A pure substance: • contains atoms of only one kind. • It has fixed physical and chemical properties like boiling point, melting point, valency, density • contains only one chemical identity, e. g. one element or one compound. • cannot be separated into 2 or more substances by physical or mechanical means • is homogeneous, ie, has uniform composition throughout the whole sample • its properties are constant throughout the whole sample • its properties do not depend on how it is prepared or purified • has constant chemical composition
Pure Substances Elements and compounds are both examples of pure substances. Pure substances cannot be separated into simpler substances by physical or mechanical means such as sifting, filtering, crystallization, distillation, etc. eg, distilling pure water (H2O) does not separate water into hydrogen and oxygen, it only produces water vapour. Pure substances display a sharp melting and boiling point. On a graph of temperature vs time, this is shown as flat line where the temperature does not change over time until all the pure substance has melted or boiled.
A mixture: • can be separated into 2 or more substances by physical or mechanical means • contain more than one chemical substance • displays the properties of the pure substances making it up • its composition can be varied by changing the proportion of pure substances making it up • they do not have a fixed composition • heterogeneous substances, ones with non-uniform composition throughout the sample, are always mixtures Mixtures Some examples of mixtures are given below: Type of Mixture |Example | |gas in gas |The atmosphere is a mixture of gases, mostly nitrogen and oxygen. | |[pic] | |liquid in liquid |Wine is a mixture of mostly ethanol and water. | |[pic] | |solid in solid |Alloys, such as brass, are made up of a mixture of metals. |[pic] | |gas in liquid |Soft drinks, such as cola, are mixtures of mainly carbon dioxide gas and water. | |[pic] | |solid in liquid |Sea Water is a mixture of salts dissolved in water. | |[pic] | |solid in gas |Smoke is mixture of tiny solid particles in atmospheric gases. |
Separating the Components of a Mixture Most laboratory work in biology requires the use of techniques to separate the components of mixtures. This is done by exploiting some property that distinguishes the components, such as their relative • size • density • solubility • electrical charge Dialysis Dialysis is the separation of small solute molecules or ions (e. g. , glucose, Na+, Cl-) from macromolecules (e. g. , starch) by virtue of their differing rates of diffusion through a differentially permeable membrane. An example:
Cellophane is perforated with tiny pores that permit ions and small molecules to pass through but exclude molecules with molecular weights greater than about 12,000. If we fill a piece of cellophane tubing with a mixture of starch and sugar and place it in pure water, the sugar molecules (red dots) will diffuse out into the water until equilibrium is reached; that is, until their concentration is equal on both sides of the membrane. Because of their large size, all the starch (blue disks) will be retained within the tubing. Chromatography Chromatography is the term used for several techniques for separating the components of a mixture.
Follow the links below for examples. Electrophoresis Electrophoresis uses a direct electric current to separate the components of a mixture by the differing electrical charge. Some methods for separating the components of a mixture include: |separation technique |property used for separation |example | |Sifting (sieving) |particle size |alluvial gold is separating from smaller soil particles using a sieve | |[pic] |Visual Sorting |colour, shape or size |gold nuggets can be separated from crushed rock on the basis of colour | |[pic] | |Magnetic Attraction |magnetism |magnetic iron can be separated from non-magnetic sulfur using a magnet | |[pic] | |Decanting |density or solubility |liquid water can be poured off (decanted) insoluble sand sediment | | | |less dense oil can be poured off (decanted) more dense water | |[pic] | |Separating Funnel |density of liquids |in a separating funnel, less dense oil floats on top of more dense water, when | | | |the valve is open the water can be poured out from under the oil | |[pic] | |Filtration |solubility |insoluble calcium carbonate can be separated from soluble sodium chloride in | | | |water by filtration | |[pic] | |Evaporation |solubility and boiling point |soluble sodium chloride can be separated from water by evaporation | |[pic] | |Crystallization |solubility |slightly soluble copper sulfate can be separated from water by crystallization | |[pic] | |Distillation |boiling point |ethanol (ethyl alcohol) can be separated from water by distillation because | | | |ethanol has a lower boiling point than water | Element ? Any substance that contains only one kind of an atom ? Elements are made up of atoms, the smallest particle that has any of the properties of the element. John Dalton, in 1803, proposed a modern theory of the atom based on the following assumptions. |1. Matter is made up of atoms that are indivisible and indestructible. | |2. All atoms of an element are identical. | |3.
