Secondary Two Chemistry: Dissolving and Solutions

• KBTC International School provides Cambridge Lower Secondary Course revision notes for Secondary Two Chemistry in the 2024-2025 academic year.

Dissolving and Solutions

• A solute is a substance that dissolves.
• A solvent is a substance with the ability to dissolve a solute.
• A solution is a mixture of two or more substances (solute) dissolved in a solvent.
• Dissolving is a process where a solute is added into a solvent and the solute disappears, also referred to as dissolution.
• A solution is a homogeneous mixture, as the solute is evenly distributed throughout.
• A mixture consists of two or more substances physically combined, not chemically.
• True solutions are clear and transparent and do not scatter light.
• Components of a true solution cannot be separated by simple filtration.
• Suspension is a heterogeneous mixture where solid particles spread throughout the liquid without dissolving, visible to the naked eye, and settle down.
• Colloids are mixtures with dispersed substances as relatively large solid particles or liquid droplets throughout, remaining dispersed without settling due to gravity.
• Colloids can scatter light.
• The Tyndall Effect is the scattering of light by particles in a colloid or suspension
• Melting involves a single substance changing its phase, while dissolving involves two different substances forming a solution.
• The mass of a solution equals the total mass of the solute and solvent.

Types of Solutions

• Solid solutions consist of a solid solute and solid solvent. An example is copper dissolved in gold to form alloys.
• Liquid solutions consist of a liquid solute and liquid solvent. An example is ethyl alcohol dissolved in water.
• Gaseous solutions consists of a gas solute and liquid solvent. An example is carbon dioxide dissolved in water (soda water).
• A concentrated solution has a large amount of solute in a given solvent.
• A dilute solution has a small amount of solute in a large amount of solvent.
• Dilution is the process of adding a solvent to a solution to reduce the concentration of the solute.
• After dilution, the amount of solute remains unchanged, but the volume of solvent and concentration changes.
• A saturated solution contains the maximum amount of solute that can be dissolved at a given temperature.
• An unsaturated solution contains less than the maximum amount of solute capable of being dissolved.
• A supersaturated solution contains more than the maximum amount of solute capable of being dissolved at a given temperature.
• A soluble solid dissolves in a solvent, while an insoluble solid does not dissolve in a solvent, such as water.
• Solubility is the maximum amount of a substance that can be dissolved in a given amount of water at a specified temperature.
• The solubility of sodium chloride is 40g in 100g of water at room temperature to form a saturated solution.
• Different substances have different solubilities.
• Most solutes dissolve more quickly and easily in hot water than in cold water.

Factors Affecting Solubility

• Temperature: Solubility of most solid and liquid solutes increases with temperature, however, the solubility will decline at a point.
• Pressure: Solubility of a gas increases as pressure increases; pressure does not affect the solubility of solids and liquids.
• Molecular Size: Solubility decreases as molecular size increases.
• Intermolecular Force: Stronger intermolecular forces between solute and solvent increase solubility.

Solubility Calculations

• The curve line on a graph showing the relationship between temperature and solubility of a substance at different temperatures defines a solubility curve or graph, showing a graphical relationship between solubility and temperature.

How to Construct a Solubility Curve

• Plot the variations in temperature on the X-axis of a graph.
• Plot the solubility on the Y-axis.
• Next, plot the points of intersection of temperature and solubility on the graph
• Draw a line by joining the plots.

Importance of a Solubility Curve

• Solubility determination: The solubility of different substances at a particular temperature can be determined.
• Solubility comparison: The solubility curve helps us to compare the solubilities of different substances at the same temperature.
• Understanding trends: It provides a clear idea of how the solubility of a substance changes with temperature.
• General trend analysis: It can tell us the general trend for the solubility of different substances.
• Water is a universal solvent used for making solutions.
• Ethanol is a solvent for perfumes and paint varnish.
• Methanol is a solvent for cleaning paint brushes.
• Acetone is used as a nail polish remover.
• Toluene is a stain remover.

Solubility Investigation:

• The independent variable is the variable that is changed in a solubility investigation.
• The dependent variable is the variable that is measured during a solubility investigation.
• Control variables are the variables that are kept constant during the investigation.
• When graphing, the independent variable goes along the horizontal axis and the dependent variable goes up the vertical axis.

