Chemistry 6092/02 Paper 2, Oct/Nov 2026 (Questions 1–5)

“Sulfur dioxide causes breathing difficulties in humans and animals. Sulfur dioxide and nitrogen oxides react with oxygen in the air to form acidic compounds which dissolve in rain water, forming solutions of strong acids. This is called acid rain.”

“However, when rain combines with excessive amounts of sulfur dioxide and nitrogen dioxide produced by human activity, it can have a pH of around 4.0.”

“Acid rain (NO₂ and SO₂)”

“Two oxides of nitrogen are air pollutants. One is acidic and the other is neutral. (a) Name the two oxides. (b) Identify the oxide that is acidic and explain how it causes harm to the natural environment.”

“(a) Nitric oxide and nitrogen dioxide (b) Nitrogen dioxide is acidic. It dissolves in rain to form acid rain which can kill plants and marine life.”

“Acid rain kills plants and aquatic life. It also corrodes buildings and structures made from limestone, marble and metals.”

“Frequent exposure to acid rain can have devastating effects on forests and water bodies. Acid rain leaches essentials nutrients like magnesium and calcium from the soil, which hampers the health and growth of trees (Figure 22.5). Acid rain can also lower the pH of soil below what many plant species can tolerate, killing them.”

“Aquatic organisms are also very susceptible to pH changes. Over time, the addition of acidic rainwater to a water body like a lake or a pond can cause the pH to drop out of the survivable range.”

“Living organisms gain the energy for life processes by breaking down glucose back into carbon dioxide and water, which are released into the atmosphere.”

“Most fossil fuels contain mainly methane. They are burnt in the presence of oxygen to generate energy. Carbon dioxide is a product of combustion.”

“Dead organisms are broken down into simpler compounds by decomposers. Aerobic decomposition occurring in the presence of oxygen releases carbon dioxide into the atmosphere.”

“The combustion of fossil fuels in factories, power plants and vehicles is a major contributor of atmospheric carbon dioxide. In 2019, these sources produced 36.7 billion tonnes of carbon dioxide, or about 4700 kg per human on Earth. The clearing of forests for agriculture is also a large contributor to carbon dioxide emissions. It has the added effect of reducing the number of plants available to absorb and retain carbon.”

“Production of CO₂ • Respiration • Decomposition • Combustion”

“(c) Respiration / decomposition / combustion (Any two)”

“Plants take in carbon dioxide and water to make glucose and oxygen in the presence of sunlight.”

“Oceans and other large bodies of water absorb carbon dioxide. Some marine plants use the dissolved gas to photosynthesize. It is also converted to carbonic acid and calcium carbonate that make up the shells of marine organisms and accumulate on the seabed when they die.”

“The carbon cycle describes a set of processes that regulates the amount of carbon dioxide in the atmosphere.”

“Removal of CO₂ • Photosynthesis • Ocean uptake”

“(b) Photosynthesis / ocean uptake (Any one)”

“Global warming has major implications for human civilisation. It is predicted that extreme weather phenomena like typhoons and tornadoes will become stronger and occur more frequently. Parts of the world that are currently fertile will quickly become arid and barren. Island nations like Singapore are also likely to face challenges related to sea levels rising from melting polar ice sheets and the thermal expansion of ocean water.”

“Shifting wind patterns can lead to changes in rainfall patterns. Lush areas might begin to receive too little rain and turn into deserts while other areas may receive too much rain and flood more often than before (Figure 22.19). Fertile land would become barren and will no longer be able to produce food, potentially causing food shortages.”

“Heat waves would also become more common. Some parts of the world would even become too hot for humans to live in.”

“The warmer the ocean, the stronger these storms become. There has been a notable increase in their strength and frequency since the 1980s.”

“Increases in average global temperatures have caused glaciers and polar ice caps to melt and shrink.”

“It has been estimated that sea levels around the world would rise by more than 60 m if the ice caps completely melted. This would permanently flood most coastal cities like Singapore since 60 m is about 20 storeys high (Figure 22.26)!”

“glacial retreat and melting of polar ice caps • Glacier-fed river levels would change. • Sea levels would rise and permanently flood coastal areas. • Climate-stabilising ocean currents would be disrupted.”

