IS MATTER AROUND US PURE?If we observe some sugar and some soil placed on two different sheets of paper with a magnifying glass, we will find that the colour, shape and size of all the particles of sugar are the same, but the soil contains particles of different colours, shapes and sizes. For example, the soil contains clay particles, some grass particles and even some dead insects, etc. Now, sugar which contains particles of only one kind is called a pure substance whereas soil which contains particles of different kinds is called and impure substance (or mixture). From this we conclude that all the matter around us is not pure. The matter around us is of two types: pure substances and mixtures. The mixtures are impure substances. We will now discuss pure substances and mixtures in a little more detail.

Matter can be classified in terms of its chemical composition as a pure substance or as a mixture. A pure substance is a single kind of matter that cannot be separated into other kinds of matter by any physical means. All samples of a pure substance contain only that substance and nothing else. Pure sucrose (table sugar) contains only that substance and nothing else.

A pure substance always has a definite and constant composition. This invariant composition dictates that the properties of a pure substance are always the same under a given set of conditions. Collectively, these definite and constant physical and chemical properties constitute the means by which we identify the pure substance. A mixture is a physical combination of two or more pure substances in which each substance retains its own chemical identity. Components of a mixture retain their identity because they are physically mixed rather than chemically combined.

Pure substances: Elements and compounds

I. Elements

Pure substances

An element is a pure substance which cannot be broken down by further chemical techniques. These include heating, cooling, electrolysis and reacting with other chemicals. (By the way, it is correct that an atom can be destroyed, but NOT by chemical means. You must use a more powerful reaction, called a nuclear reaction, to destroy or change atoms. That is a topic for a lesson in a different unit.)

A sample of an element contains only one kind of atom in the sample. Suppose you had a lump of copper in your hand. The ONLY type of atom in the lump is copper. In the lump there are trillions and trillions and trillions of copper atoms. NOTHING else. (I am ignoring impurities such as a tiny piece of rock occluded within the lump, some zinc atoms randomly trapped among the copper atoms, grease from your skin sticking to the surface of the lump or oxygen atoms from the atmosphere absorbed onto the surface of the copper.)

If you were to heat the lump of copper, it would melt and eventually vaporize. The smallest unit of the copper, called the atom, would remain unaffected by this. The atoms of copper would be in the solid state, the liquid state or the gaseous state, but they would be EXACTLY the same in each state.

The atom is the smallest subdivision of an element which still retains the properties of that element. In fact, a very good definition of an atom is:

The smallest part of an element that can enter into a chemical combination

There are around 118 elements known to man, of which 20-30 are really, really important. Almost every element that exists has some form of use. There are some which are so unstable they only last for seconds or even tiny fractions of a second and no use has yet been found for them.

Elements have names and symbols. For example hydrogen has the symbol H and iron has the symbol Fe. Please note that Fe is one symbol, not two. Also, make sure to use lower case for the second letter. Writing BR for bromine is incorrect, writing it as Br is correct.

II. Compounds

A compound is a pure substance composed of two or more different atoms chemically bonded to one another. A compound can be destroyed by chemical means. It might be broken down into simpler compounds, into its elements or a combination of the two. The key distinction is that compounds break down whereas the same techniques do not cause an element to break down.

The molecule is smallest subdivision of a compound that still retains the properties of that compound. The parallel definition (to the element one above) for the molecule is:

Water is a typical example of a compound. One molecule of water is composed of two hydrogen atoms and one oxygen atom, chemically bonded together. It is identified with its formula: H2O.

An important point to remember is that the compound is going to have distinctly different properties than its elements. Hydrogen has a set of properties, as does oxygen. However, the set of properties that water has in no way like the two elements. For example, at room temperature (about 20-25 °C) water is a liquid while hydrogen and oxygen are gases.

Another classic example is sodium chloride (formula = NaCl). Sodium metal (Na) and chlorine gas (formula = Cl2) have very, very different properties from each other and neither one of the two is like sodium chloride at all.

Compounds have names and formulas. The formula is made from the symbols of the elements in the molecule and how many of each element there are. For example, glucose's formula is C6H12O6.

There are something over 12 million known chemical compounds. Well over 75% of them are mentioned in only one scientific article. Of the remaining bunch, there are several thousand of great interest and usefulness to science.

The need for pure

Many industries need pure substances to make products such as foods, steel, computer chips and medicines.

A pure substance is a single substance not mixed with anything else.

The need for pure

In the pharmaceutical industry, medicines must be pure. Impurities can be dangerous as they may poison people.

In the computer industry, silicon is used to make silicon chips. Extremely pure silicon (99.999999 % pure) has to be used.

In the steel industry, impurities can make it weaker than it should be.

How to know if a substance is pure?

Chemist use some complex methods to check purity. But there is one simple method you can use in the lab, you can check melting and boiling points.

A pure substance has a definite, sharp, melting point and boiling point. You can look these up in data tables.When the substance contains an impurity, its melting point falls and its boiling point rises. And melting and boiling no longer take place sharply, but over a range of temperature.The more impurity present, the bigger the change in melting and boiling points, and the wider the temperature range over which the substance melts and boils.

For example:

How can we obtain a pure substance?

This sulphur sample melts sharply at 119°C and boils at 445°C. So it is pure.

This water freezes around – 0.5°C and boils around 101°C. So it is not pure.

When you carry out a reaction to make something, you usually end up with a mixtures of pure substances. Then you have to separate the one you want.

The table below shown some separation methods. These can give quite pure substances. For example when you filter off a solid, and rinse it really well with distilled water, you remove a lot of impurity. But it is just not possible to remove every tiny particle of impurity.

FilterSolid from a liquid

CentrifugeSolid from a liquid

EvaporateSolid from it’s

solution

CrystalliseA solid from its

solution

DistillA solvent from a solution

Fractional Distillation

Liquids from each other

ChromatographyA mixture of substances

from a solution

Integrantes:

Castillo Cortez, Juan MiguelLlanos Tuesta, YesseniaPisfil Zapata, SilvanaSabino Graus, Katherine del Pilar

Curso:

Inglés

Docente:

Nelver Vera

Fecha:

02/07/13

Facultad de Ingeniería QuímicaPure substances and

impurities