Paper Chromatography

• Paper chromatography is an analytical technique used to separate and analyze mixtures of soluble substances based on their solubility.
• Mixtures of colored inks, food dyes, or plant pigments can be separated using paper chromatography.
• Solvents like water or alcohol are used as the mobile phase in paper chromatography.
• The stationary phase is paper, made of cellulose, a polymer of glucose.
• The solvent carries the soluble mixture with different distances through the paper.
• The more soluble substances are carried further from the base line.
• The resulting image on the paper is called a chromatogram.

Methods for Separating Mixtures

• Filtration separates an insoluble solid from a liquid.
• Decantation is a process to separate mixtures by removing a liquid layer that is free of a precipitate or solids deposited from a solution.
• Evaporation separates a soluble solid from a liquid by evaporating the solvent.
• Crystallization separates a soluble solid from a solution and collect crystals on the surface.
• Magnetic separation separates magnetic components of mixtures using a magnet.
• Centrifugation separates disperse solids from a liquid by spinning them in a centrifuge at high speed.
• Simple distillation separates a solvent from a solution by evaporating and condensing the solvent.
• Fractional distillation is used for separating two liquids that are mixed together, based on their boiling points, using a fractionating column.
• Chromatography can be used to separate mixtures of soluble substances.

Atomic Structure

• Atoms are the fundamental building blocks of matter, originating from a Greek word meaning "cannot be split".
• All atoms of an element are identical, and different elements have different atoms.
• Atoms consist of protons, neutrons, and electrons.
• Protons and neutrons are arranged closely in the center (nucleus); electrons move around the nucleus.
• Protons carry a positive charge and have a mass of 1 atomic mass unit (amu).
• Neutrons have no charge and have a mass of 1 amu.
• Electrons carry a negative charge and have a mass of 1/1840 amu.
• Most of an atom is empty space.
• Electrostatic attraction between positive protons and negative electrons holds the atom together.
• An atom is electrically neutral because the number of protons equals the number of electrons.
• The atomic number is the number of protons in an atom.
• Mass number is the total number of protons and neutrons in the nucleus.
• J.J. Thompson discovered the electron and proposed that electrons were scattered throughout the structure of an atom.
• Rutherford discovered the proton in 1909 and the nucleus in 1911. He also the carried out the gold foil experiment.
• James Chadwick proved that neutrons existed in 1932.

Properties of an Atom Models

• Thomsons model states that negatively charged electrons are embedded into the sphere of uniform positive charge, uniformly distributed and neutral.
• Rutherfords model says electrons move around a positively charged heavy nucleaus, neutral with the mass in its nucleus
• Bohrs model states that electrons move around the nucleus in certain fixed shells called orbits, neutral with the mass in its nucleus

Purity of an Element

• An element is pure if all its atoms are the same type of atom; alloys of gold may contain other metals like copper or silver.
• The unit for gold purity is the carat.
• Pure gold is 24 carats; 18 carat or 9 carat golds are alloys.
• Higher gold content means higher purity.
• 18 carat gold has 18 parts gold out of 24; the rest is other metals like silver or copper.
• Diamonds are made of carbon atoms in a specific arrangement; pure diamonds are colorless and translucent.
• Impurities in diamond result in different colors in diamond.
• Seawater contains water and salts like sodium chloride; sodium chloride can be obtained by evaporating the water off.
• About 350g of salts is present in 1000g of seawater; around 68% is sodium chloride, and the remainder is made up of magnesium chloride, sodium sulfate, calcium chloride, and other salts.
• Sodium chloride obtained from seawater is only 68% pure.
• The mass of sodium chloride in 1000 g seawater is 23.8%.
• Salts are compounds made from acids.
• Sodium chloride from hydrochloric acid.
• Sodium sulfate from sulfuric acid.
• Some salts are soluble in water, while others are insoluble.

Solubility Rules

• All nitrates are soluble.
• Most chlorides are soluble, except silver chloride, mercuric chloride, lead chloride.
• Most sulfates are soluble, except barium sulfate, lead sulfate, calcium sulfate.
• Most carbonates are insoluble, except sodium carbonate, potassium carbonate, and ammonium carbonate.
• In chemical reactions, product purity is often very important.
• Chemical reactions are processes where two or more substances interact to form new substances with different compositions
• A chemical equation represents a chemical reaction.
• Reactants are starting materials written on the left-hand side of the equation.
• Products are new substances formed after a reaction, written on the right-hand side of the equation.