Atoms of different elements have different weights and different chemical properties. | |4. Atoms of different elements combine in simple whole numbers to form compounds. | |5. Atoms cannot be created or destroyed. When a compound decomposes, the atoms are recovered unchanged | ? cannot be broken down into simpler substances ? is a chemical substance that is made up of a particular kind of atoms and hence cannot be broken down or transformed by a chemical reaction into a different element, though it can be transmitted into another element through a nuclear reaction. ? all of the atoms in a sample of an element have the same number of protons, though they may be different isotopes, with differing numbers of neutrons. elements can be divided into three categories that have characteristic properties: metals, nonmetals, and semimetals ? Some properties of an element can be observed only in a collection of atoms or molecules of the element. These properties include color, density, melting point, boiling point, and thermal and electrical conductivity. ? While some of these properties are due chiefly to the electronic structure of the element, others are more closely related to properties of the nucleus, e. g. , mass number. Compounds • The relative proportions of the elements in a compound are fixed. • . Two or more elements combined into one substance through a chemical reaction form a chemical compound.
All compounds are substances, but not all substances are compounds. • The components of a compound do not retain their individual properties. Both sodium and chlorine are poisonous; their compound, table salt (NaCl) is absolutely essential to life. • Properties of compound is different from the elements that made it up • The mass of the compound is determined by the mass of the elements that made it up. • Compounds cannot be separated by physical means: using magnet, filtration, etc. It takes large inputs of energy to separate the components of a compound Compounds can be broken back into elements by chemical reaction, exposure to light, etc. When compounds are formed heat and light is given out or absorbed. • Compounds are homogeneous forms of matter. Their constituent elements (atoms and/or ions) are always present in fixed proportions (1:1 depicted here). The elements can be divided into three categories that have characteristic properties: 1. Metals 2. Nonmetals 3. Metalloids Most elements are metals, which are found on the left and toward the bottom of the periodic table. A handful of nonmetals are clustered in the upper right corner of the periodic table. The semimetals can be found along the dividing line between the metals and the nonmetals Properties of an element are sometimes classed as either chemical or physical.
Chemical properties are usually observed in the course of a chemical reaction, while physical properties are observed by examining a sample of the pure element. The chemical properties of an element are due to the distribution of electrons around the atom’s nucleus, particularly the outer, or valence, electrons; it is these electrons that are involved in chemical reactions. A chemical reaction does not affect the atomic nucleus; the atomic number therefore remains unchanged in a chemical reaction. Some properties of an element can be observed only in a collection of atoms or molecules of the element. These properties include color, density, melting point, boiling point, and thermal and electrical conductivity. While some of hese properties are due chiefly to the electronic structure of the element, others are more closely related to properties of the nucleus, e. g. , mass number. The elements are sometimes grouped according to their properties. One major classification of the elements is as metals, nonmetals, and metalloids. Elements with very similar chemical properties are often referred to as families; some families of elements include the halogens, the inert gases, and the alkali metals. In the periodic table the elements are arranged in order of increasing atomic weight in such a way that the elements in any column have similar properties. Chemical properties Chemical properties of elements and compounds Atomic number – Atomic mass – Electronegativity according to Pauling – Density – Melting point – Boiling point – Vanderwaals radius – Ionic | |radius – Isotopes – Electronic schell – Energy of first ionisation – Energy of second ionisation – Standard potential | |Atomic number | | | |The atomic number indicates the number of protons within the core of an atom. The atomic number is an important concept of chemistry and | |quantum mechanics. An element and its place within the periodic table are derived from this concept. |When an atom is generally electrically neutral, the atomic number will equal the number of electrons in the atom, which can be found around | |the core. These electrons mainly determine the chemical behaviour of an atom. Atoms that carry electric charges are called ions. Ions either| |have a number of electrons larger (negatively charged) or smaller (positively charged) than the atomic number. | |Atomic mass | | | |The name indicates the mass of an atom, expressed in atomic mass units (amu). Most of the mass of an atom is concentrated in the protons and| |neutrons contained in the nucleus.
Each proton or neutron weighs about 1 amu, and thus the atomic mass in always very close to the mass (or | |nucleon) number, which indicates the number of particles within the core of an atom; this means the protons and neutrons. Each isotope of a | |chemical element can vary in mass. The atomic mass of an isotope indicates the number of neutrons that are present within the core of the | |atoms. The total atomic mass of an element is an equivalent of the mass units of its isotopes. The relative occurrence of the isotopes in | |nature is an important factor in the determination of the overall atomic mass of an element. In reference to a certain chemical element, the| |atomic mass as shown in the periodic table is the average atomic mass of all the chemical element’s stable isotopes.
The average is weighted| |by the relative natural abundances of the element’s isotopes. | |Electronegativity according to Pauling | | | |Electro negativity measures the inclination of an atom to pull the electronic cloud in its direction during chemical bonding with another | |atom. | |Pauling’s scale is a widely used method to order chemical elements according to their electro negativity. Nobel prize winner Linus Pauling | |developed this scale in 1932. | |The values of electro negativity are not calculated, based on mathematical formula or a measurement.
