Chemistry Definitions M-R

Chemistry Definitions  of words beginning with the letters M to R.

M

Magma     Molten rock beneath the surface of the Earth. When magma spills onto the surface of the Earth (maybe from a volcano) it is called lava.

Magnetic stripes     Tectonic plates move around the Earth. It is believed that the ocean floors are spreading so that the continents are moving further apart. Measurements of magnetic properties along the ocean floor show that the rock has been formed at different times.

Mantle     The layer of the Earth immediately below the crust. Beneath the mantle is the core.

Melting point     The temperature at which a solid turns into a liquid.

Metal     A metal is a substance whose atoms are so close together that their outer shells overlap. This allows electrons to wander from atom to atom as part of a delocalised sea of electrons. This allows the metal to conduct electricity. Metals also conduct heat, are shiny, malleable and ductile, sonorous. They also (usually) have high melting points and boiling points.

Metamorphic     Referring to rock types it means that the rock has been changed by the action of heat and / or pressure. Examples include marble (which is formed from limestone).

Minerals     The chemical substances from which rocks are made.

Mixture     Two or more substances loosely joined together. The substances are not chemically joined together. Mixtures can usually be separated into their components quite easily.

Molecule     Two or more atoms joined together by a covalent bond. Molecules can be elements (eg H₂ , O₂ etc) or compounds (eg CH₄ , H₂SO₄).

Molecular formula     This shows the number of each kind of atom in a molecule. (eg CH₄ has one carbon atom and four hydrogen atoms; H₂SO₄ has two hydrogen atoms per molecule, one sulphur atom and four oxygen atoms per molecule)

Mortar     It is made by mixing sand and cement with water. It sets hard and is used to hold bricks etc together.

Mudstone     A sedimentary rock made from small particles of mud.

N

Native     Used to describe a metal that is found as an element even though it may be mixed with other substances. Gold is often (!) found native. Even though it may not be in large lumps, it is not chemically combined with another element.

Natural gas     The fossil fuel that is formed (usually along with crude oil) when the remains of dead sea creatures decompose over millions of years. It is composed mainly of methane (CH₄) and it is the gas that is used in the home and in school for Bunsen burners.

Neutral     A substance that is neither acidic nor alkaline. It has a pH of 7.

Neutralisation     A reaction where an acid "cancels out" an alkali. 

Neutral oxide     Although most metal oxides are bases and most non-metal oxides are acidic, a few form neutral solutions in water. One example of this kind of oxide is carbon monoxide.

Nitrogen oxides     When nitrogen enters the cylinders of a car engine, it can be burned to form a variety of oxides. Rather than  deal with all the different ones individually, it is useful to discuss them together as "nitrogen oxides". They are sometimes written as NO_x. They are removed from the exhaust gases of a car by using a catalytic converter.

Non-metals     Although this sounds like it ought to be everything that is not a metal, it really means the elements that are not metals. The non-metals are usually dull, have low melting and boiling points and do not conduct electricity There are some exceptions though like the graphite form of carbon which can conduct electricity.

O

Observation     What you see during an experiment. You should look carefully at the substances before they react, while they are reacting and after they have reacted. Observations should include a colour and a physical state. A good observation could be something like "a pale brown liquid was formed near the positive electrode". That is far better than, "it went brown".

Some people make the mistake of saying something like, "hydrogen was produced" instead of , "a colourless gas was produced which, when tested later, burned with a squeaky pop".

Oil     Crude oil is the decayed remains of plants and animals that lived in the sea. The oil has been formed over a few million years. Crude oil is the major source of alkanes.

Ore     A substance that can be used as a source of a metal. Haematite is an ore of iron. To be useful, an ore has to have a high enough percentage of the metal in it to be economically viable to extract.

Oxidation     A reaction in which oxygen is added to a substance. For every oxidation, there is always a reduction (something losing oxygen) of some kind. 

Oxide     The new substance that is formed when a substance has reacted with oxygen.

Metal oxides are bases...they neutralise acid

Non-metal oxides are usually acids. A few oxides are neutral.

