Air Pollution and Greenhouse Gases PDF

# Air Pollution Increasing carbon dioxide is causing climate change. But carbon dioxide isn't the only gas released when fossil fuels burn you can also get other nasties like oxides of nitrogen, sulfur dioxide and carbon monoxide. ## Combustion of Fossil Fuels Releases Gases and Particles 1. F...

# Air Pollution Increasing carbon dioxide is causing climate change. But carbon dioxide isn't the only gas released when fossil fuels burn you can also get other nasties like oxides of nitrogen, sulfur dioxide and carbon monoxide. ## Combustion of Fossil Fuels Releases Gases and Particles 1. Fossil fuels, such as crude oil and coal, contain hydrocarbons (see page 146). 2. Hydrocarbons can combust (burn in oxygen). There are two types of combustion: * **Complete combustion** - when there's plenty of oxygen around all of the fuel burns. * **Incomplete combustion** - when there's not enough oxygen around some of the fuel does not burn. 3. Both types of combustion release carbon dioxide and water vapour into the atmosphere. 4. During incomplete combustion, solid particles (called particulates) of soot (carbon) and unburnt fuel are also released. Carbon monoxide gas is also produced. 5. Particulates in the air and carbon monoxide can cause all sorts of problems: * **Particulates** * If particulates are breathed in they can get stuck in the lungs and cause damage. This can lead to respiratory (breathing) problems. * They're also bad for the environment - they reflect sunlight back into space. This means that less light reaches the Earth - causing global dimming * **Carbon monoxide** * Carbon monoxide (CO) is really dangerous because it can stop your blood from carrying enough oxygen around the body. * A lack of oxygen in the blood can lead to fainting, a coma or even death. * Carbon monoxide doesn't have any smell or smell so it's very hard to detect. This makes it even more dangerous. ## Sulfur Dioxide and Oxides of Nitrogen Can be Released 1. Other pollutants are also released from burning fossil fuels. 2. Sulfur dioxide (SO₂) is released during the combustion of fossil fuels that contain sulfur impurities. 3. Nitrogen oxides form in a reaction between nitrogen and oxygen in the air. This reaction is caused by the heat of the burning fossil fuels. 4. These gases mix with clouds and cause acid rain. 5. Acid rain kills plants. It also damages buildings, statues and metals. 6. Sulfur dioxide and nitrogen oxides also cause respiratory problems if they're breathed in. # Greenhouse Gases and Climate Change Greenhouse gases are important but can also cause problems - it's all about keeping a delicate balance. ## Carbon Dioxide is a Greenhouse Gas 1. Greenhouse gases include carbon dioxide, methane and water vapour. 2. Greenhouse gases keep the Earth warm enough to support life. Here's how they work: * This radiation is reflected back by the Earth as long wavelength radiation. This is thermal (heat) radiation. It's then absorbed by greenhouse gases. * Greenhouse gases then give out this radiation in all directions. * Some radiation heads back towards the earth and warms us the surface. This is the greenhouse effect. 3. Some forms of human activity increase the amount of greenhouse gases in the atmosphere. For example: * **Deforestation:** fewer trees means that less carbon dioxide is taken in for photosynthesis. * **Burning fossil fuels:** releases carbon dioxide. * **Agriculture:** more farm animals produce more methane when they digest their food. * **Creating waste:** more landfill sites and more waste from farming means more carbon dioxide and methane is released when the waste breaks down. ## Increasing Carbon Dioxide is Linked to Climate Change 1. Recently, the average temperature of the Earth's surface has been going up. 2. Scientists agree that this has been caused by the extra carbon dioxide from human activity. 3. They believe this will lead to climate change. 4. Evidence for this has been peer-reviewed (see page 1). This means that the information is reliable. 5. However, the Earth's climate is very complex. So, it's very hard to make a model that isn't oversimplified. 6. This has led to people forming their own theories and opinions, particularly in the media. These stories may be biased or only give some of the information. ## Climate Change Could Have Dangerous Consequences 1. Higher global temperatures are causing ice in the Arctic and Antarctic to melt - causing sea levels to rise. If sea levels keep rising, this will lead to more flooding. 2. Changes in rainfall are causing some regions to get too much or too little water. 