1.1.1 Formulae, word and balanced equations

Chemical formulae: symbols and subscripts fix what a substance is

1. A symbol with no subscript means one atom of that element in the particle.
2. Most elements are written as the bare symbol, such as carbon $C$, sulfur $S$, iron $Fe$ and copper $Cu$.
3. Seven non-metal elements exist as diatomic molecules, so their formulae are $H_2,N_2,O_2,F_2,{Cl}_2,{Br}_2$ and $I_2$.
4. A compound contains atoms of two or more different elements chemically bonded together.
5. Common compound formulae include water $H_{2}O$, carbon dioxide ${CO}_2$, carbon monoxide $CO$, methane ${CH}_4$ and ammonia ${NH}_3$.
6. Common acid and salt formulae include $HCl$, $H_2{SO}_4$, ${HNO}_3$, $NaOH$, $NaCl$, ${CuSO}_4$ and ${CaCO}_3$.
7. Brackets group a set of atoms, and the subscript outside them multiplies everything inside, so ${Ca(OH)}_2$ has one calcium atom, two oxygen atoms and two hydrogen atoms.

Ions: atoms and groups of atoms that carry a charge

1. Losing electrons leaves a positive ion, and gaining electrons makes a negative ion.
2. Common positive ions include $H^{+},{Na}^{+},K^{+},{Ag}^{+},{NH}_4^{+},{Mg}^{2+},{Ca}^{2+},{Cu}^{2+},{Zn}^{2+}$ and ${Al}^{3+}$.
3. Common negative ions include ${Cl}^{-},{Br}^{-},I^{-},O^{2-},{OH}^{-},{NO}_3^{-},{SO}_4^{2-}$ and ${CO}_3^{2-}$.
4. The ammonium, hydroxide, nitrate, sulfate and carbonate ions are groups of atoms that share one charge between them.
5. The charge is written as a superscript after the formula, and it is part of what identifies the ion.

Formulae of ionic compounds: the charges have to cancel

1. Write the symbol and charge of each ion, then find the smallest whole-number ratio of ions that cancels the charges.
2. ${Na}^{+}$ and ${Cl}^{-}$ balance in a $1:1$ ratio, giving sodium chloride $NaCl$.
3. ${Mg}^{2+}$ needs two ${Cl}^{-}$ ions to cancel its charge, giving magnesium chloride ${MgCl}_2$.
4. Two ${Al}^{3+}$ ions balance three $O^{2-}$ ions, giving aluminium oxide ${Al}_2{O}_3$.
5. Put brackets round a group of atoms whenever more than one of that group is needed, as in ${Ca(OH)}_2$ and ${Al(NO}_3{)}_3$.
6. The charges are not written in the finished formula, because the compound as a whole is neutral.

Word equations: names on each side of the arrow

1. A word equation shows a reaction using the names of the substances rather than their formulae.
2. The reactants are the substances present at the start, and they go on the left of the arrow.
3. The products are the new substances formed, and they go on the right of the arrow.
4. A plus sign separates two or more substances on the same side of the arrow.
5. The arrow means “reacts to form”, so it is never replaced by an equals sign.
6. Magnesium burning in oxygen is written as: $$magnesium+oxygen\to magnesium\; oxide$$
7. A word equation names the substances but shows nothing about the numbers of atoms involved.

Balanced symbol equations: change the coefficients, never the formulae

1. A symbol equation shows the same reaction using the formulae of the substances.
2. An equation is balanced when each element has the same number of atoms on both sides of the arrow.
3. Balancing matters because atoms are only rearranged in a reaction, so none are created or destroyed.
4. Balance by changing coefficients only, because changing a subscript changes the substance itself, turning water $H_{2}O$ into hydrogen peroxide $H_2{O}_2$.
5. Write the correct formulae first, count each element on both sides, adjust the coefficients, then count again.
6. Balance an element that appears in one formula on each side before one that appears in several formulae.

