Science Revision

DNA stands for deoxyribonucleic acid

Science Study for Yearly Exam

DNA  Is the molecule that determines characteristics

DNA is made up of molecules called nucleotides.

DNA

Nucleotide molecules have three main parts: phosphate group deoxyribose sugar one of four nitrogen-rich bases.

nucleotides are organised in a way that makes DNA a double helix.

The nitrogen-rich bases (commonly called bases) pair up to form the rungs. The four bases adenine (A), thymine (T), guanine (G) and cytosine (C)

These bases can only pair up in one way, a characteristic known as complementary base pairing. For example, adenine can only form a complementary base pair with thymine (A-T) and guanine can only pair with cytosine (G-C).

Chromosomes are long, thin, threadlike structures found in the nucleus of cells.

Chromosomes are made of DNA and protein. The cells in the human body each contain 46 chromosomes (in 23 pairs). The only exceptions are the sperm and egg cells, which only contain 23 chromosomes (one of each pair) and red blood cells, which have no nucleus. Other organisms have different numbers of chromosome pairs in their cells.

Genes are sections of DNA. Each chromosome can have over 1000 genes. The difference between one gene and the next is the: order of bases along the DNA strand length of the DNA strand.

The order of the bases along the DNA strand is the genetic code. Each gene codes (contains instructions) for a specific protein. Proteins control many characteristics or functions in the body. Proteins include the structural materials that build up your cells and tissues, most hormones and all enzymes.

James Watson and Francis Crick are credited with the discovery of DNA in 1953, But the history of DNA goes back further than that.  In 1869, Johannes Friedrich Miescher (1844-95), a Swiss physician and biologist, isolated a previously unknown chemical from the nuclei of dead white blood cells. Miescher was looking for proteins when he identified a substance that was chemically very different. He called this new chemical nuclein because it was found in the cell nucleus.

Rosalind Franklin obtained this image of DNA in 1953. James Watson and Francis Crick used it to work out the structure of the molecule James Watson and Francis Crick explain the structure of their model of DNA.

The process of copying DNA is known as replication.

In the first step of replication, the strands of the double helix separate from each other in much the same way as a zip opens. The bases are then exposed. Within the nucleus there are individual nucleotides that are not part of a DNA chain. In step 2, these nucleotides pair up with the exposed bases following the rules of complementary base pairing. In step 3, the sugar and phosphate molecules bond with neighbouring nucleotides and new strands of DNA are formed.

Replication occurs on both of the exposed strands of DNA, and the result is two identical double helices of DNA.

Mitosis is a continuous process. However, scientists have identified several distinct stages in the process.In the period between the actual divisions of the cell, the DNA replicates. At this stage, individual chromosomes are not visible. When the cell begins to divide, the DNA coils up and separate chromosomes become visible. Each chromosome comprises two chromatids. The membrane surrounding the nucleus breaks down. Chromosomes line up across the equator (middle) of the cell and a network of fibres appears, extending from the poles of the cell to each chromosome. The chromatids separate to become two independent chromosomes. The network of fibres contracts, pulling the chromosomes to opposite poles (ends) of the cell. A nuclear membrane encloses the chromosomes at each pole. The chromosomes uncoil and are no longer visible as individual strands. Division of the nucleus is complete. The cytoplasm divides and the result is two identical daughter cells. The daughter cells grow in size in preparation for the next round of cell division.

Meiosis is the process of cell division that produces gametes. The chromosomes replicate in preparation for division just as they do for mitosis.The nuclear membrane breaks down and then, in preparation for the first part of meiosis, the homologous pairs of chromosomes line up on the equator of the cell. A network of fibres extends from the poles of the cell to each chromosome pair. The fibres contract, drawing one chromosome from each pair to opposite poles of the cell. At this stage, each chromosome is still two chromatids. A new network of fibres forms at right angles to the first. The fibres attach to the chromosomes that have lined up on the equator of the cell. This time when the fibres contract, the chromatids are pulled apart towards the poles of the cells. There are now bundles of 23 chromosomes. New nuclear membranes form and the cytoplasm divides to produce four new cells, each containing the haploid number of chromosomes. These cells are the gametes or sex cells.