3. Storms may become more frequent and severe 4. Changes in temperature and rainfall are having an effect on the production of food in certain places. # Purity and Formulations In a perfect world, every compound a chemist made would be pure. Unfortunately, in the real world it doesn't always work out like that. Luckily, there are ways to find out how pure a substance is ## Purity Has a Different Meaning in Chemistry to Everyday 1. Usually when you say that a substance is pure you mean that nothing has been added to it. So it's in its natural state. For example: pure milk or beeswax. 2. In chemistry, a pure substance is something that only contains one compound or element all the way through. It's not mixed with anything else. ## The Boiling or Melting Point Tells You How Pure a Substance Is 1. A chemically pure substance will melt or boil at a specific temperature. 2. You can test how pure a known substance is by measuring its melting or boiling point. You then compare this value with the melting or boiling point of the pure substance, You can find this in a data book. 3. The closer your measured value is to the actual melting or boiling point, the purer your sample is. 4. Impurities in your sample will lower the melting point. They may also cause the sample to melt across a wider range of temperatures. 5. Impurities in your sample will increase the boiling point. They may also cause the sample to boil across a range of temperatures. ## Formulations are Mixtures with Exact Amounts of Its Parts 1. Formulations are useful mixtures that have been designed for a particular use. 2. They are made by following a 'formula' (a recipe). 3. Each part of a formulation is measured carefully so that it's there in the right amount - This makes sure the formulation has the right properties for it to work as it's supposed to. 4. For example, paints are formulations. They are made up of: * **Pigment** - gives the paint colour. * **Solvent** - used to dissolve the other parts and change how runny the paint is. * **Binder** - holds the pigment in place after it's been painted on. * **Additives** - added to change the properties of the paint. 5. The chemicals used and their amounts can be changed so the paint made is right for the job. 6. In everyday life, formulations can be found in cleaning products, fuels, medicines, cosmetics, fertilisers, metal alloys and even food and drink. # Cracking Crude oil fractions from fractional distillation can be split into smaller molecules. This is called cracking. It's super important, otherwise we might not have enough fuel for cars and planes and things. ## Cracking Means Splitting Up Long-Chain Hydrocarbons 1. There is a high demand for fuels with small molecules. 2. This is because short-chain hydrocarbons tend to be more useful than long-chain hydrocarbons. 3. So, lots of longer alkane molecules are turned into smaller, more useful ones. This is done by a process called cracking. 4. Some of the products of cracking are useful as fuels, e.g. petrol for cars. 5. Cracking also makes alkenes. Alkenes are a lot more reactive than alkanes. They're used a starting material when making lots of other compounds and can be used to make polymers (see p.118). ## Bromine water can be used to test for alkenes: 1. When orange bromine water is added to an alkane: no reaction will happen and it'll stay bright orange. 2. If it's added to an alkene, the bromine reacts with the alkene to make a colourless compound. So, the bromine water turns colourless. ## There are Different Methods of Cracking 1. Cracking is a thermal decomposition reaction. This means the molecules are broken down by heating them. 2. This can be done by catalytic cracking or by steam cracking: * **Steam cracking** 1. Long-chain hydrocarbons are heated to turn them into a gas. 2. The hydrocarbon vapour is mixed with steam. 3. They are then heated to a very high temperature which splits them into smaller molecules. * **Catalytic cracking** 1. Long-chain hydrocarbons are heated to turn them into a gas. 2. Then the vapour is passed over a hot powdered aluminium oxide catalyst. 3. The long-chain molecules split apart on the surface of the specks of catalyst. 3. You might be asked to work out the formula of the products or reactants involved in a cracking reaction. You can do this by balancing the number of carbons and hydrogens on each side of the reaction. # Fractional Distillation Crude oil can be used to make loads of useful things, such as fuels. But you can't just put crude oil in your car. First, the different hydrocarbons have to be separated. That's where fractional distillation comes in. ## Fractional Distillation Can be Used to Separate Hydrocarbon Fractions 1. Crude oil is a mixture of lots of different hydrocarbons, most of which are alkanes. 