It is more like a pragmatic range. | |Pauling gave the element with the highest possible electro negativity, fluorine, a value of 4,0. Francium, the element with the lowest | |possible electro negativity, was given a value of 0,7. All of the remaining elements are given a value of somewhere between these two | |extremes. | |Density | | | |The density of an element indicates the number of units of mass of the element that are present in a certain volume of a medium. | |Traditionally, density is expressed through the Greek letter ro (written as r).
Within the SI system of units density is expressed in | |kilograms per cubic meter (kg/m3). The density of an element is usually expressed graphically with temperatures and air pressures, because | |these two properties influence density. | |Melting point | | | |The melting point of an element or compound means the temperatures at which the solid form of the element or compound is at equilibrium with| |the liquid form. We usually presume the air pressure to be 1 atmosphere. | |For example: the melting point of water is 0 oC, or 273 K. |Boiling point | | | |The boiling point of an element or compound means the temperature at which the liquid form of an element or compound is at equilibrium with | |the gaseous form. We usually presume the air pressure to be 1 atmosphere. | |For example: the boiling point of water is 100 oC, or 373 K. | |At the boiling point the vapor pressure of an element or compound is 1 atmosphere. | |Vanderwaals radius | | | |Even when two atoms that are near one another will not bind, they will still attract one another. This phenomenon is known as the | |Vanderwaals interaction. |The Vanderwaals forces cause a force between the two atoms. This force becomes stronger, as the atoms come closer together. However, when | |the two atoms draw too near each other a rejecting force will take action, as a consequence of the exceeding rejection between the | |negatively charged electrons of both atoms. As a result, a certain distance will develop between the two atoms, which is commonly known as | |the Vanderwaals radius. | |Through comparison of Vanderwaals radiuses of several different pairs of atoms, we have developed a system of Vanderwaals radiuses, through | |which we can predict the Vanderwaals radius between two atoms, through addition. |Ionic radius | | | |Ionic radius is the radius that an ion has in an ionic crystal, where the ions are packed together to a point where their outermost | |electronic orbitals are in contact with each other. An orbital is the area around an atom where, according to orbital theory, the | |probability of finding an electron is the greatest. | |Isotopes | | | |The atomic number does not determine the number of neutrons in an atomic core. As a result, the number of neutrons within an atom can vary. | |Then atoms that have the same atomic number may differ in atomic mass.
Atoms of the same element that differ in atomic mass are called | |isotopes. | |Mainly with the heavier atoms that have a higher atomic number, the number of neutrons within the core may exceed the number of protons. | |Isotopes of the same element are often found in nature alternately or in mixtures. | |An example: chlorine has an atomic number of 17, which basically means that all chlorine atoms contain 17 protons within their core. There | |are two isotopes. Three-quarters of the chlorine atoms found in nature contain 18 neutrons and one quarter contains 20 neutrons. The mass | |numbers of these isotopes are 17 + 18 = 35 and 17 + 20 = 37. The isotopes are written as follows: 35Cl and 37Cl. |When isotopes are noted this way the number of protons and neutrons does not have to be mentioned separately, because the symbol | |of chlorine within the periodic chart (Cl) is set on the seventeenth place. This already indicates the number of protons, so that one can | |always calculate the number of neutrons easily by means of the mass number. | | | |A great number of isotopes is not stable. They will fall apart during radioactive decay processes. Isotopes that are radioactive are called | |radioisotopes. | |Electronic shell | | | |The electronic configuration of an atom is a description of the arrangement of electrons in circles around the core.
These circles are not | |exactly round; they contain a wave-like pattern. For each circle the probability of an electron to be present on a certain location is | |described by a mathematic formula. Each one of the circles has a certain level of energy, compared to the core. Commonly the energy levels | |of electrons are higher when they are further away from the core, but because of their charges, electrons can also influence each another’s | |energy levels. Usually the middle circles are filled up first, but there may be exceptions due to rejections. | |The circles are divided up in shells and sub shells, which can be numbered by means of quantities. |Energy of first ionisation | | | |The ionisation energy means the energy that is required to make a free atom or molecule lose an electron in a vacuum. In other words; the | |energy of ionisation is a measure for the strength of electron bonds to molecules. This concerns only the electrons in the outer circle. | |Energy of second ionisation | | | |Besides the energy of the first ionisation, which indicates how difficult it is to remove the first electron from an atom, there is also an | |energy measure for second ionisation. This energy of second ionisation indicates the degree of difficulty to remove the second atom. | | |As such, there is also the energy of a third ionisation, and sometimes even the energy of a fourth or fifth ionisation. | |Standard potential | | | |The standard potential means the potential of a redox reaction, when it is at equilibrium, in relation to zero. When the standard potential | |exceeds zero, we are dealing with an oxidation reaction. When the standard potential is below zero, we are dealing with a reduction | |reaction. The standard potenti |