“(c) An increase in atmospheric greenhouse gases leads to an enhanced greenhouse effect, which causes global warming. This leads to extreme weather conditions and melting of polar ice caps. (Accept other possible answers.)”

“Yeast is added to dough during the bread-making process to help the bread rise. Yeast contains enzymes that break down the glucose in the dough into carbon dioxide gas and alcohol.”

“Enzymes are part of a special group of substances called catalysts. A catalyst increases the rate of chemical reactions without being chemically changed at the end of the reaction.”

“Catalysts provide an alternative pathway of lower activation energy for the reaction to occur. This speeds up the rate of reaction.”

“Due to the alternative pathway with lower activation energy, more reactant particles will have energy greater than or equal to activation energy. This increases the frequency of effective collisions. Hence, the rate of reaction increases.”

“Catalysts have the following characteristics: • They increase reaction rates but not the yield. • They are not used up in the reaction. • They are selective (i.e. they only catalyse certain reactions). • Impurities can poison them. • Their physical appearance may change but they remain chemically the same. • They provide an alternative pathway with a lower activation energy for the reaction.”

“Biological catalysts are known as enzymes.”

“They are temperature-sensitive. A lot of enzymes operate best at body temperature (i.e. between 35 °C and 40 °C). Below these temperatures, the enzymes are dormant or can also be denatured. This reduces the catalytic activity. At high temperatures, enzymes are denatured and are unable to catalyse reactions.”

“At higher temperatures, the rate of reaction is affected in two ways: 1. At higher temperatures, reactant particles have more kinetic energy. This means that they move and vibrate faster, increasing the frequency of effective collisions. 2. At higher temperatures, more reactant particles possess energy that is greater than or equal to activation energy. This increases the frequency of effective collisions.”

“Hence, higher temperatures result in faster reaction rates (Figure 17.18). An increase in the temperature of reactants results in an increase in the rate of reaction.”

“We have learnt in Chapter 17 that high temperatures would denature the enzymes in yeast.”

“Ensure that the temperature of the solution does not exceed 37 °C.”

“The concentration of a solution shows the number of particles in a given volume. As the concentration of reactants increases, the number of reacting particles per unit volume increases. They collide more frequently and hence, the frequency of effective collisions also increases (Figure 17.10).”

“When more effective collisions occur between reactant particles, the rate of reaction increases.”

“An increase in concentration of reactants increases the rate of reaction.”

“Flask A has the steepest initial gradient followed by flask B, then flask C. This shows that the rate of reaction decreases with decreasing reactant concentration.”

“They have a specific catalytic action. Enzymes are specific in their catalytic activity. Living cells contain many different types of enzymes, and each catalyses a specific reaction. As seen in Figure 17.26, the active site of each enzyme is specific. The active site can only fit certain reactant molecule(s) in a specific orientation.”

“Protons and neutrons are found clustered together in the nucleus of an atom. The number of protons is fixed for all atoms of a given element. However, the number of neutrons in the nucleus can vary. Neutrons do not affect the electrical charge of the atom, although they affect the total mass.”

“Isotopes are atoms of the same element that have the same proton (atomic) number but different nucleon (mass) numbers. This means that they have different numbers of neutrons.”

“The relationship between the proton and nucleon numbers can be used to determine the number of neutrons in an atom: Number of neutrons = Nucleon number (A) – Proton number (Z)”

“Complete the following table of isotopes.”

“bromine-79 • 35 • 44 • 35 • ⁷⁹₃₅Br bromine-81 • 35 • 46 • 35 • ⁸¹₃₅Br”

“Isotopes • Atoms of the same element with different numbers of neutrons • Same proton (atomic) number (Z) • Different nucleon (mass) numbers (A)”

“Because isotopes of the same element have the same number of protons, they also have the same number of electrons. This means that they undergo the same kind of chemical reactions but may have different physical properties. Such physical properties may include density, melting point, and boiling point.”

“Elements in the same group have the same number of valence electrons. The chemical properties of an element is largely dependent on the number of valence electrons it has. Hence, elements in the same group tend to exhibit similar chemical properties.”

“Since isotopes of the same element have the same proton number, they also have the same number of electrons, and the same electronic configuration.”