2. The different compounds in crude oil are separated by fractional distillation. Here's how it works: * The oil is heated until most of it has evaporated (turned into gas). The gases enter a fractionating column (and the liquid bit is drained off). * In the column it's hot at the bottom and gets cooler as you go up. * The shorter hydrocarbons have low boiling points. This means that they're still gases at low temperatures. So they don't condense and turn back into liquids until they move up near the top of the column, where they cool down a lot. * The longer hydrocarbons have high boiling points. This means that they'll only stay a gas if it's very hot. As they move up the fractionating column, it gets cooler. So they condense back into liquids and drain out of the column slowly, when they're near the bottom. 3. You end up with the crude oil mixture separated into different fractions (parts), e.g. petrol and diesel oil. 4. Each fraction contains a mixture of hydrocarbons. All of the hydrocarbons in one fraction contain a similar number of carbon atoms. This means they'll have similar boiling points. # Crude Oil Crude oil has fuelled modern life - it would be a very different world if we hadn't discovered oil. ## Crude Oil is Made Over a Long Period of Time 1. Crude oil is a fossil fuel found in rocks. 2. Fossil fuels are natural substances. They can be used as a source of energy. 3. Crude oil formed mainly from the remains of plankton, as well as other plants and animals. These died millions of years ago and were buried in mud. * Fossil fuels like coal, oil and gas are called non-renewable fuels. This is because they take so long to make that they're being used up much faster than they're being formed. * They're finite resources - one day they'll run out. ## Crude Oil has Various Important Uses in Modern Life 1. It provides the fuel for most modern transport - cars, trains, planes, the lot. 2. Diesel oil, kerosene, heavy fuel oil and LPG (liquefied petroleum gas) all come from crude oil. 3. Petrochemicals are compounds that come from crude oil. The petrochemical industry uses some of the compounds from crude oil as a feedstock to make new compounds for use in things like... * Polymers (e.g. plastics) * Lubricants * Detergents 4. All the products you get from crude oil are examples of organic compounds. Organic compounds are compounds containing carbon atoms. 5. Most of the organic compounds in crude oil are hydrocarbons (see previous page). 6. You can get a large variety of products from crude oil. This is because carbon atoms can bond together to form different groups called homologous series. 7. These groups contain similar compounds which have many properties in common. Alkanes and alkenes are both examples of different homologous series. ## Hydrocarbon Properties Change as the Chain Gets Longer 1. The hydrocarbons in crude oil are a range of different sizes. 2. As the length of the carbon chain changes, the properties of the hydrocarbons change. 3. The shorter the hydrocarbon chain * ...the more runny a hydrocarbon is - that is, the less viscous (gloopy) it is. * ...the lower its boiling points will be. * ...the more flammable (easier to ignite) the hydrocarbon is. 4. The properties of hydrocarbons affect how they're used for fuels. # Hydrocarbons Organic chemistry is about compounds that contain carbon. Hydrocarbons are the simplest organic compounds. ## Alkanes Only Have C-C and C-H Single Bonds 1. Hydrocarbons are compounds formed from carbon and hydrogen atoms only. 2. Alkanes are the simplest type of hydrocarbon. They have the general formula C_nH_2n+2. 3. In alkanes, each carbon atom forms four single covalent bonds. 4. The first four alkanes are methane, ethane, propane and butane. ## Complete Combustion Occurs When There's Plenty of Oxygen 1. The complete combustion of a hydrocarbon in oxygen releases lots of energy. This makes them useful as fuels. 2. The only waste products are carbon dioxide and water vapour. 3. During combustion, both carbon and hydrogen from the hydrocarbon are oxidised. Oxidation is the gain of oxygen. 4. You need to be able to give a balanced symbol equation for the complete combustion of a simple hydrocarbon when you're given its molecular formula. Here's an example: # Reversible Reactions Reversible reactions are what they sound like - reactions that can be reversed. So they can go backwards ## Reversible Reactions Go Both Ways 1. This equation shows a reversible reaction: A + B <=> C + D 2 The products (C and D) react to form the reactants (A and B) again. 