State symbols: showing the physical state of every substance

1. A state symbol is written straight after a formula to show that substance's physical state.
2. $(s)$ means solid, $(l)$ means liquid and $(g)$ means gas.
3. $(aq)$ means aqueous, so the substance is dissolved in water.
4. The state symbol goes immediately after the formula, with any coefficient in front of the whole formula.
5. Magnesium burning in oxygen, written with state symbols, is: $$2Mg(s)+O_2(g)\to 2MgO(s)$$
6. Hydrochloric acid neutralising sodium hydroxide, written with state symbols, is: $$HCl(aq)+NaOH(aq)\to NaCl(aq)+H_{2}O(l)$$
7. Water made in a reaction is $H_{2}O(l)$ rather than $H_{2}O(aq)$, because the water is the liquid itself and not something dissolved in water.

1.1.2 Balanced ionic equations

Ionic equations: show only the particles that change

1. Ionic equations are written for reactions in solution, where the ions of a dissolved ionic compound are free to move apart.
2. A balanced ionic equation conserves every element and has the same total charge on each side.
3. A coefficient in front of a formula multiplies the particles, and it multiplies the charge they carry as well.
4. Balance an ionic equation using coefficients, never by changing a subscript or a charge.
5. Cancelling the spectator ions leaves a short equation that holds for every reaction of that type, whichever soluble compounds were mixed.

Writing an ionic equation: split the aqueous substances, then cancel

1. Start from a balanced symbol equation with a state symbol on every substance.
2. Split each aqueous ionic substance into its separate ions, keeping each ion's charge.
3. Keep solids, liquids, gases and precipitates as complete formulae, because their particles are not free to move apart.
4. Cancel any ion that appears unchanged on both sides of the arrow.
5. Write out the particles that are left, then check the atoms and the total charge.

Common ionic equations: match the ions to the product they form

1. An acid produces $H^{+}$ ions in aqueous solution, and an alkali produces ${OH}^{-}$ ions.
2. In neutralisation, hydrogen ions and hydroxide ions join to make water: $$H^{+}(aq)+{OH}^{-}(aq)\to H_{2}O(l)$$
3. With a carbonate, hydrogen ions release carbon dioxide and water: $${CO}_3^{2-}(aq)+2H^{+}(aq)\to {CO}_2(g)+H_{2}O(l)$$
4. Two hydrogen ions are needed there because they balance the carbonate ion's $2-$ charge and supply the two hydrogen atoms in the water.
5. With a metal oxide, hydrogen ions release the metal ion into solution and form water: $$CuO(s)+2H^{+}(aq)\to {Cu}^{2+}(aq)+H_{2}O(l)$$
6. The copper oxide equation has one copper atom, one oxygen atom and two hydrogen atoms on each side, with a total charge of $+2$ on each side.

Checking an ionic equation: atoms and total charge both balance

1. Count each element on the left and the right once the spectator ions have been cancelled.
2. Add up the charges on each side, including any coefficients written in front of the ions.
3. Check that every state symbol still matches its substance, especially for solids, gases and water.
4. Use the smallest whole-number coefficients that balance both the atoms and the total charge.
5. An equation whose atoms balance but whose charges do not is still wrong, so check both every time.

1.1.3 Hazard symbols and risk in practical work

Hazard symbols: the label names the kind of danger

1. Read the symbol and the rest of the label before you open or use a substance.
2. The symbol tells you what could go wrong, so it points you towards precautions that keep the substance off your skin, away from flames and out of the drains.
3. The symbol names the type of danger but not the amount of danger, which depends on the concentration and on what you are doing with the substance.
4. Two substances can carry the same symbol and still need different precautions, so the full label and the practical instructions still apply.

The nine pictograms: what each one warns you about

1. Flammable, shown by a flame: the substance catches fire easily. Keep it away from naked flames, sparks and hot equipment.
2. Oxidising, shown by a flame over a circle: the substance makes other materials burn more readily. Keep it away from flammable materials and use only the quantity stated in the method.
3. Corrosive, shown by liquid damaging a hand and a metal surface: the substance can cause serious burns and attack materials. Wear eye protection, avoid skin contact and clear up spills using the stated procedure.
4. Toxic, shown by a skull and crossbones: a small exposure can cause serious illness or death. Prevent swallowing, inhalation and skin contact, and use a fume cupboard when the method says so.
5. Harmful or irritant, shown by an exclamation mark: the substance can irritate the skin, eyes or airways. Avoid direct contact and wear the eye protection or other personal protective equipment the method specifies.
6. Explosive, shown by an exploding object: the substance may explode if it is heated, struck or rubbed. Keep it away from heat and handle it only as instructed.
7. Gas under pressure, shown by a gas cylinder: the container holds compressed gas and may burst if heated. Keep the container secured and away from any heat source.
8. Serious health hazard, shown by a person with a star shape on the chest: the substance may cause serious or long-term damage to health. Avoid exposure and follow the special handling instructions on the label.
9. Environmental hazard, shown by a dead tree and fish: the substance may harm aquatic organisms. Prevent spills and dispose of the substance as instructed rather than pouring it down the sink.

Hazard and risk: the same chemical, a different risk in each procedure

1. The hazard belongs to the substance or the equipment, so it does not change when you change the method.
2. The risk does change with the method, because it depends on the concentration, the volume, the temperature and how long you are exposed.
3. Concentrated sulfuric acid in an open beaker is a far higher risk than the same acid sealed in a bottle on a shelf, although the hazard is identical.
4. A precaution reduces the risk, and it never removes the hazard.
5. Evaluating a risk means weighing how likely the harm is against how serious it would be.

Risk assessment: identify, judge, control, then check again

1. Identify the hazards: read every chemical label and look for hot equipment, sharp edges, electricity, pressure and any gas the reaction produces. Include the substances made during the practical, not only the ones you start with.
2. Work out how harm could happen: decide whether a substance could be swallowed, inhaled, splashed into the eyes or brought onto the skin.
3. Judge the likelihood: think about the concentration, the volume, the temperature, the length of exposure and the chance of a spill.
4. Judge the severity: think about the injury, illness, fire, explosion or environmental damage that could follow.
5. Choose control measures: each one should cut the likelihood of harm, its severity, or both.
6. Check the remaining risk: start the practical only once the risk that is left is low enough to work safely, and reassess if the method changes.

Risks in common chemistry practicals

1. Using acids and alkalis: these can irritate or burn the skin and eyes, depending on their concentration. Check the label, wear eye protection, use small volumes and rinse any splash as the method instructs.
2. Making crystals from an acid: the method brings together chemical splashes, hot solutions and hot glassware. Heat gently, keep the container stable, wear eye protection and let the apparatus cool before you move it.
3. Heating a substance in a test tube: hot glass burns, and the contents can spit from the open end. Use a test-tube holder, point the opening away from people and stand hot equipment on a heatproof mat.
4. Collecting a gas: acid can splash, connections can work loose, and pressure rises if the gas path is blocked. Clamp the apparatus, check that the syringe plunger moves freely and keep the gas path clear.
5. Carrying out electrolysis: liquids sit close to electrical connections, and the products can include flammable or toxic gases. Use the low-voltage supply stated, keep the connections dry and switch off before you adjust anything. Keep flames away if hydrogen may form, and work in a fume cupboard if the method warns that chlorine may be produced.
6. Using a chromatography solvent: some solvents are flammable or harmful, and their vapour can be inhaled. Use a small volume, replace the lid on the container and keep the solvent away from naked flames.

Answering a risk question: tie each precaution to a named hazard

1. Name the specific hazard, including the chemical, the equipment or the product that could cause harm.
2. Describe the accident that could happen and the harm it would do, such as acid splashing into an eye or solvent vapour catching fire.
3. Judge the risk from the likelihood of that accident and the severity of the harm together.
4. Give a precaution that acts on that risk, such as removing the flames, using a smaller volume or putting a barrier between your eyes and a splash.
5. Say what the precaution does, so a fume cupboard draws harmful vapour away before you can breathe it in.
6. Give more than one precaution when a procedure carries separate hazards, such as a corrosive acid and a flammable gas.
7. Personal protective equipment is one control measure among several, and it does not replace careful handling, secure apparatus or removing an ignition source.