“The outermost shell is also called the valence shell, and electrons in that shell are called valence electrons.”

“Will bromine-79 and bromine-81 atoms undergo different types of chemical reactions? Explain your answer.”

“Table 3.1 provides a summary of the mass, charge and location of these three particles, relative to each other.”

“proton • 1 • +1 • nucleus neutron • 1 • 0 • nucleus electron • 1/1840 • –1 • electron shell”

“Protons and neutrons are heavier particles grouped together in the centre of the atom, which is called the nucleus.”

“Protons • Carry a positive electrical charge. Neutrons • Neutral particles with no net electrical charge • Found in the nucleus between protons”

“The proton number (Z) of an atom is the number of protons in its nucleus.”

“Protons • Number identifies the element • Relative charge: +1 • Relative mass: 1 • Shown by the proton (atomic) number. Neutrons • Keep the nucleus together • Relative charge: 0 • Relative mass: 1”

“We can represent the proton and nucleon numbers of an atom using nuclide notation (Figure 3.7).”

“For example, ¹²₆C represents a specific type of carbon atom, with 12 nucleons and 6 protons. Other types of carbon atoms with different numbers of nucleons can be represented as ¹³₆C and ¹⁴₆C using nuclide notation.”

“The atomic symbol and the proton number both tell us the type of atom we are referring to. Atoms of different elements have different numbers of protons.”

“The proton number (Z) of an atom is the number of protons in its nucleus. The nucleon number (A) is the total number of protons and neutrons in the nucleus. Nucleon number (A) = proton number (Z) + number of neutrons”

“Since its nucleon number is 184, an atom of tungsten has 184 nucleons, or the total number of protons and neutrons. The proton number of 74 tells us that it has 74 protons. We can subtract the proton number from the nucleon number to find the number of neutrons.”

“X: atomic symbol of element • A: nucleon (mass) number • Z: proton (atomic) number”

“Lead(II) chloride is insoluble in water, so it must be prepared by the precipitation method.”

“Step 2 Filter to remove unreacted lead(II) carbonate. The filtrate is lead(II) nitrate solution. Step 3 Add excess aqueous potassium chloride to the filtrate to precipitate lead(II) chloride. 2KCl(aq) + Pb(NO₃)₂(aq) → PbCl₂(s) + 2KNO₃(aq)”

“Precipitation involves mixing two solutions to form an insoluble solid that separates out from the reaction mixture. The general equation for the preparation of salts by precipitation is: aqueous solution AB + aqueous solution XY → insoluble salt AY + aqueous solution XB”

“barium nitrate + sodium sulfate → barium sulfate + sodium nitrate Ba(NO₃)₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaNO₃(aq)”

“Although salts are ionic compounds, not all salts are soluble in water. The solubility of a salt must be determined before we can choose a suitable method to prepare the salt.”

“all sodium salts • all potassium salts • all ammonium salts • all nitrates”

“The state symbol shows the physical state of the substance during the reaction. Common states are: • solid (s) • liquid (l) • gas (g) • aqueous (aq)”

“Polyatomic ions are ions which are made up of more than one atom covalently bonded together. They carry overall charges, and their overall valences is the charge on the ions (Table 6.7).”

“Write the chemical formula of magnesium nitrate. Thought Process: Magnesium is in Group 2 and has a valency of 2. The nitrate ion is a polyatomic ion with a valency of 1. Overall, two NO₃⁻ ions are needed to exactly cancel out the +2 charge on the Mg²⁺ ion. Answer: Hence, the chemical formula of magnesium nitrate is Mg(NO₃)₂.”

“Transition metals can form more than one stable ion and so they have variable valences. Their valences are indicated using Roman numerals in brackets (Table 6.5).”

“The percentage mass of an element in a compound can be calculated using this formula: Percentage mass = (number of atoms of the element in the formula × Ar of the element) / Mr of compound × 100 %”

“Find the percentage mass of nitrogen in ammonium sulfate, (NH₄)₂SO₄. Answer: Relative formula mass of (NH₄)₂SO₄ = 2[14 + 4(1)] + 32 + 4(16) = 132. Percentage mass of nitrogen in (NH₄)₂SO₄ = (14 × 2) / 132 × 100 % = 21.2 %”

“Ionic compounds do not exist as molecules. We use the term relative formula mass to refer to the relative mass of an ionic compound. The relative formula mass (Mr) of an ionic compound is the average mass of one unit of that compound relative to 1/12 the mass of an atom of carbon-12. Relative formula mass is calculated the same way as relative molecular mass and also has the symbol Mr (Table 7.2).”