3. You can tell it's a reversible reaction because of the symbol <=>. 4. The reaction of A and B is called the forward reaction. The reaction of C and D is the backward reaction. ## Reversible Reactions Will Reach Equilibrium 1. As the reactants react, their concentrations fall. The forward reaction slows down. 2. As more and more products are made the backward reaction will speed up. 3. After a while the forward reaction and backward reaction will be going at exactly the same rate. The system is at equilibrium. 4. Equilibrium doesn't mean that there are the same amounts of products and reactants. It just means that the amounts of products and reactants aren't changing any more. 5. Equilibrium is only reached if the reaction takes place in a 'closed system'. A closed system just means that none of the reactants or products can escape and nothing else can get in. ## Reversible Reactions Have an Overall Direction 1. Once a reaction is at equilibrium, there could be more of the products than reactants. When this happens, we say the reaction is going in the forwards direction. 2. If there are more reactants than products then the reaction is going in the backwards direction. 3. You can change the direction by changing the conditions (the temperature, pressure or concentration). ## Reversible Reactions Can Be Endothermic and Exothermic 1. If the reaction is endothermic (takes in heat) in one direction, it will be exothermic (give out heat) in the other. 2. The amount of energy taken in by the endothermic reaction is the same as the amount given out during the exothermic reaction. # Working Out Reaction Rates As well as doing experiments and drawing graphs, you need to be able to do some calculations to work out reaction rates. Don't worry though, they're not too bad. Read on and all will be explained. ## Here's How to Work Out the Rate of a Reaction **Mean Rate of Reaction = Amount of reactant used or amount of product formed / Time** This equation is for mean rate of reaction. So, it lets you work out the average rate over an amount of time. ## You Can Calculate the Mean Reaction Rate from a Graph 1. To find the mean rate for the whole reaction, start by working out when the reaction finished. This is when the line goes flat. 2. Then work out how much product was formed (or how much reactant was used up). 3. Then divide this by the total time taken for the reaction to finish. # Rates of Reaction Rates of reaction are pretty important. In the chemical industry, the faster you make chemicals, the faster you make money (and the faster everyone gets to go home for tea). ## The Speed of a Reaction is Called its Rate 1. The rate of a chemical reaction is how fast the reactants are changed into products. 2. Some reactions are very slow, for example, rusting. Others, like burning, are fast. 3. Graphs can show you how the rate (speed) of a reaction changes. 4. The steeper the line on the graph, the faster the rate of reaction. 5. Over time the line becomes less steep as the reactants are used up. ## Reaction Rates Can Change when the Reaction Conditions Change 1. Faster reactions have steeper lines to begin with and become flat more quickly. 2. This graph shows how the speed of a particular reaction changes under different conditions. ## Particles Must Collide with Enough Energy in Order to React 1. Reaction rates can be explained by an idea called collision theory. 2. Collision theory says that a reaction will only take place when particles collide (crash into each other). 3. The particles also have to have a certain amount of energy when they collide, otherwise they won't react. 4. The minimum (smallest) amount of energy they need is called the activation energy. 5. Collision theory can explain rates of reactions in a bit more detail too... * The more often the particles collide, the faster the reaction will happen. For example, if the reactant particles in a certain reaction collide with enough energy twice as often, the reaction will happen twice as fast. * The more energy the particles have, the faster the reaction will be. This is because there's more chance that they'll have at least the activation energy. # Reaction Profiles Reaction profiles are handy little diagrams which show you the changes in energy during a reaction. ## Activation Energy is Needed to Start a Reaction 1. The activation energy is the minimum amount of energy the reactants need to have to react when they collide with each other. 