Types of Chemical Reactions

• Synthesis involves combining of two or more reactants
• Decomposition involves the breakdown of a compound.
• Single displacement involves one element replacing another in a compound.
• Double displacement involves the exchange of ions between two compounds.
• Combustion involves burning a substance with oxygen.
• Acid-base neutralization involves the reaction between an acid and a base.

Atmospheric Reactions

• Synthesis: A+B → AB
• Carbon + oxygen (excess) → carbon dioxide
• Potassium + iodine → potassium iodide
• Sodium + fluorine → sodium fluoride
• Hydrogen + nitrogen → ammonia
• Metal + water → metal hydroxide + hydrogen
• Calcium + water → calcium hydroxide + hydrogen
• Magnesium + water → magnesium hydroxide + hydrogen
• Metal + steam → metal oxide + hydrogen
• Aluminium + steam → aluminium oxide + hydrogen
• Zinc + steam → zinc oxide + hydrogen
• Decomposition: AB → A + B
• Water → hydrogen + oxygen
• Calcium carbonate → calcium oxide + carbon dioxide
• Single Displacement: A + B-C → A-C + B
• Chlorine + sodium bromide → sodium chloride + bromine
• Copper + silver nitrate → copper nitrate + silver
• Iron + copper sulfate → iron sulfate + copper
• Metal + dilute acid → salt + hydrogen
• Magnesium + hydrochloric acid (dilute) → magnesium chloride + hydrogen
• Zinc + sulfuric acid → zinc sulfate + hydrogen
• Double Displacement: AB + CD → AD + BC
• Silver nitrate + sodium chloride silver chloride + sodium nitrate
• Magnesium + sodium chloride/carbonate → magnesium carbonate + sodium chloride
• These reactions are also called precipitation reactions because one type of salt precipitates after the reaction.
• Combustion: Metal/nonmetal + oxygen → metal/nonmetal oxides
• Magnesium + oxygen → magnesium oxide
• Sulfur + oxygen → sulfur dioxide (or) sulfur trioxide
• Methane + oxygen → carbon dioxide + water
• Glucose + oxygen → carbon dioxide + water + energy
• Acid-Base Neutralization: Base/alkali + acid → salt + water

Acids and Weather

• Sodium hydroxide + hydrochloric acid → sodium chloride + water
• Zinc oxide + sulfuric acid → zinc sulfate + water
• Metal carbonate + acid → salt + water + carbon dioxide
• Calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
• Copper carbonate + sulfuric acid → copper sulfate + water + carbon dioxide
• Weather is the state of the atmosphere at a particular place during a short time.
• Climate is the state of atmosphere at a particular place during a longer time.
• Weather is affected by temperature, humidity, cloudiness, and precipitation.
• Climate is affected by temperature and precipitation.
• Meteorology studies weather.
• Climatology studies climate. Each climate zone has a characteristic climate.
• The polar climate is very cold and dry all year.
• The temperate zone has cold winters and mild summers.
• The arid climate is hot and dry all year.
• The tropical zone is hot and wet all year.
• The mediterranean climate has mild winters and hot, dry summers.
• Mountains/tundra/taiga are very cold all year.