“To find the relative molecular mass (Mr) of a substance, we add together the relative atomic masses (Ar) of all the atoms in its chemical formula (Table 7.1).”

“Transition elements are the block of metals found in Groups 3 to 12 of the periodic table (Figure 14.17). They are also called the transition metals. Examples of some common transition metals include chromium (Cr), manganese (Mn), iron (Fe) and copper (Cu).”

“When a compound is made up of only covalently-bonded carbon and hydrogen atoms, we call it a hydrocarbon.”

“Hydrocarbons are compounds that only contain hydrogen and carbon atoms.”

“Alkanes are hydrocarbons that contain only carbon–carbon single bonds and carbon–hydrogen bonds.”

“Step 1 Assuming that 100 g of the substance is present, the masses of each element would then be equal to their percentage mass. Step 2 Fill in the relative atomic mass values from the periodic table. Calculate the number of moles of each element present. Step 3 Divide each mole value by the smallest of the mole values. This gives the mole ratio of the elements. Step 4 Write out the empirical formula using the simplest mole ratio derived in step 3.”

“The simplest ratio of elements in a compound is called its empirical formula.”

“Hydrocarbons are compounds that only contain hydrogen and carbon atoms. A certain hydrocarbon contains 80.0 % carbon by mass. What is its empirical formula?”

“Plexiglass is a plastic that is commonly used for making spectacle lenses and optical panels. Plexiglass has the following composition by mass: 59.9 % carbon, 8.1 % hydrogen and 32.0 % oxygen. Find its empirical formula.”

“Hexane is an alkane whereas hexene is an alkene (Figure 19.23). Although both alkanes and alkenes are hydrocarbons, their chemical properties are very different. Alkanes are saturated hydrocarbons as they only contain single covalent bonds between the carbon atoms. In saturated hydrocarbons, all carbon atoms are bonded to four other atoms.”

“Aqueous bromine is a reagent commonly used in laboratories to test for unsaturation. Aqueous bromine is a brown solution which consists of bromine dissolved in water. Figure 19.24 shows how aqueous bromine can be used to distinguish alkenes from alkanes.”

“aqueous bromine remains brown when shaken with alkanes • aqueous bromine decolourises when shaken with alkenes”

“On the other hand, alkenes are unsaturated hydrocarbons as they contain double covalent bonds between the carbon atoms. In unsaturated hydrocarbons, more atoms can be added across the carbon–carbon double bond.”

“The general molecular formula of alkanes is CₙH₂ₙ₊₂, where n is the number of carbon atoms in each molecule (n = 1, 2, 3, 4 and so on).”

“The molecular formula is a multiple of the empirical formula. The molecular formula of a compound reflects the exact number of atoms of each element found in one molecule of the compound.”

“We know that the molecular formula is a multiple of the empirical formula. We can find the multiple, n, using the formula below: n = relative molecular mass (Mr) of compound / Mr from empirical formula. By multiplying the numbers of elements in the empirical formula by n, we can then work out the molecular formula.”

“In an addition reaction, two reactants combine to form one product.”

“The carbon–carbon double bonds in alkenes are reactive. Hence, alkenes will readily undergo addition reactions.”

“The carbon-carbon double bond breaks to form new single bonds. As a result, each carbon atom which was originally part of the C=C bond forms four single covalent bonds instead.”

“Alkenes can react with steam to produce alcohols. The conditions required are a temperature of 300 °C and a pressure of 60 atm. The reaction also requires a catalyst (phosphoric(V) acid, H₃PO₄).”

“ethene + steam → ethanol • C₂H₄(g) + H₂O(g) → C₂H₅OH(aq)”

“The carbon-carbon double bond breaks to form new single bonds. As a result, each carbon atom which was originally part of the C=C bond forms four single covalent bonds instead.”