2. The greater the activation energy, the more energy needed to start the reaction. This energy has to be given, e.g. by heating the reaction mixture. ## Reaction Profiles Show Energy Changes Reaction profiles are diagrams that show the difference between the energies of the reactants and products in a reaction, and how the energy changes over the course of the reaction. ## Exothermic Reactions 1. The reaction profile on the right shows an exothermic reaction. 2. You can tell because the products are at a lower energy than the reactants. 3. The difference in height between the reactants and the products shows the overall energy change in the reaction (the energy given out). 4. The rise in energy at the start shows the energy needed to start the reaction - This is the activation energy. ## Endothermic Reactions 1. In this reaction profile, the products are at a higher energy than the reactants. So the reaction is endothermic. 2. The difference in height shows the overall energy change during the reaction (the energy taken in). 3. The rise in energy at the start is the activation energy # Exothermic and Endothermic Reactions In all chemical reactions, there's a change in energy. This means that when chemicals get together, things either heat up or cool right off. I give you a heads up - this page is a good 'un. ## Energy is Moved Around in Chemical Reactions 1. Chemicals store a certain amount of energy and different chemicals store different amounts. 2. Sometimes, the products of a reaction store more energy than the reactants. This means that the products have taken in energy from the surroundings during the reaction. 3. But if the products store less energy, then the extra energy was transferred (given out) to the surroundings during the reaction. 4. The amount of energy transferred is the difference between the energy of the products and the energy of the reactants. 5. The overall amount of energy doesn't change. This is because energy stays the same (is conserved) in reactions - it can't be made or destroyed, only moved around. 6. This means the amount of energy in the universe always stays the same. ## In an Exothermic Reaction, Energy is Given Out 1. An EXOTHERMIC reaction is one which gives out energy to the surroundings. 2. This is shown by a rise in temperature of the surroundings. 3. Examples of exothermic reactions include: * Burning fuels - also called COMBUSTION. * Neutralisation reactions (acid + alkali). * Many oxidation reactions. 4. Exothermic reactions have lots of everyday uses. For example: * Some hand warmers use an exothermic reaction to release energy. * Self heating cans of hot chocolate and coffee also use exothermic reactions between chemicals in their bases. ## In an Endothermic Reaction, Energy is Taken In 1. An ENDOTHERMIC reaction is one which takes in energy from the surroundings. 2. This is shown by a fall in temperature of the surroundings. 3. Examples of endothermic reactions include: * The reaction between citric acid and sodium hydrogencarbonate. * Thermal decomposition (when a substance breaks down when it's heated). 4. Endothermic reactions also have everyday uses. For example: * Endothermic reactions are used in some sports injury packs. The chemical reaction allows the pack to become instantly cooler without having to put it in the freezer. # Electrolysis of Aqueous Solutions When you electrolyse a salt that's dissolved in water, you also have to think about the ions from the water. ## You Can Predict what Forms when a Salt Solution is Electrolysed 1. Water can break down into H⁺ and OH^- ions. 2. So in solutions that contain water, there will be the ions from the ionic compound as well as hydrogen ions (H⁺) and hydroxide ions (OH^-) from the water. 3. H⁺ ions and metal ions will move to the cathode. 4. If the metal's more reactive than hydrogen, hydrogen gas will form. 5. If the metal is less reactive than hydrogen, a solid layer of the pure metal will form. 6. If the salt contains halide ions (Cl^-, Br^-, I^- ), chlorine, bromine or iodine will form at the anode. 7. If no halide ions are present, then the OH^- ions lose electrons and oxygen will form at the anode. ## Electrolysis of Molten Ionic Solids Forms Elements 1. Molten ionic compounds can be electrolysed because the ions can move freely and conduct electricity. 2. Molten ionic liquids are always broken up into their elements. 3. The metal forms at the cathode. The non-metal is formed at the anode. 