Chemistry Definitions S-Z

Chemistry Definitions  of words beginning with the letters S to Z.

S

Sacrificial anode     Iron can be protected from rusting by connecting a more reactive metal (such as magnesium) to it. The magnesium reacts in preference to the iron and, if the magnesium is placed in an easily accessible location, it can be replaced more easily than the iron structure.

Salt     In general this is a compound formed when an acid reacts with a metal, a metal oxide, a metal hydroxide or a metal carbonate. You need to know these general equations:

METAL + ACID       goes to           SALT   +   HYDROGEN

METAL OXIDE  + ACID       goes to           SALT   +   WATER                      

      METAL CARBONATE  +   ACID       goes to           SALT   +   WATER   +   CARBON DIOXIDE

Hydrochloric acid gives chlorides
Sulfuric acid gives sulphates
Nitric acid gives nitrates

Common Salt has the chemical name sodium chloride. It is the salt that you might use to put on chips ! 

You could make common salt by reacting sodium hydroxide with just the right amount of hydrochloric acid. Be careful to use just the right amount though!

Sandstone     Grains of sand can be formed into this sedimentary rock by the processes of deposition and cementation.

Saturated    This has many different meanings depending on the context.

1.     Saturated solutions cannot dissolve any more solute at that temperature.

2.     Saturated hydrocarbons (alkanes) have only carbon and hydrogen atoms and use only single bonds.

Sediment     The grains of rock or animal remains that are dropped to the bottom of a river, lake or sea.

Sedimentary rock     Rock formed from layers of sediment after the grains of sediment have been squashed together and have  undergone cementation.

Shape memory alloys     These can be bent into a variety of shapes but will return to their original shape when heated. They can be used for medical applications where even the warmth of the body can cause a row of sutures made of them to pull a wound together.

Slaked lime     Calcium hydroxide, it is made by adding a limited amount of water to quicklime (calcium oxide). It can be used as a soil additive to reduce the acidity of soil.  

Smelting     The extraction of a metal from its ore.

Structural formula     This is sometimes called a displayed formula. It shows the arrangement of the atoms within a molecule. When drawing the structural formula for a hydrocarbon, remember that all carbon atoms must have four bonds and every hydrogen atom has only one bond.

Sulphur compounds     They are present in crude oil and so find their way into petrol and diesel. When they are burned, sulphur dioxide is released. 

Sulphur dioxide     This gas is a major contributor to acid rain. It is formed by the combustion of sulphur and sulphur compounds. It is an irritant too.

Symbol (chemical symbol)     This may be one, two (or even three) letters. The first letter is always a capital. The others are lower case. The three letter symbols are for elements that have not yet been assigned a name (eg ununquadium which is atomic number 114 and has the symbol Uuq).

Symbol equation     If a chemical equation is done using symbols it MUST be balanced. There must be the same number of each kind of atom on each side of the equation.

T

Theory     A scientific explanation. A scientist uses imagination to produce a theory which seems to explain the observations that have been made. An experiment is then performed to try to either confirm or disprove the theory. The theory is then modified as these results are taken into account. The whole process continues. 

Although it may seem that science is a list of facts, it is continually evolving. Some of the ideas that were considered perfectly acceptable one hundred years ago are now considered too simple or even wrong. It is possible, though, that some of the ideas we believe today might turn out to be mistaken. Somebody reading this website today could be the scientist who will produce the new theory! 

Thermal conductivity     One of the properties of metals, it is the ability to pass heat energy. Metal are good thermal conductors.

Thermal decomposition     A chemical reaction where one substance is broken down into more than one substance by the action of heat. A good example is the thermal decomposition of limestone (calcium carbonate).

Transparent     Light can travel through transparent substances. "See-through" is an alternative description (but not as good).

Transition metal     One of the metals from the central block of the periodic table. Iron is a good example. Transition metals have all the usual properties of metals but in addition usually have particularly high melting and boiling points, high density and usually form coloured compounds. They are often active as catalysts too.

Transport     Movement of particles of rock. Together with weathering, transport makes up the process off erosion.

U

Universal indicator     This is a mixture of indicators which shows the strength of an acid or alkali as well as simply showing which it is. By comparison with a colour chart, a pH number can be selected. pH numbers less than 7 show an acid (the smaller the number, the stronger the acid) and pH numbers bigger than 7 show an alkali (the larger the number, the stronger the alkali).

Unreactive     A substance for which reactions are difficult. Unreactive substances come at the bottom of a reactivity series. A typical unreactive metal is gold. Iodine is a typical unreactive non-metal.

Unsaturated     An unsaturated hydrocarbon is a substance that contains only carbon atoms and hydrogen atoms and it must contain at least one carbon-carbon double bond. Alkenes are unsaturated hydrocarbons. So are alkynes. 

V

Vacuum     A completely empty space. The collapsing can experiment shows the effect of the pressure of gas particles. The gas particles on the inside of the can are removed, leaving a vacuum. The pressure of the particles on the outside squash the can. 

A common mistake is to think that the vacuum sucks the can inwards.

Variable     A factor that can be changed in an experiment. There are different ways to describe variables and these are important for "How Science Works".

Discrete variable:     Only whole numbers are possible (such as how many layers of insulation). 
The values are steps.

Continuous variable:     Any number is possible (such as the thickness of insulation).
The values are gradual.

Categoric variable:     A description is used to identify the variable (such as cork insulation or polystyrene)

Independent variable:     The variable that is adjusted to change the conditions of the experiment so that we can find out the value of the dependent variable.

Dependent variable:     The one we find out about. Its value depends on the value of the variable that we adjust.

Fixed variable:     A factor that could have been changed but we decided to hold constant in order to make the experiment a fair test.

Vibration     Solids are made of particles that are vibrating. The particles are fixed in position but can wriggle about.  

W

Water vapour     This is water in the form of a gas. Although there is a technical difference between a vapour and a gas but that does not matter at GCSE. When a kettle is boiling, many people call the white clouds "steam". This is not correct.  The true steam is the invisible gas that is right by the mouth of the kettle. The white clouds are actually made of droplets of liquid water that have condensed as the steam met the cooler air. 

The scalds from steam are much worse than scalds from liquid water because the energy released as the gas turns into liquid is added to the energy released as a hot substance becomes cooler.

Weathered     Rocks that have been worn away are said to have been weathered. This does not have to be because of the weather! The action of roots, people walking  etc are all weathering just as much as the effect of wind-borne grit and acid rain.

Weathering     The breakdown of rocks. This does not include the deliberate extraction by mining or quarrying.

Word equation     A way to record the changes that have taken place in a chemical equation. If a word equation is asked for, be careful to use the correct words (eg do you mean chlorine or chloride, sulphide or sulphate etc). Make sure that you include all the substances. Do not use "=" (which would mean that the substances are the same but rather use an arrow to show that the substances have changed.

X

X-ray     These are released when electrons move between the inner shells. They are used in medicine to study broken bones. Bone is opaque to X-rays but soft tissue is transparent. The digestive system can be investigated by giving the patient a meal containing an X-ray opaque material (usually a barium compound such as barium sulphate). 

X-rays are actually harmful and so the operators work behind a lead shield. Patients are given the minimum dose of X-rays that will give the information required. Sensitive areas (the genital areas in particular) are protected by a lead sheet.

Y

Yellow    Sulphur (actually sulphur these days) is one of the oldest known substances. It forms two crystal types (allotropes) depending on the temperature at which the crystals are formed.

Z

Zwitterion     A form of an amino acid which has both a positive and negative charge. The amine group has an extra H⁺ and the carboxyl group loses its H⁺