“When there are two carbon atoms in each molecule, n = 2. So, the number of hydrogen atoms will be 2(2) + 1 = 5. Hence, when n = 2, the formula of the alcohol is C₂H₅OH (Figure 20.4).”

“A carbon-carbon double bond is a double covalent bond between two carbon atoms (Figure 19.14).”

“Like endothermic reactions, we can represent an exothermic reaction using an energy level diagram (Figure 16.15) and an energy profile diagram (Figure 16.16).”

“reactants • activation energy (Ea) • enthalpy change (ΔH = negative) • products • Progress of reaction”

“Since exothermic changes involve a net release of energy to the surroundings, the products have less energy than the reactants. Hence, exothermic changes are indicated by a negative enthalpy change.”

“Exothermic changes have a negative enthalpy change (ΔH < 0).”

“Energy level diagrams show the overall enthalpy change of a reaction. Energy profile diagrams show the enthalpy change and the activation energy of a reaction.”

“Enthalpy change, ΔH, measures the difference in energy content of the reactants and products. • ΔH = total energy of products – total energy of reactants”

“The activation energy, Ea, is the minimum amount of energy that colliding reactant particles must possess to react with each other.”

“When reactant particles with insufficient energy (i.e. energy less than the activation energy) collide, they are unable to break their bonds. Hence, no chemical reaction occurs (Figure 16.7).”

“The activation energy, Ea, is the minimum amount of energy that colliding reactant particles must possess to react with each other.”

“For an effective collision to occur, the reactant particles must collide with energy that is greater than or equal to the activation energy (Figure 17.9).”

“At higher temperatures, more reactant particles possess energy that is greater than or equal to activation energy. This increases the frequency of effective collisions.”

“If the anode is replaced with an inert electrode, some pure copper can still be deposited at the cathode. This copper is produced from the Cu²⁺ ions in the electrolyte. The colour intensity of the electrolyte gradually fades from blue to colourless. The deposition of copper at the cathode ceases once the reservoir of Cu²⁺ ions is exhausted.”

“After Cu²⁺ and OH⁻ ions are discharged, H⁺ and SO₄²⁻ ions remain in the solution. • Hence, the resulting solution becomes increasingly acidic as electrolysis progresses.”

“Since OH⁻ ions are lower than SO₄²⁻ ions in the electrochemical series, OH⁻ ions are selectively discharged. • Since copper is below hydrogen in the reactivity series, Cu²⁺ ions are selectively discharged over the H⁺ ions.”

“2Cu²⁺(aq) + 4OH⁻(aq) → 2Cu(s) + O₂(g) + 2H₂O(l)”

“pH <7: The concentration of H⁺ ions is higher than that of OH⁻ ions, so the solution is acidic.”

“Strong acids such as hydrochloric acid, nitric acid and sulfuric acid exist around pH 1 to 3.”

“Number of moles of a substance = (mass of substance in g) / (molar mass of the substance in g/mol)”

“Concentration in mol/dm³ = (number of moles of solute) / (volume of solvent in dm³)”

“The concentration of a solution is the amount of a solute dissolved in a unit volume of the solvent.”

“In Chapter 1, we learnt that 1 dm³ is equivalent to 1 litre. There are 1000 cm³ in 1 dm³.”

“A 200 cm³ solution was made by dissolving 1.74 g of potassium sulfate (K₂SO₄) in water. Find the concentration of the potassium sulfate solution in mol/dm³.”

“The relative atomic masses of elements are given in the periodic table.”

“The copper anode dissolves to form Cu²⁺ ions and decreases in mass. • Red-brown copper is deposited onto the copper cathode which increases in mass.”

“These processes do not have a net reaction as the metal that is being oxidised at the anode is simultaneously being reduced at the cathode.”

“Reactive electrodes, unlike inert electrodes, can take part in the electrolysis process. In industries, reactive electrodes are often used to introduce a metal cation into the electrolyte. When a reactive metal is used as the anode, it is oxidised to form its cation. This cation may then be reduced back to the original metal on the cathode.”

“Cu(s) → Cu²⁺(aq) + 2e⁻ • Cu²⁺(aq) + 2e⁻ → Cu(s)”

“The copper deposited onto the cathode comes mainly from the electrolyte. • The copper anode is constantly oxidising and dissolving into the electrolyte, replenishing the Cu²⁺ ions which are reduced at the cathode. • Since the concentration of Cu²⁺ ions remains unchanged, the colour intensity of the blue electrolyte also remains constant.”