In your body cells, there are 46 chromosomes, half of which came from your father and half from your mother. The number of chromosomes in your body cells is the diploid number. The diploid number is also described as 2N, which means two sets. In your gametes, there has to be half this number of chromosomes. If each parent passed on their complete set of genetic information, then their offspring would have 4N chromosomes and then the next generation would have 8N and so on. By halving the number of chromosomes in the gametes, the number of chromosomes from generation to generation is kept constant at 2N. Of the 46 chromosomes in your cells, two are sex chromosomes—the ones that determine whether you are male or female. The other 44 chromosomes are not sex chromosomes and are known as autosomes. In human females, the sex chromosomes are a pair of X chromosomes (XX). In males, the sex chromosomes are one X and one Y chromosome (XY). The autosomes in your cells are grouped into 22 pairs. The chromosomes in the pair are homologous. Homologous chromosomes are the same length, have the centromere (the point where the two chromosomes join) in the same position. Homologous chromosomes also have genes for particular characteristics at the same location along their length. For example, the gene for hair curliness is found in the same position on the pair of homologous chromosomes shown in Figure 1.2.5. Therefore, each new cell formed by mitosis of the zygote has two copies of the gene for each characteristic, one on each chromosome of the homologous pair. One chromosome from each homologous pair must end up in each gamete that is produced. Therefore, the gametes have 23 chromosomes in total. This is the haploid number or N. The female sex chromosomes are a homologous pair. The male X and Y chromosomes are not homologous but they behave as a pair during meiosis.

There are plants and animals that sometimes reproduce asexually. This means that offspring are produced through mitosis of particular cells without any union of gametes. Hydra and grasses are examples of organisms that use asexual reproduction

Genetics is the study of inherited characteristics called traits. In Austria in 1856, a monk called Gregor Mendel (1822-84) (Figure 1.3.1) carried out experiments on pea plants. The results of these experiments led him to construct theories that became the basis for the study of modern genetics

Mendel worked with pure-breeding red-flowered pea plants and pure-breeding white-flowered pea plants. In pure-breeding lines, all the individuals have the same genetic information. Therefore, only red-flowered offspring could be produced from red-flowered parents and white-flowered offspring from white-flowered parents. When pollen from red-flowered plants was used to cross-pollinate the white-flowered plants, all the plants in the next generation produced red flowers. Mendel called the red characteristic the dominant characteristic. The dominant characteristic is the characteristic that can be observed in the appearance of the individual. The other characteristic he called the recessive characteristic—the one that remained hidden. Mendel then cross-pollinated these red flowers of the first generation, or F1 generation. Each set of crosses that he performed produced both white and red flowers in the next generation—the F2 generation.The conclusion from these results is that genes work in pairs to determine which characteristic is shown or expressed. The gene that was studied in this situation was the gene for flower colour and this gene came in two varieties. Variations of genes are known as alleles. In this instance, the gene for flower colour had an allele for the red flower trait and an allele for the white flower trait. When studying crosses and potential characteristics of offspring, geneticists use shorthand conventions. The dominant allele is represented by an upper-case letter related to the name. In this case, the red flower allele could be represented by the letter R (pronounced as 'big r'). The recessive characteristic (white flower) is then represented by the lower-case of the same letter—r (pronounced as 'little r'). By using R and r, it shows that a particular gene is being discussed.

Punnett squares - are one way of showing all the possible types of offspring that could result from a cross. You cannot assume that the offspring will appear in exactly this order and in this exact ratio. It represents a probability. In a Punnett square the possible gametes produced by one parent are shown across the top. The gametes from the other parent are shown down the side. In each square is a possible outcome of fertilisation.

