R

Reactants     The chemicals that react together to form the new materials. The word reagent is sometimes used instead. 

Reactive      A substance that is easily able to react. When considering metals, the reactive ones are at the bottom of the periodic table. When considering non-metals, the reactive ones are at the top.

Reactivity series     A list of metals or non-metals that has the most reactive at the top. Sometimes hydrogen and carbon are included in the metal series. Carbon is included because of the importance of carbon in the reduction of metal oxides. Hydrogen is included because only metals more reactive than it can release hydrogen from an acid.

Reagents     The chemicals that react together to form the new materials. The word reactant is sometimes used instead.

Recycling     Metals (and other things like plastics and paper) can be re-used. Think about an aluminium drinks can. Recycling involves the can being melted and re-shaped into another object. This requires some energy. If the same amount of aluminium had to be extracted from its ore, that would require a huge amount more energy. So, recycling aluminium makes economic sense.

Also, although not a particular problem for aluminium, there is only a fixed amount of metal oxides in the world, so it makes sense to avoid using it all up if we don't need to.

Reduction     1.     The removal of oxygen from a substance. This is the main process when metals are extracted from their ores.

Reduction      2.     The addition of one or more electrons, especially in terms of an electrode reaction during electrolysis. An example would be Na⁺ gaining an electron at the cathode to form Na.

Respiration     All living cells carry out this reaction. Glucose and oxygen are converted to carbon dioxide and water releasing energy. The energy is used to carry out useful reactions in the cell.

Reversible change     A change that can be undone fairly easily. Often called a physical change or a temporary change.

Reversible reaction     Some chemical reactions can be undone. An example is the removal of water from copper sulfate crystals. Heating the crystals (blue) loses water and the crystals become a white powder. If the powder is left to cool and then cold water is added, the powder turns blue and gets very hot. The reaction is reversed and the energy change is reversed too.

Reversible reactions in a closed system can set up an equilibrium. This is usually studied in the context of the Haber process.

Rock cycle     The interconversion of igneous, metamorphic and sedimentary rocks and all the processes such as weathering, transport and cementation are all part of the rock cycle.

Rust     When iron reacts with water and oxygen, rust is formed. Chemically it could be called hydrated iron oxide. Although other metals may corrode, only iron can be said to rust.

The need for water AND oxygen can be demonstrated in an experiment using boiled water to provide water but no oxygen and using a sealed test tube containing anhydrous calcium chloride to provide air with no trace of water.

Prevention of rust is an important topic. Techniques include coating the iron (eg paint, oil, grease etc) or using electrical protection (such as sacrificial anode).

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