“None of the components in the electrolyte are oxidised if the anode is reactive.”

“The percentage mass of an element in a compound can be calculated using this formula: • Percentage mass = (number of atoms of the element in the formula × Ar of the element) / (Mr of compound) × 100 %”

“The percentage mass of an element in a compound allows us to quickly calculate how much of that element is present in a given sample. For example, if a compound is 62.5% carbon by mass, then 1000 kg of that compound should contain 625 kg of carbon.”

“One mole of any substance has a mass equal to its relative atomic mass (Ar), relative molecular mass or relative formula mass (Mr) in grams. This mass is the molar mass of the substance.”

“Number of moles of a substance = (mass of substance in g) / (molar mass of the substance in g/mol)”

“The relative atomic masses of elements are given in the periodic table.”

“One mole of any substance has a mass equal to its relative atomic mass (Ar), relative molecular mass or relative formula mass (Mr) in grams. This mass is the molar mass of the substance.”

“Number of moles of a substance = (mass of substance in g) / (molar mass of the substance in g/mol)”

“The relative atomic masses of elements are given in the periodic table.”

“An increase in oxidation state (more positive, less negative) refers to the loss of electrons, and hence oxidation. A decrease in oxidation state (more negative, less positive) refers to a gain of electrons, and hence reduction.”

“For a compound with no net charge, all the oxidation states must add up to zero. For an ion with a net charge, the oxidation states will add up to that net charge.”

“Rule 0 (all elements) • Atoms of the same element, such as: single atoms; molecules of an element; and metallic elements. • 0 • Atoms of the same element have no net charge.”

“(a) Group 1 elements in their compounds • +1 • Atoms from Group 1 will lose an electron (e.g. Na⁺, K⁺).”

“(c) oxygen in its compounds (usually) • –2 • Oxygen, after fluorine, is next most likely to gain electrons. It usually has an oxidation state of –2.”

“elements from Groups 3 to 12 in their compounds, and elements with proton numbers 57 to 71 or 89 to 103 • always positive • Elements in Groups 3 to 12 can have different oxidation states in their compounds. Roman numerals indicate the oxidation state present in the named species (e.g. chromium(III) indicates that chromium has an oxidation state of +3).”

“Oxygen’s oxidation state is –2, based on rule 2(c). • Since this is a compound, the overall charge must be 0.”

“The oxidation state of iodine increased from –1 in potassium iodide to 0 in aqueous iodine. • The oxidation state of chlorine decreased from 0 in chlorine gas to –1 in potassium chloride.”

“From the reactions that occur at the anode and cathode, the overall equation for this electrolysis is: • 4H⁺(aq) + 4OH⁻(aq) → 2H₂(g) + O₂(g) + 2H₂O(l)”

“Write the overall equation for the reaction. • 2Cu²⁺(aq) + 4OH⁻(aq) → 2Cu(s) + O₂(g) + 2H₂O(l)”

“Electrolytic Cell • Converts electrical energy to chemical energy • Simple Cell • Converts chemical energy to electrical energy”

“Today, the battery has evolved into a variety of more portable forms. Disposable alkaline batteries and rechargeable nickel metal-hydride cells (NiMH) are widely used to power low-voltage electronics like remote controls. Our laptops and mobile phones are powered by lithium ion (Li-ion) batteries.”

“However, in graphite, each carbon atom is bonded to three other carbon atoms. This leaves 1 unbonded electron per carbon atom, which is free and mobile to conduct electricity.”

“Graphite is slightly different from other giant covalent molecules because of its layered structure. Strong covalent bonds are found within each layer, but weak intermolecular forces of attraction exist between the layers.”

“This means that graphite layers can be easily separated from each other, but each layer is hard to break apart.”

“Only a small amount of energy is needed to overcome the weak intermolecular forces of attraction between the layers of carbon atoms. Hence, these layers of carbon atoms can slide over each other easily.”