4. When molten lead bromide is electrolysed, lead forms at the cathode and bromine forms at the anode. ## Metals Can be Extracted From Their Ores Using Electrolysis 1. Aluminium is extracted from an ore that contains aluminium oxide, Al₂O₃. 2. Aluminium oxide has a very high melting point so it's mixed with a substance called cryolite. This lowers the melting point. 3. The positive Al³⁺ ions are attracted to the negative electrode where they form aluminium atoms. 4. The negative O^2- ions are attracted to the positive electrode where they react to form O₂ molecules. # Reactions of Metals Metals react to form salts. And you, my friend, need to be able to predict the salt that'll form from a reaction. ## Metals React With Acids 1. Some metals react with acids to produce a salt and hydrogen gas: * Acid + Metal => Salt + Hydrogen * Hydrochloric acid + magnesium => magnesium chloride + hydrogen * Sulfuric acid + zinc => zinc sulfate + hydrogen * Hydrochloric acid + iron => iron chloride + hydrogen 2. Very reactive metals like potassium, sodium, lithium and calcium react explosively with acids. 3. Less reactive metals such as magnesium, zinc and iron react less violently with acids. 4. In general, copper won't react with cold, dilute acids. ## Metals Also React with Water 1. Many metals will also react with water. * Metal + Water => Metal Hydroxide + Hydrogen 2. For example, calcium: Ca + 2H₂O => Ca(OH)₂ + H₂ 3. The metals potassium, sodium, lithium and calcium will all react with water. 4. Less reactive metals like zinc, iron and copper won't react with water. ## You Can Work Out a Reactivity Series From the Reactions of Metals 1. If you put metals in order from most reactive to least reactive based on their reactions with either an acid or water, the order you get is the reactivity series (see the previous page). 2. To compare the reactivities of metals, you could watch how quickly bubbles of hydrogen are formed in their reactions with water or acid. The more reactive the metal, the faster the bubbles will form. 3. You can also measure the temperature change of the reaction in a set time period. The more reactive the metal, the greater the temperature change should be. ## More Reactive Metals Can Displace Less Reactive Metals from Salts 1. Displacement reactions involve one metal kicking another one out of a compound. Here's the rule: A MORE REACTIVE metal will displace a LESS REACTIVE metal from its compound. 2. For example, iron is more reactive than copper. So if you add solid iron to copper sulfate solution, you get a displacement reaction. 3. The iron kicks the copper out of copper sulfate. You end up with iron sulfate solution and copper solid. * Iron + copper sulfate => iron sulfate + copper * Fe + CuSO₄ => FeSO₄ + Cu # The Reactivity Series and Extracting Metals You can place metals in order of reactivity. This can be a lot more useful than it sounds, promise. ## The Reactivity Series - How Easily a Metal Reacts 1. The reactivity series lists metals in order of how reactive they are (their reactivity). 2. Metals react to form positive ions. 3. So for metals, their reactivity depends on how easily they lose electrons and form positive ions. 4. The higher up the reactivity series a metal is, the more easily it forms positive ions. ## Metals Often Have to be Separated from their Oxides 1. Lots of common metals, like iron and aluminium, react with oxygen to form oxides. 2. This process is an example of oxidation. 3. These oxides are often the ores that the metals are removed (extracted) from. 4. A reaction that separates a metal from its oxide is called a reduction reaction. ## Some Metals Can be Extracted by Reduction with Carbon 1. Some metals can be extracted from their ores using a reaction with carbon. 2. In this reaction, the ore is reduced as oxygen is removed from it. Carbon gains oxygen, so it is oxidised. 3. For example: 2Fe₂O₃ + 3C => 4Fe + 3CO₂ 4. The reactivity series can tell you if a metal can be extracted with carbon. * Metals above carbon in the reactivity series are extracted using electrolysis. This is expensive as it takes lots of energy to melt the ore and to produce the electricity. Electrolysis is also used to extract metals that react with carbon. * Metals below carbon in the reactivity series can be extracted by reduction using carbon. For example, iron oxide is reduced in a blast furnace to make iron. This is because carbon can only take the oxygen away from metals which are less reactive than carbon itself is. 5. Some metals are so unreactive they are found in the earth as the metal itself. For example, gold. # Acids and Bases Testing the pH of a solution means using an indicator - and that means pretty colours... ## The pH Scale Goes From 0 to 14 1. The pH scale is a measure of how acidic or alkaline a solution is. 2. The lower the pH of a solution, the more acidic it is. 3. The higher the pH of a solution, the more alkaline it is. 4. A neutral substance (e.g. pure water) has pH 7. ## You Can Measure the pH of a Solution 1. An indicator is a dye that changes colour depending on whether it's above or below a certain pH. 2. Wide range indicators are substances that gradually change colour as pH changes. 