years after Wegener proposed his theory, when scientists found that the seafloor is spreading apart in some places between continents. The idea that the continents used to form one land mass is supported by evidence from coastline geometries, fossil data and rock structures.
Sticking with the topic if the Earth’s make-up; the crust and upper mantle of the Earth are made from tectonic plates.
Evidence from fossils and rock structures, as well as the geometry of the coast lines across the Earth, tell us that South America and Africa used to form one land mass and are still gradually moving apart.
This movement is due to the movement of tectonic plates. Deep within the Earth, radioactive decay produces vast amounts of energy which heat up the mantle. Minerals in the mantle expand and rise to the surface as they become less dense.
Cooler, more dense minerals sink to take their place. These movements create forces called convection currents, which move tectonic plates slowly across the Earth.
If a reaction takes places and it transfers energy from the reactants to the surroundings, this is known as an exothermic reaction.
We can exploit the energy transferred during a reaction. For example, energy released from the exothermic re-crystallisation of sodium ethanoate is used in hand warmers.
Soaking filter paper in blue cobalt chloride and then leaving it to dry makes cobalt chloride paper. The paper turns pink when water is added, so this can be used as a test for the presence of water.
The cobalt chloride in the paper is anhydrous (blue), because all of the water has evaporated. The water can be added back into the cobalt chloride in the reverse reaction to form a pink, hydrated compound. The forward reaction is exothermic and the reverse reaction is endothermic.
During a chemical reaction, energy is always transferred when chemical bonds are broken and new ones are formed. In an exothermic reaction, energy released from the formation of the products often heats up the surroundings and we can measure a rise in temperature as the reaction progresses.
If a reaction transfers energy from the surrounding to the reactants this is an endothermic reaction. In an endothermic reaction, the surroundings decrease in temperature as the reaction progresses.
Collision theory tells us that, for a reaction to take place, particles must collide with enough energy to react. The minimum amount of energy needed for a reaction to happen is known as the activation energy.
A top tip to remember is that using powdered reactant instead of solid reactant increases the number of collisions between particles because the powder has a larger surface area.
One final thing to know about chemical changes, we are able to separate liquids and gases using fractional distillation.
Fractional distillation separates liquids according to their boiling points. In industry, the air is compressed to 150 times the atmospheric pressure and passed over pipes carrying cold water because the air warms up when it is compressed.
When the pressure is released, the air expands rapidly and cools down so much that it condenses into a liquid. Carbon dioxide and water in the air are cooled down to solids at this temperature and are removed.
The remaining gases in the liquid are heated up gradually so that nitrogen and oxygen vaporise in turn and are collected as pure gases.