“two-dimensional layers (strong covalent bonds between carbon atoms, but weak intermolecular forces of attraction between the layers of carbon atoms) • conducts electricity along layers”

“The first shell can hold a maximum of 2 electrons and cannot accommodate any more. • The third electron must occupy a new, higher energy electron shell. • This arrangement is called the electronic configuration or electronic structure of lithium. We can write it as 2, 1.”

“A positive ion or cation is formed when an atom loses one or more electrons. It is no longer an atom, since atoms are overall electrically neutral. The ion formed is positively charged, because it has more protons than electrons. Metal atoms tend to lose electrons and form positive ions.”

“Positive ions (cations) have a net positive charge and usually have a noble gas electronic configuration.”

“This is because the size of the atom increases going down a group. Hence, the outermost electrons of the element will be further away from the attractive force of the nucleus.”

“As we go down Group 1, the size of the atom increases, as seen in Figure 14.9. It is easier to lose the valence electron from bigger atoms.”

“lithium • Reacts quickly. Lithium floats on the water. • lithium + water → lithium hydroxide + hydrogen • 2Li(s) + 2H₂O(l) → 2LiOH(aq) + H₂(g)”

“Alkali metals are highly reactive. They are stored in oil to prevent them from reacting with air and water.”

“All Group 1 elements, or the alkali metals, react with cold water to form hydrogen and an alkali.”

“Like alkanes, alkenes burn in excess oxygen to produce carbon dioxide and water.”

“Complete combustion of alcohol produces carbon dioxide and water.”

“Plastics are mostly flammable. When plastics are incinerated, they produce poisonous gases. For example, PVC produces corrosive hydrogen chloride gas on combustion.”

“Plastics do not decompose. Thus, burying plastic waste in landfills leads to an increasing amount of built-up waste.”

“Plastics thrown into the sea endanger marine animals. For example, turtles often mistake plastic bags for food and choke on them. Plastics may also clog up rivers and drains, which might become breeding grounds for mosquitoes.”

“However, most plastics are non-biodegradable. This means that they cannot be broken down by bacteria and other living organisms in the soil. Hence, the biggest problem with the use of plastics is how to dispose of them after they are used or no longer required.”

“The overlap shows that this pair of electrons is shared between the two atoms. These electrons are called a bonding pair. A single covalent bond has been formed.”

“The overlap shows two bonding pairs of electrons. A double covalent bond has been formed.”

“A carbon atom has 4 electrons in its outermost or valence shell. It has a valency of 4. • An oxygen atom has 6 electrons in its outermost or valence shell. It has a valency of 2. • Thus, a carbon atom will form four covalent bonds and each oxygen atom will form two covalent bonds.”

“The dot-and-cross diagram should show the following: • a carbon atom with 4 electrons spaced around its outermost shell • two oxygen atoms, each sharing 2 electrons with the carbon atom • the carbon atom sharing 2 electrons with each oxygen atom”

“A carbon-carbon double bond is a double covalent bond between two carbon atoms (Figure 19.14). Figure 19.15 shows how two pairs of electrons are shared between the carbon atoms to form the double bond.”

“Carboxylic acids are organic compounds with the carboxyl (–COOH) functional group.”

“A weak acid is an acid that is only partially ionised in an aqueous solution.”

“Only some CH₃COOH molecules will ionise in an aqueous solution (Figure 8.12).”

“The strength of an acid is its degree of ionisation whereas concentration is related to the number of acid molecules present in a given volume of water.”

“Carboxylic acids are weak acids because they partially ionise in water to form a carboxylate ion and hydrogen ion. The partial ionisation of carboxylic acids could be represented by the following equation: • carboxylic acid ⇌ carboxylate ion + hydrogen ion • RCOOH(aq) ⇌ RCOO⁻(aq) + H⁺(aq)”

“CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)”

“oxygen (O₂) • –218 • –183 • gas”

“Since simple covalent substances typically exist as neutral molecules, they do not have any mobile charge carriers to conduct electricity.”

“Silicon dioxide consists of many silicon and oxygen atoms linked to each other by strong covalent bonds. The atoms are joined to each other in a regular arrangement, forming a giant covalent structure.”

“Ionic compounds have giant ionic crystal lattice structures (Figure 5.6). The lattice consists of large numbers of alternating positive and negative ions held together by electrostatic forces of attraction.”

“The weak intermolecular forces of attraction between molecules are easily overcome with a small amount of energy. Thus, simple covalent molecules have low melting and boiling points.”

“At certain temperatures, heating or cooling a substance results in changes of state instead of changes in temperature. These temperatures are the freezing, melting and boiling points of the substance, also known as transition temperatures.”

“When an ionic compound is in the solid state, the ions in the lattice are only able to vibrate about their fixed positions. Hence, the ions are not mobile. However, when the compound is molten (in the liquid state) or in the aqueous state, the ions are mobile and can conduct electricity. Therefore, ionic compounds can conduct electricity in the liquid and aqueous states but not in the solid state (Figure 5.9).”

“Giant covalent structures with no mobile electrons, like diamond and silicon dioxide, do not conduct electricity.”

“To melt these giant covalent substances, a large amount of energy is required to break the strong covalent bonds. Hence, they are solids at room temperature, with very high melting and boiling points.”

“Because the mutual electrostatic attraction between oppositely charged ions are strong, a large amount of energy is needed to overcome these forces. Hence, ionic compounds tend to have very high melting and boiling points. They are solids at room temperature.”

“Because the mutual electrostatic attraction between oppositely charged ions are strong, a large amount of energy is needed to overcome these forces. Hence, ionic compounds tend to have very high melting and boiling points. They are solids at room temperature.”

“Ionic compounds have giant ionic crystal lattice structures (Figure 5.6). The lattice consists of large numbers of alternating positive and negative ions held together by electrostatic forces of attraction.”

“The weak intermolecular forces of attraction between molecules are easily overcome with a small amount of energy. Thus, simple covalent molecules have low melting and boiling points.”

“This is very different from simple covalent molecules that have weak intermolecular forces of attraction.”

“The particles with increased energy can overcome the forces of attraction in the solid state.”

“The metallic bond is the mutual electrostatic attraction between positively charged metal ions and the “sea of delocalised electrons”. In a giant metallic lattice, the atoms occupy fixed positions that are evenly distributed throughout the solid.”

“Metals usually have high melting and boiling points because the atoms are held together in a lattice by strong metallic bonds.”

“The “sea of delocalised electrons” found in both metals and alloys make them good electrical conductors.”

“Pure metals have a regular structure, as shown in Figure 5.27. If enough force is applied, the layers of atoms can slide over one another easily. Thus, when hammered, pure metals can be bent or flattened into a thin sheet (malleable). They can also be pulled into a wire without breaking (ductile). The “sea of delocalised electrons” will continue to hold the metal ions together unless a much greater force is applied.”

“The metallic bond is the mutual electrostatic attraction between positively charged ions in a metal and the “sea of mobile (or delocalised) electrons”.”

“In an alloy, the atoms are of different sizes. The regular lattice arrangement in the pure metal is disrupted, so a larger force is needed to make the layers slide over each other (Figure 5.28).”

“An alloy is a mixture of a metal with one or more other elements.”

“The regular arrangement of copper atoms is disrupted by the randomly distributed tin atoms, which are about 1.6 times the size of a copper atom.”

“Ionic compounds have giant ionic crystal lattice structures (Figure 5.6). The lattice consists of large numbers of alternating positive and negative ions held together by electrostatic forces of attraction.”

“When an ionic compound is in the solid state, the ions in the lattice are only able to vibrate about their fixed positions. Hence, the ions are not mobile.”

“Hence, the metallic lattice structure is described as a lattice of positive ions surrounded by a “sea of mobile electrons”.”

“The metallic bond is the mutual electrostatic attraction between positively charged ions in a metal and the “sea of mobile (or delocalised) electrons”.”

“Alkali metals: • are soft and can be cut easily (Figure 14.8); • have low melting and boiling points; and • have low densities.”

“lithium • 180 • 0.53 • sodium • 98 • 0.97 • potassium • 63 • 0.86 • rubidium • 39 • 1.53”

“Notice that the melting points and densities of the transition metals are higher than those of the Group 1 and Group 2 metals.”

“One of the most significant features of transition metals is that they usually form coloured compounds.”

“Describe typical transition elements as metals having high melting points, high densities, variable oxidation states and forming coloured compounds.”