Some genes do not have dominant and recessive alleles. Some alleles show incomplete dominance, and the appearance of a heterozygous individual results from a 'blending' of two such alleles. A heterozygote will look different from both its homozygous parents. One example is in domestic chickens in which black feathers are incompletely dominant to white feathers. The heterozygous chicken has blue-grey feathers. The blue-grey colouring of cockatiels (Figure 1.3.6) is also the result of incomplete dominance. The shorthand convention in this situation is to use uppercase letters related to the two different alleles such as B for black and W for white feathers. This is a reminder that neither allele is dominant to the other

Sex determination Your two sex chromosomes determine which sex you are. Inheritance of these chromosomes can be seen clearly in Figure 1.3.7. All of the eggs produced by a female will have one X chromosome. Half the male's sperm will carry an X chromosome and the other half will have a Y chromosome. If a sperm containing an X chromosome fertilises an egg, then the offspring will be a female (XX). If a sperm carrying a Y chromosome fertilises an egg, then the offspring will be male (XY).

Sex linkage Some genes are found on the X chromosome and not on the Y chromosome. These are called sex-linked genes because they are present on one of the chromosomes that are also responsible for the determination of sex.The red-green colour-blindness gene is carried on the X chromosome. Normal vision (N) is dominant to red-green colour-blindness (n). Females who are heterozygous (XNXn) for colour-blindness will still have normal vision because the dominant allele masks the effect of the recessive allele. However, they are carriers of the allele. Carriers are able to pass the trait on to their children. The Y chromosome does not carry a colour-blindness gene. Therefore, the only possible genotypes for a male are XNY and XnY. In the genotype XnY, the recessive allele is the one that is expressed in the phenotype and the male is colour blind.

Chromosomal abnormalities Mistakes can happen as DNA is copied. The base sequence is changed and mistakes occur in the manufacture of proteins. This type of change is called a mutation

BIG BANG THEORY

Astronomers refer to the brightness of a star as its magnitude. The colour of a star is due to its temperature.

A star's apparent magnitude is a measure of how bright it will appear to an observer on Earth.

The very brightest stars are given negative magnitudes. For example, Alpha Centauri (the brighter of the two Pointers to the Southern Cross) has an apparent magnitude of -0.27. In most cities, the dimmest stars that can be observed with the naked eye have a magnitude of 3.5.

the human eye can see stars down to a magnitude of 6.5.

Apparent magnitude is measured on a logarithmic scale.

Astronomers often measure interstellar distances (the distances between stars) in light-years (l.y.). One light-year is the distance that light will travel in one year. This distance is a little under 9.5 trillion kilometres. The distance to Betelgeuse is 650 l.y. This means that the light you see when you look at Betelgeuse was emitted by the star nearly 650 years ago.

Another commonly used astronomical unit of length is the parsec (pc), which is equivalent to 3.26 light-years.

Each star emits light at a range of different wavelengths. Some of this light is in the visible part of the electromagnetic spectrum (Figure 7.1.5), while some of it will be in the invisible infrared or ultraviolet range. Your eyes collect the visible light from stars and your brain performs a complex averaging process that results in you perceiving the star as a particular colour. Rather than rely on the human eye and the brain to interpret the colour, scientists analyse the light from a star by using filters. By comparing the magnitude of the star when viewed through coloured filters, its colour can be precisely measured.

A star's spectrum is mainly determined by its surface temperature. Cooler stars emit most of their energy in the infrared and red parts of the spectrum and therefore appear red to your eyes. Very hot stars emit a lot of energy in the violet and ultraviolet part of the spectrum and appear blue. Stars with temperatures in between these extremes emit light across a range of wavelengths and can appear orange, yellow or white.