Oxide layer     Some metals (like iron) have oxides that crumble away to expose new metal. Some metals (aluminium and titanium are two good examples) forms oxide layers that are tough and protect the metal from further attack. Think about the position of aluminium in the reactivity series (very high) but yet it is used to make window frames that have to resist corrosion for many years. If you want to see the true reactivity of aluminium ask your teacher to show you aluminium in water once it has had its oxide layer removed by rection with mercury.

P

Particles     The bits of matter from which everything is made. If the particles stick together in fixed places and vibrate, we have a solid; if they can roll and slide about but remain in contact with each other, we have a liquid; if they fly around and are free from each other, we have a gas.

The particle theory (the particulate nature of matter) can explain many properties of the three states of matter. Our understanding of diffusion of gases and liquids, dissolving, solids maintaining their own shape, melting and boiling etc all depend on the particle theory.

Particulate     When fossil fuels are burned, some tiny bits of carbon can be left. These small particles are a source of air pollution and can lead to global dimming.

Particulate theory     Matter is made of particles.

Periodic table     Most people use the form of periodic table formulated by Mendeleev. This shows all the elements that have so far been discovered. They are in order of atomic number. the sets of elements with similar properties are arranged in vertical columns called GROUPS. The horizontal rows are called PERIODS.

When Mendeleev created the table, he was brave enough to leave gaps for elements that had not at that time been discovered. later, scientists looked for elements to fill the gaps hoping that they would have the properties Mendeleev predicted. This worked out well.

Permanent change     The new substance that you have made cannot be changed back into the starting materials. Chemical changes are permanent changes. Some people use the term "irreversible change" too.

pH     NOT the "power of hydrogen"!!! It is a mathematical expression:

pH = -log₁₀[H⁺]                   [H⁺] means "the concentration of hydrogen ions in mol dm^-3 "

At GCSE you need to know that 
Acids have pH less than 7.
Alkalis have pH greater than 7.

Weak acids have a pH nearer to 7 than strong ones. 
Weak alkalis have pH nearer to 7 than strong ones.

Here is a chance to prove your English teacher wrong! You CAN start a sentence with a small letter:

pH is a measure of the strength of an acid.    This is correct because the p of pH is always lower case.

Phlogiston     An old theory said that when metals burn they lose phlogiston. The theory explained the fact that metals get heavier when they burn by stating that phlogiston had negative mass. Although interesting as a theory to show how science has developed, we no longer believe the phlogiston theory.

Photosynthesis     Plants take in carbon dioxide and water. They use the energy from the Sun to make new substances glucose and oxygen. Glucose and oxygen have a higher energy content than carbon dioxide and water. 

Physical change     No new substances are made. It is fairly easy to reverse this kind of change. 
An example would be dissolving salt in water. The change can be reversed simply by evaporating the water.

Physical weathering     Physical processes (such as temperature change, wind-borne grit etc) that can cause a rock to be broken down into smaller pieces.

Porous     Porous rocks have tiny holes in them that can soak up water or other liquids. Crude oil is found soaked into porous rocks.

Potassium     Watch out for the spelling of this word. It is a silvery metal that reacts violently with water. It is one of the alkali metals. By comparing the reactivity of lithium, sodium and potassium you can see tone of the trends in the groups of the periodic table.

Precipitate     An insoluble solid which is formed when two solutions react together.

Pressure     The force caused by particles hitting the walls of a vessel. 

Product     The new substance or substances that are formed during a chemical reaction. 

Property     A feature of a substance that can be described or measured. Chemical properties refer to how the substance behaves in a chemical reaction (eg it is an acid, it burns in air) while a physical property is to do with the substance itself (eg it is a hard, shiny solid with a melting point of 465^oC).

Pure     A pure substance contains only one substance. Pure substances have a single melting point and a single boiling point. 

Many people wrongly believe that "natural" and "organic" things are pure. 
"Natural" spring water contains many salts.
 "Organic" in this context means that it has been grown without artificial fertiliser or weedkillers etc.

Q

Quartzite     This is a metamorphic rock that is made by the action of heat and pressure on sandstone.

Quicklime     Calcium oxide (it is formed by heating limestone (calcium carbonate) very strongly)

Quicksilver     An old name for mercury. 

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 sulphate 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).