ELEMENT, COMPOUNDS AND MIXTURES

Zambian Secondary School Science Syllabus G10-12

Matterexists either as in form of an element, compound or a mixture. An element is the simplest form of a substance because is made up of one type of atoms. Elements can further be classified into metals and non-metals. Compounds are formed between two or more elements that have chemically combined. Many different substances out there do exist as compounds.

A mixture is a combination of two or more substances that have not chemically combined. This implies that they can easily be separated using physical means. Some common methods of separating mixtures are distillation, evaporation, filtration, chromatography, centrifugation, magnetization, crystallisation and decanting.

ELEMENTS

The simplest form of a substance is called an element. It cannot be split into other simpler substances by any chemical process. An element is made up of only one kind ofatom. There are 115 elements and 24 of them do not occur in nature. They form what we have as the periodic table. The elements are classified into metallic and non-metallic categories.

Metals often have high density, high melting points, highboiling points, malleable, ductile, shiny (lustrous), high thermal and electrical conductivity. Non-metallic elements have low values for the stated characteristics. Each element is given a special chemical symbol to characterize it, e.g. Carbon (C), Sulphur (S), Sodium (Na), Potassium (K), Chlorine (CL) and Lead (Pb).

Symbols consists of one or two letters of the alphabet, the first being capital letter- A, B, C. Several elements with the same initial letter, a second letter or subsequent letter is added e.g. C for carbon, calcium (Ca), Chlorine (Cl). Other use Latin names e.g. sodium (Na) from natrium, lead (Pb) from plumbum etc

Isotopes

The number of electrons in an atom is equal to the number of protons. The number of protons in an atom gives the atomic (proton) number for that atom. The number of neutrons for the same element is often equal. When atoms for an element have the same number of protons (same number of electrons), with the same chemical properties but the number of neutrons vary, one atom type is stable while the other atom is called an isotope for the stable atom. They however show different physical properties.

Some examples of elements with isotopes are Cl (35, 37), H (1, 2, 3), C (12, 13, 14), O (16, 17)

Allotropes

Some substances can crystallize in two or more forms. Some of these substances are unstable in one form and therefore change from one form to another under some circumstances. In situations where they change form due totemperature, the specific temperature under which they change is called the transition temperature. Some allotropes include

• Calcium Carbonate (Calcite and aragonite)
• Iodine mono-chloride (red, brown)
• Sulphur (rhombic, monoclinic)
• Phosphorous (red, white, black).
• Carbon (amorphous, graphite, diamond).
• Oxygen (0₂, Ozone)
• “fin (grey, white)

Allotropes always exhibit different physical properties and at times they may have even different chemical properties. These properties make the allotropes be used for different purposes.

COMPOUNDS

Compounds are pure substances which are formed due to a chemical reaction between l wo or more elements. When a new substance is formed during the chemical reaction, chemical change takes place. This combining of elements can be represented by word equation:hydrogen + oxygen → water. Water molecules contain two atoms of hydrogen and one atom of oxygen and hence the chemical formulaH₂O.

The formula of a compound is made up from the symbols of the elements present and the ratio in which the different atoms are present.Carbon dioxidehas the formula CO₂. This shows that it contains one carbon atom for every two oxygen atoms. The 2 shows that there are 2 oxygen atoms present in each molecule of carbon dioxide.

Compounds are made up of fixed proportion of elements: They have a fixed proportion. Chemists call this the “Law of constant composition”. The forces that hold atoms together in compounds are called “Chemical bonds”. Reaction between elements and oxygen is called an oxidation reaction as the substance gains oxygen. Some examples

2Mg (_S) + O₂(_g) → 2MgO(_S

C(s) + O₂(g)→ CO₂(_g)

When substances react such that there is a loss of oxygen, the process is called reduction. Some examples:

CuO (_S) + H₂(_g)→Cu(_S) + H₂0(_g).

Fe₂O_3(S) + 3CO(_g) → 2Fe(_S) + 3CO₂(_g)

Look at theseREDOXreactions

1. This is an example of oxidation reaction

Carbon + Oxygen → Carbon dioxide

C(s) + O₂(g) → CO₂₍g)

Oxidation reaction is gain of oxygen, loss of hydrogen or electrons.

2. This is an example of a reduction reaction

Copper Oxide + Hydrogen → Copper + Water

CuG_(s)+ H₂(_g) → Cu (_S) + H₂O(_g)

deduction reaction is loss of oxygen, gain of hydrogen or electron.

MOLECULES

The atoms are joined together in small groups. These small groups are called“molecules”.For example the atoms of Hydrogen, Iodine, Chlorine, Nitrogen, Oxygen, Fluorine and Bromine are each joined in pairs and are known as “diatomic”molecules, Phosphorus atoms are joined in four and sulphur in eight. In chemical shorthand the molecule of Chlorine is written s Cl₂.

The gaseous element helium, neon, argon, krypton, Xenon and Radon are composed of separate and individual atoms. They are “monatomic”; they are shorthanded Ne, Ar, Kr, Xe and Rn respectively.

Molecules are not always formed by atoms of the same type joining together. For example, water exists as molecule containing Oxygen and hydrogen atoms.

MIXTURES

Mixtures contain more than one substance (element or compound). The substances of the mixture have their specific physical properties. Our world is very complex, owing to the vast range of pure substances available, and to the variety of ways in which these pure substances can mix with each other. In everyday life we do not ‘handle’ pure substances very often.

The air we breathe is not a single, pure substance – and we could not live in it if it were! Water would be rather tasteless if we drank it distilled – indeed, some companies do market distilled water that has had salts added back to it.

It is therefore possible to separate these substances provided there are suitable differences in their physical properties. If the mixture is made to undergo a chemical reaction, then separation thereafter becomes more complicated.

Types of mixtures

Mixtures may be Homogeneous or Heterogeneous

• Ahomogeneous mixture hasthe same composition of elements/or compounds throughout – evenly distributed. It is also called solution. An example of such a mixture is copper II sulphate solution.

Aheterogeneoushas different regions or areas with different compositions and properties – not evenly distributed. An example of such a mixture is concrete.

Each mixture must be made from at least two parts, or phases, which may be solid, liquid or gas.

The temperature stays constant while the gas condenses, and while the liquid freezes. A cooling mixture of ice and salt could be used to lower the temperature below 0 °C.T

There are a number of different ways in which the three states can be combined. In some, the states are completely mixed to become one single state or phase – ‘you cannot see the join’. This is ahomogeneous mixture. Technically, the term ‘solution’ is used for this type of mixture. Solid salt dissolves in liquid water to produce a liquid mixture – a salt solution. In other types of mixture, the states remain separate (aheterogeneous mixture). One phase is broken up into small particles, droplets, or bubbles, within the main phase.

Solutions and homogeneous mixtures

Table 2.3 shows us some of the ways in which substances in different states can combine. Perhaps the most important idea here is that of one substance dissolving in another – the idea of asolution. We most often think of a solution as being made of a solid dissolved in a liquid. Two- thirds of the Earth’s surface is covered by a solution of various salts in water. The salts are totally dispersed into the water and cannot be seen. However, other substances that are not normally solid are dissolved in sea water. For example, the dissolved gases, oxygen and carbon dioxide, are important for life to continue in the oceans.

Less obvious perhaps, but quite common, are solutions of one liquid in another. Alcohol mixes (dissolves) completely with water. Beer, wine and whisky do not separate out into layers of alcohol and water (even when the alcohol content is quite high). Alcohol and water are completelymiscible: they make a solution.

+

When a solute dissolves in a solvent, the solute particles are completely dispersed in the liquid.

Table 2.3 Important examples of different types of mixture

These solutions are particularly important in chemistry.

Technically the air itself could be described as a solution of several gases in nitrogen, though this would be an unusual everyday use of the term. However, it is interesting to note that different gases always mix completely with each other. Likewise, alloys are homogeneous mixtures of metals, though we do not usually call them solutions. They are made by mixing the liquid metals together (dissolving one metal in the other) before solidifying the alloy.

Suspensions and heterogeneous mixtures

There are a great number of combinations of substances that do not mix: solids that do not dissolve in water; liquids that do not mix with each other (they areimmiscible). Flour does not dissolve in water but forms aslurryorsuspension. The particles of solid are simply dispersed (spread) throughout the water and will eventually settle out if left to stand. Sand does not dissolve in water either. The ocean tides shift and deposit it on beaches and sandbars throughout the world. The great estuaries such as the Nile or Mississippi deltas are silted up by suspended soil deposited from the river.

Oil and water do not mix. Salad dressing made from olive oil and vinegar (a mixture of ethanoic acid and water) will settle out into two layers. When shaken, it forms anemulsionwith droplets of oil suspended throughout the vinegar. Unlike pure liquids, emulsions are cloudy (opaque) so you cannot see through them. To stop an emulsion, such as mayonnaise, separating out into layers, anemulsifieris added.

This prevents the small droplets of oil coming together to form larger droplets and then a separate layer. In traditional mayonnaise the emulsifier is egg yolk, which contains lecithin. Other food products contain other emulsifiers. In the European system for labelling food additives, emulsifiers are given E numbers from E322 to E494. E322 is lecithin.

The world of cosmetics is full of examples of emulsions. Skin moisturising creams are emulsions of oils in water; the oils prevent the skin from drying out. It is possible to switch the balance of the two phases in a cosmetic preparation. Cold cream and cleansing cream are emulsions of water in oils. They have an oilier‘feel’or texture than hand or face creams.

There are two ways in which mixtures can be formed between different substances:

• homogeneous mixtures,where the substances are totally mixed together and are indistinguishable – examples include solutions of salts and sugars in water.
• heterogeneous mixtures,where the substances remain separate and one substance is spread throughout the other as small particles, droplets, or bubbles – examples include suspensions of insoluble solids in water.

Differences between Mixtures and Compounds

Colloids and suspensions

The heterogeneous mixtures listed in table 2.3 can be divided into two types – suspensions and colloids. The difference depends on the size of the particles suspended, or dispersed, in the surrounding medium.

• Suspensionscontain relatively large particles (over l000nm, where 1 nm = 10^-9m) of an insoluble solid, or droplets of an insoluble liquid. In time, the particles or droplets settle out.
• Colloidscontain smaller particles (1-1000nm) and take various forms. Emulsions, sols, gels, aerosols and foams are all examples of colloids. The particles of a colloid are too small to be seen by eye, but they do scatter light.

Colloids fill an important place in our lives. Our blood contains proteins spread throughout a watery medium; milk is a complex colloid containing fat droplets and proteins dispersed in water. We also manufacture a large number of colloids to serve a wide range of purposes. ‘Emulsion’ paint is an example of a colloidal system.

In white ‘emulsion’ paint, the dispersed particles are solid (for example titanium(iv) oxide particles), mixed up in water but not dissolved in it. A true emulsion is made from one liquid dispersed in another. ‘Emulsion’ paint is not in fact a true emulsion. Nor is the photographic ‘emulsion’ coated on films. The light-sensitive layer of a film consists of fine crystals of silver bromide trapped in a gelatine gel.

Every colloid has at least two parts :

• thedispersed phaseis split into very small particles – for example the fat in milk or the water droplets in mist.
• thecontinuous phasecontains the dispersed phase spread throughout it – for example the water in milk or the air in mist.

Table 2.4 The dispersed and continuous phases of different types of colloid