Another device that is used to analyse starlight is a spectrometer. This is a device that splits light into a spectrum to reveal its component colours. Scientists can determine what chemical elements are present in a star from distinctive lines that appear in its spectrum. Particular elements emit colours of particular wavelengths. These can be measured precisely to determine the elements in the star. When studying the spectra from stars, scientists also see dark lines showing missing colours. The Sun has dark lines called Fraunhofer lines in its spectrum. You can see them in Figure 7.1.6. The lines are due to light interacting with atoms in the outer layers of the star. The light energy is absorbed by electrons in atoms of all the elements in the outer gas layers. These electrons absorb light energy of particular wavelengths. The absorption occurs at exactly the same wavelength that the same element would emit when it is extremely hot.

The source of energy that keeps stars at these extraordinarily high temperatures is a process known as nuclear fusion.

Data from spectral analysis has indicated that three-quarters of the material in a typical star is hydrogen. Most of the remaining quarter consists of helium and small amounts of iron and other heavy elements. The enormous gravitational forces within a star can heat the material at its centre to a temperature of almost 15 million degrees Celsius. Hydrogen is the simplest element in the periodic table, consisting usually of a single proton and an electron. At the enormous temperatures inside a star, the electrons have too much energy to stay bound to the protons so the material takes the form of plasma. Plasma is a state of matter consisting of a 'soup' of positively charged ions and free electrons. Protons are positively charged and so they strongly repel each other. However, in the centre of a star, the massive gravitational force is enough to bring individual protons close enough so that they will fuse together into a new nucleus.

Hertzsprung-Russell diagrams In the early part of the 20th century, two astronomers, Ejnar Hertsprung of Denmark and Henry Norris Russell of the United States, independently came up with the idea of plotting stars on a diagram. Absolute magnitude (brightness) was placed on one axis and spectral class (colour) on the other. When they did this, they noticed that the stars fell into a number of clearly defined groups. This type of diagram became known as the Hertzsprung-Russell or H-R diagram. A typical H-R diagram is shown in Figure 7.1.10. The H-R diagram revolutionised astronomy because it showed that there was a relationship between the brightness and temperature of stars. H-R diagrams were also interpreted as showing that stars were changing from one 'type' to another. These changes became known as the 'life cycle' of a star. Using an H-R diagram is a bit like going into a forest and seeing all the trees at different stages of growth and concluding that the stages represent different life stages of the species. You can't actually see a tree at one stage grow into the other, but it is clear that they must. It became apparent that as stars develop and change, they move from one section of the H-R diagram to another. Thus the H-R diagram acts as a map of the life-cycle of a star.

Main sequence On an H-R diagram, most stars fall on a broad line running from the top left-hand corner to the bottom right-hand corner. This line is known as the main sequence. The structure of any star is determined by the balance of two opposing forces. One of these forces is gravity, which causes the material within the star to fall in towards the centre of the star. Opposing gravity is the radiation pressure, which is produced by the heat generated by nuclear fusion. In a main sequence star, gravity and radiation pressure are in equilibrium—they balance each other out, giving the star a constant radius and brightness. This equilibrium can last for millions or even billions of years until the hydrogen in the core of the star starts to run out.

Red giants When the hydrogen in the core of a medium-sized star runs out, fusion stops and the outward radiation pressure also stops. Gravity causes the star to collapse inwards and the outer layers of the star to start to fuse. Heat from this fusion produces radiation pressure, which causes these outer layers to expand and cool. The star becomes a red giant with a small dense core and a large, relatively cool outer atmosphere. Fusion in the outer layers of a red giant occurs at a lower temperature than in a main sequence star. Therefore, a red giant produces more light in the red part of the spectrum, giving the star its distinctive colour. As the hydrogen in the outer layers of the red giant fuses, the helium produced sinks into the core of the star. As more and more matter is added to the core, its gravitational force and temperature increase until helium atoms start to fuse into heavier elements such as beryllium and carbon.

Neutron stars If the amount of material left behind by a supernova is 1.4-3 times the mass of our Sun, then gravitational forces are strong enough to cause the structure of the atoms within it to break down. Electrons and protons combine to form neutrons. The resulting neutron star has an enormous density since its entire mass can be compressed into a sphere about 10-15 km across. Black holes For supernova remnants that are more than three times the mass of our Sun, the process of collapse after a supernova does not end with the formation of a neutron star. The immense gravitational forces cause the star to shrink even further into what scientists refer to as a singularity or black hole. The gravitational field of a black hole is so strong that not even light is fast enough to escape from it. This makes black holes very hard to detect as they do not emit any visible light. However, it is possible to find black holes indirectly by the effect they have on other stars. One method of detection occurs when a black hole is part of a binary star system (Figure 7.1.15). This occurs when two stars form close to one another and orbit a common centre of mass between them. If one of these stars becomes a black hole, its enormous gravitational field will start to strip material from the other star. As this material spirals into the black hole, it emits a distinctive high-energy X-ray signal, which indicates the presence of the black hole.

EVOLUTION

Fossils of the lobe-finned fish and amphibians of the Devonian period show many similar bones in the limbs. However, these limbs also have more than just bones in common. They also show a gradual change in the structure of the whole limb over geological time. Each different species seemed to have small changes in its general structure, such as bone shapes and the number and position of toes. This apparent change in species over time is called evolution. Evolution is defined as a genetic change in the characteristics of a species over many generations, resulting in the formation of new species. A generation is the time between the birth of an individual and when that individual produces their own offspring. The fossil history of the horse is a good example of changes occurring over many generations. Fossil skeletons have been found of a horse-like animal that was about the size of a small dog. The scientific name of the genus of this animal is Hyracotherium (Figure 3.1.2 on page 70). It is not classified as a horse, but is similar enough that biologists consider it to be a likely ancestor of horses. Radioactive dating methods show that Hyracotherium lived about 52 million years ago.

Structure and relationships When organisms are classified on the basis of their structure, some groups seem very similar. An example is cats and lions. Others, such as cats and jellyfish, seem quite different. The first biologists who studied evolution over 150 years ago proposed that organisms that were very similar in structure must be related. This view was based on the knowledge that organisms seemed to inherit their characteristics from their parents. However, at that time nothing was known about genetics. Genetics has since shown us that species with the same basic structure have many genes the same or genes that are similar in their effect. It is the genes that control structure and function in organisms. Organisms with some identical genes must be related. This is because particular genes are copied from previously existing genes during meiosis. The obvious inference is that two species that share genes must have had the same ancestor at some stage. Many of the same genes have then been passed down to both species.

Homologous structures In related species, characteristics that have the same basic structure are called homologous characteristics. Biologists have discovered that these are controlled by particular inherited genes. For example, the foot bones of the different fossil horses are homologous. A cat's paw and a lion's paw are considered homologous, but a cat's paw and an insect's foot are not homologous. A cat's paw and an insect's foot may have the same function, but their structure is very different. In the last few decades, scientists have been able to isolate genes and study their chemical structures and how genes function. It has been discovered that the more alike two organisms are, the more genes they share. As you move from higher levels of classification to the lower levels, the more alike those genes become. A homologous structure does not necessarily have the same function in all the groups that share it. For example, humans, whales and bats all have five digits at the end of their limbs. Humans have five digits (fingers and toes) on each of our hands and feet. Their function is to grip things and to get traction when walking. Five digits also form the bony structure of each of a whale's flippers, which are used to propel themselves through water. In contrast, the five digits that make up each of a bat's wings form the structure of their wings, allowing them to fly. A human hand, whale flipper and bat wing are homologous structures, despite having different functions. Analogous structures Not all similar structures are homologous structures. For example, the dolphin and shark in Figure 3.1.5 have similar streamlined bodies and similar dorsal fins on their backs. However, these are not homologous structures because different genes are involved in their inheritance. Dolphins and sharks differ in most other structures. This shows that these animals are not very similar other than at the simplest (phylum) level—the fossil record shows that sharks evolved over 460 million years ago, while dolphins evolved about 10 million years ago. Dolphins and sharks have similar body shapes and fins because they evolved in similar marine environments. Structures that look similar on genetically very different organisms are known as analogous structures.

Artificial selection For many centuries, humans have selectively bred different animals and crossed different plants to gradually change the features of a species. Artificial selection is the process by which we choose to breed particular organisms with desirable features. One example is breeding of budgerigars. Wild budgerigars are green and yellow.  All the different pet budgerigars we have today have come from this wild type of budgerigar. Figure 3.1.7 shows some of the different colours. The different colours and patterns are the result of breeders choosing particular budgerigars as parents and breeding from them. They used wild budgerigars that showed small differences in colour, patterning and body size. These variations in the wild budgerigar population were all originally the result of mutations.

Selective breeding methods Selective breeding is used in two main ways. The first method is called cross-breeding. This is the process of combining in the offspring a desirable feature of one individual with a different desirable feature from another individual. An example is the creation of the dog breed called labradoodle. This is a cross between a labrador and a poodle, combining the features of both dogs. You can see a labradoodle

Another method of selective breeding is inbreeding, or line-breeding. In this process, related individuals are allowed to mate. This method is not often used in animal breeding, as there can be health issues in the offspring. Deformities, sterility and genetic disease can be caused by inbreeding.

In 1858, the English biologist Charles Darwin proposed a process by which species change over many generations. Darwin called his process natural selection. He had no knowledge of genetics because it had not been discovered at the time. Since then genetics has provided evidence to support natural selection as the most likely process by which evolution occurs.

Darwin's ideas of natural selection The following example will help you understand what Charles Darwin (Figure 3.2.1) meant by natural selection. Consider a population of mice being preyed upon by owls. The mice have two different coat colours, dark brown and light brown. These colours are inherited. The owls swoop down to catch mice that are in the fields. Imagine that there are equal numbers of dark-brown and light-brown mice. In areas where the ground colour is dark brown, the owls would find the light-brown mice easier to see. The owls would catch a greater number of light-brown than dark-brown mice. As a result, there will be more dark-brown mice surviving and breeding. The next generation would have more dark-brown mice than light-brown mice. The dark-brown mice have been 'naturally selected' by the owls, as opposed to artificially selected by humans. Darwin meant that the selection was done by 'nature', not humans. The dark-brown mice had been selected to breed, but it was not intentional. They were favoured by selection to produce the next generation of offspring. Over many generations, this process would continue and the population would gradually become all dark brown and therefore better adapted to its dark brown environment. This story of the mice shows natural selection at work. Different studies of mice have shown that natural selection also works on real mice, changing their population and characteristics.

Natural selection is the process where an environmental factor acts on a population and results in some organisms having more offspring than others. Biologists call the environmental factor that acts on the population the selective agent.

Variation Darwin concluded that natural selection could only act if there is variation (natural differences) in the population. However, genetics was unknown in his time and so he did not know how or why this variation happened. Since then, scientists have shown that variation is caused by differences in genes, which result in different characteristics. Since genes are inherited, so too are the characteristics they carry. Hence, variation is inherited too. Variation in most organisms is relatively easy to see. For example, humans show variation in height, nose shape, hairiness, baldness, leg length, and hair, eye and skin colour

A more modern definition of natural selection can be expressed in terms of genetics. Natural selection is the change in proportion of a particular genetic make-up (genotype) of a species over many generations due to environmental selection of a particular characteristic (phenotype). In simpler terms, this means the proportion of a particular characteristic (phenotype) in a species changes because individuals with a particular genetic make-up (genotype) within it are being favoured to breed.

Evidence for natural selection The peppered moth One of the first studies to collect evidence for natural selection was conducted earlier last century in England. Henry Bernard Kettlewell studied the peppered moth, which existed in two forms. The normal colour was white with black specks, although occasionally all-black mutant moths were born. Kettlewell found that in the cities, almost all the peppered moths were black. In rural areas, they were almost all white. He concluded that this difference was due to a selective agent acting on the populations. The selective agents he observed preying on the moths were birds such as the flycatcher and nuthatch. Kettlewell explained the process as follows: In the cities, all the building and tree trunks had been blackened by soot from over 150 years of industrial pollution. Any white moths resting on the trees could be seen more easily than the black moths (Figure 3.2.6). So the birds removed white moths faster than they removed the black ones. Black moths produce black offspring and so the population eventually became mainly black. So, the black form was considered to be better adapted to its polluted city environment.

SALTS

Bonding types The types of bonds formed depend on the type of atoms that are bonding. Metallic bonding occurs between metal atoms. Ionic bonding occurs between metal atoms and non-metal atoms. Covalent bonding occurs between atoms of non-metals.

Ions Ions are atoms (or groups of atoms) that have become charged because they have had electrons removed from them or because they have removed electrons from other atoms. Atoms are neutral (no charge) because they have equal numbers of protons and electrons. The transfer of electrons destroys this balance.

Number of electrons in an ion ? number of protons This imbalance gives ions a charge. Positively charged ions (+) have more protons than electrons. They form when metal atoms lose their outer-shell electrons. Negatively charged ions (-) have more electrons than protons. They form when atoms of non-metals gain electrons. The ions now have the same electron configuration and stability of noble gases.

Metallic bonding Metal atoms have a weak hold on their outer-shell electrons. This gives the outer-shell electrons the freedom to move throughout the metal without being bound to any one atom. Each metal atom becomes a positively charged ion. Opposite charges attract and this electrostatic force provides multidirectional bonding between the positive ions and the 'sea' of loose electrons surrounding them. This bonding holds the metal together and is known as metallic bonding

Ionic bonding Ionic bonding occurs when metallic elements bond with non-metallic elements. Metal atoms have only a weak hold on their outer-shell electrons. In contrast, non-metallic atoms have a strong hold on their own electrons, and tend to remove outer-shell electrons from any metal atoms nearby. This causes ions to form. Electrostatic forces pull the positive and negative ions together to form a strong ionic bond. Each ion is surrounded by ions of the opposite charge, building up a three-dimensional structure called a lattice. The ionic bonding model explains all the important properties of ionic substances, including how they conduct electricity. When solid, ionic substances don't conduct because the ions are bonded within their lattice. When molten or dissolved in water, these ions separate from one another. This allows the ions to conduct an electric current.

Covalent bonding Covalent bonding happens when non-metallic atoms bond with each other. Non-metals have the ability to remove electrons from metals but they can't do this to other non-metals. Instead, they share some of their outer-shell electrons. Covalent bonds happen when two non-metals share one or more pairs of outer-shell electrons. If one pair is shared, then one electron from each atom forms the bond. The shared grip on these electrons holds the two atoms together.

Showing states The reactants and products in a chemical reaction can be in one of four states—solid, liquid, gas or aqueous solution (dissolved in water). Chemists give these states the symbols: (s) for a solid (l) for a liquid (g) for a gas (aq) for an aqueous solution. The symbols can be added to the formula equations to give even more information about the chemical reaction. In the previous example, calcium carbonate and calcium sulfate are both solids, sulfuric acid is an aqueous solution, water is a liquid and carbon dioxide is a gas. All of this information can be included in the formula equation by writing the symbol for the state next to the formula name: CaCO3(s) + H2SO4(aq) → CaSO4(s) + H2O(l) + CO2(g

Balanced chemical equations The formula equation for the reaction of calcium carbonate with sulfuric acid is a balanced equation. This means that it has the same number of atoms of each element on both sides of the equation. You can easily check this by counting the number of atoms in the reactants and products: Reactants = 1 × Ca, 1 × C, 7 × O, 2 × H, 1 × S Products = 1 × Ca, 1 × C, 7 × O, 2 × H, 1 × S Balanced equations are consistent with the law of conservation of mass. This law states that: During a chemical reaction, atoms cannot be created or destroyed. You cannot create or destroy atoms in a chemical reaction. But you can rearrange them. As a result, the number of atoms in the reactants must equal the number of atoms in the products. Also, the mass of the reactants must equal the mass of the products.

However, not all formula equations will be balanced when you first write them. For example, when hydrogen gas reacts with oxygen gas, the product is water. The word and formula equations for this reaction are: hydrogen + oxygen → water H2(g) + O2(g) → H2O(l) Counting the number of atoms on both sides of the equation shows that the equation is not balanced. Reactants = 2 × H, 2 × O Products = 2 × H, 1 × O This means that if one molecule of hydrogen reacts with one molecule of oxygen, then an oxygen atom is left over.

However, if two hydrogen molecules react with one oxygen molecule, then the atoms can rearrange to produce two complete molecules of water. This is shown in Figure 5.1.4. Chemists represent this reaction as a balanced formula equation by writing: 2H2(g) + O2(g) → 2H2O(l) Placing a 2 in front of the chemical formula for hydrogen and water indicates that the reaction uses two hydrogen molecules and produces two water molecules. Re-counting the number of atoms in the reactants and products shows that this equation is now balanced. Reactants = 4 × H, 2 × O Products = 4 × H, 2 × O Consider another chemical reaction in which calcium metal (Ca) reacts with oxygen gas (O2) to produce solid calcium oxide (CaO). The reactants in this reaction are calcium and oxygen gas. The only product is calcium oxide. Therefore, the general equation: reactants → products becomes the word equation: calcium + oxygen → calcium oxide

Replacing the chemical names with their formulas gives the formula equation: Ca + O2 → CaO Adding the states to the formula equation gives: Ca(s) + O2(g) → CaO(s) Checking the atoms of each element on both sides shows that the equation is unbalanced: Reactants = 1 × Ca, 2 × O Products = 1 × Ca, 1 × O However, it will be balanced if two calcium atoms react with one oxygen molecule to produce two CaO molecules. So the final balanced equation can be written as: 2Ca(s) + O2(g) → 2CaO(s)

Decomposition reactions When a single reactant breaks apart to form several products, the reactant is said to decompose. The general form of a decomposition reaction can be written as: XY → X + Y An everyday example of a decomposition reaction is the chemical reaction that puts the fizz in soft drinks like the one shown in Figure 5.2.1. Soft drinks contain dissolved carbonic acid (H2CO3). When carbonic acid decomposes, it forms water (H2O) and bubbles of carbon dioxide gas (CO2). The carbon dioxide gas formed by this reaction remains dissolved in the soft drink until the lid is removed. The balanced equation for the decomposition of carbonic acid is:

Combination reactions Combination reactions occur when two reactants combine to form a single product. The general equation for a combination reaction can be written as: X + Y → XY Combination reactions are important in industry. For example, a combination reaction is used to create hydrochloric acid for industry and laboratories. First, hydrogen gas (H2) and chlorine gas (Cl2) are combined to form hydrogen chloride gas (HCl) in large chemical plants like the one in Figure 5.2.3. The balanced equation for the combination of hydrogen and chlorine is: The hydrogen chloride gas that is produced is then bubbled through de-ionised water to produce hydrochloric acid.

Precipitation reactions Occasionally when two clear solutions are mixed together, they react to form a solid. The solid is said to precipitate out of the solution. As shown in Figure 5.2.4, most precipitates quickly fall to the bottom of the beaker. These types of reactions are known as precipitation reactions. For example, the scale that builds up in kettles, taps and pipes is solid calcium carbonate (CaCO3) that has precipitated out of the tap water.

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