Ice Age and Atmosphere

• The ice age is characterized by glaciers and extensive ice sheets, leading to colder global temperatures.
• The Earth's climate cycles between glacial and interglacial periods in the ice age.
• In an interglacial period, permanent ice is near the North and South Poles.
• In a glacial period, ice spreads further from the North and South Poles.
• Scientists know Earth was colder based on boulders, fossils of cold-adapted organisms and pollen in peat bogs.
• An auger is used to take samples of peat bogs.
• The atmopsphere is a core layer of gas above Earths surface
• Deeper peat is older with the upper area being the youngest.
• The atmosphere is a layer of gas above the Earth's surface with five major layers: troposphere, stratosphere, mesosphere, thermosphere, and exosphere.
• Weather conditions occur in the troposphere layer.
• The stratosphere is also called the ozone layer and filters UV radiation from the Sun.
• Gases evolved from volcanic eruptions include water vapor and sulfur dioxide.
• Calcium carbonate makes up animal shells and limestone, a sedimentary rock.
• Fossils are preserved remains buried in sediments, such as sand and mud.
• Fossil fuels are hydrocarbons formed from dead plants and animals in Earth’s crust.
• Fossils are analyzed using radiometric carbon dating.
• Atmospheric change means changes in the composition of Earth's atmosphere.
• Changes can be natural (volcanic eruption) or from human activity.
• Natural processes and human activity increase carbon dioxide in the atmosphere.
• Fossil fuels, like oil and coal, release carbon dioxide when burned.
• Calcium carbonate decomposes to calcium oxide and carbon dioxide with heat.
• Greenhouse gases (GHGs) are naturally in the atmosphere and trap heat.
• Examples: carbon dioxide, methane, nitrous oxide, chlorofluorocarbons.
• The greenhouse effect is the process of heat being trapped near Earth's surface by GHGs
• More GHGs trap more heat, and the Earth becomes warmer, leading to global warming.
• Global warming is a gradual, general increase in Earth's temperature due to an increased green house effect causes by greenhouse gases.
• Causes for global warming: volcano eruption, burning of fossil fuels, deforestation, emission of GHGs from industry and using nitrogen fertilizers in agriculture
• A carbon footprint is a measure of total GHGs in the atmosphere.
• Renewable resources can be replenished within a human lifetime.
• Examples are: wind, tidal, and solar power.
• Non-renewable resources cannot be replaced after use, like coal, petroleum, and natural gas.
• Photovoltaic cells can convert solar energy to electricity.
• Bioplastic is a biodegradable material that comes from renewable sources, unlike conventional plastics.
• Bioplastics can reduce reliance on fossil fuels and prevent environmental damage from single-use plastics.
• Biogas refers to a mixture of gases produced by the anaerobic decomposition of organic matter and made largely of methane gas

Exothermic and Endothermic Chemical Reactions

• An exothermic reaction releases heat (thermal energy) into its surrounding such as combustion, neutralization, respiration, rusting iron and more
• Temperature of the surrounding is expected to increase in exothermic reactions
• Burning, or combustion, occurs when a substance combines with oxygen to form an oxide.
• The substance reacting with oxygen is a fuel like charcoal, wood, natural gas, or oil.
• Burning is an exothermic reaction as it releases heat.
• When fuel burns, chemical energy is transformed into thermal, light and sound energy.
• Methane + oxygen → carbon dioxide + water
• A combustion requires oxygen, heat and fuel.
• Evidences of burning or combustion: the substances combine with oxygen in air to form oxides, surroundings temperature increases/ energy changes occur
• Hydrogen combustion is an exothermic reaction and releases kinetic, thermal, sound and light energy.
• Metal combustion is an exothermic reaction combined with oxygen to form a metal oxide, giving off heat and light.
• A metal reacts with water to form metal hydroxide or oxide and hydrogen where stored chemical energy transfer to thermal
• A metal reacting with water is classified exothermic.
• When a metal reacts with acid, metal salt and hydrogen form
• Reaction of metal and acid is classified as exothermic because heat is given off into the surrounding.
• Increased gas during a reaction indicates a more reactive metal.
• An endothermic reaction absorbs heat from its surroundings.
• Sodium hydrogen carbonate, citric acid, photosynthesis, and thermal decomposition are all examples of endothermic reactions.
• An endothermic reaction experiences a decrease in surrounding temperature and is classified as absorbing thermal energy from the surroundings.

Endothermic v Exothermic Chemical Reactsion

• The temperature of an endothermic reaction decreases
• Energy is stored in the form of chemical bonds through the process of thermal energy in chemcial energy
• Termite reaction is an exothermic reaction
• Sel-heating can is classified as exothermic
• Ice pad used to treat injury is an endothermic relaction
• Metal reacts to oxygen to form metal oxide
• Metal with oxygen reaction is labeled oxidation reactions
• Sodium and potassium need to be stored under oil because its is easily cut witha knife during the process
• Metal with dilute acid form is considered products of salt and hydrogen