3. They're useful for estimating the pH of a solution. 4. For example, universal indicator is a wide range indicator. It gives the colours shown above. 5. A pH probe attached to a pH meter can also be used to measure pH electronically. 6. The probe is put in the solution and the pH is shown as a number. This means it's more accurate than an indicator. ## Acids and Bases Neutralise Each Other 1. When acids dissolve in water, they form solutions with a pH of less than 7. Acids form H⁺ ions in water. 2. Bases have pH greater than 7. 3. Alkalis are bases that dissolve in water to form solutions with a pH greater than 7. Alkalis form OH^- ions in water . For example, soluble metal hydroxides are alkalis. 4. The reaction between acids and bases is called neutralisation: acid + base => salt + water. 5. Neutralisation between acids and alkalis can be shown using H⁺ and OH^- ions like this: H⁺ + OH^- => H₂O. 6. The products of neutralisation reactions have a pH of 7. This means they're neutral. 7. You can add an indicator to the acid or alkali you're neutralising. Then gradually add the other substance. The indicator will change colour when the neutralisation reaction is over. If you use Universal indicator, add the substance until the Universal indicator is green. This is when the pH of the solution is neutral. # Concentrations of Solutions Lots of reactions take place between substances that are dissolved in a solution. And sometimes it's useful to find out the mass of a substance that's dissolved in a solution. Hold onto your hats and concentrate.... ## Concentration is a Measure of How Crowded Things Are 1. The amount of a substance (e.g. the mass) in a certain volume of a solution is called its concentration. 2. The more substance that's dissolved in a certain volume, the more concentrated the solution. ## Concentration Can be Measured in g/dm³ 1. You can find the concentration of a solution if you know the mass of the substance dissolved and the volume of the solution. 2. The units will be units of mass/units of volume. For example, g/dm³. 3. Here's how to calculate the concentration of a solution in grams per decimetre cubed (g/dm³): ## You Can Calculate the Percentage Mass of an Element in a Compound To work out the percentage mass of an element in a compound, you need to use this formula: # Conservation of Mass You've probably realised by now that you can't magic stuff out of thin air. It can't magically disappear, either. ## In a Chemical Reaction, Mass Always Stays the Same 1. During a chemical reaction no atoms are lost and no atoms are made. 2. This means there are the same number and types of atoms on each side of a reaction equation. 3. Because of this, no mass is lost or gained - we say that mass is conserved (stays the same) in a reaction. 4 You can see that mass stays the same if you add up the relative formula masses of the substances on each side of a balanced symbol equation. 5. The total Mr of all the reactants will be the same as the total Mr of the products. ## You Can Calculate the Mass of a Reactant or Product 1. You can use the idea of conservation of mass to work out the mass of a reactant or product in a reaction. 2. You need to know the masses of all the reactants and products except for one. 3. You can work out the total mass of everything on one side of the equation. 4. You can also work out the total mass of everything on the other side of the equation, except for the thing you don't know the mass of. 5. The mass of the thing you don't know is the difference between these two totals. # Relative Formula Mass Calculating relative formula mass is important for lots of calculations in chemistry. It might sound a bit hard to begin with, but it gets easier with practice. We'd better get cracking... ## Compounds Have a Relative Formula Mass, Mr To find the relative formula mass, Mr, of a compound, add together the relative atomic masses of all the atoms in the molecular formula. ## You Can Calculate the Percentage Mass of an Element in a Compound To work out the percentage mass of an element in a compound, you need to use this formula: