Photographic emulsions are made of fine grains of silver bromide suspended in a thin layer of gelatin. When negative film is processed, the developer converts light-sensitive crystallites of silver bromide in the emulsion layers into metallic silver. At the atomic level, a photon hits the silver bromide crystal and frees an electron from the bromide ion. The liberated electron moves around the crystal, eventually lodging in the crystal lattice defects, turning silver ions into silver atoms. Slowly, as this process continues, tiny specks of elemental silver start to appear in the crystal lattice, darkening its surface from the inherent yellow color. A similar photochemical decomposition process occurs if the crystals are composed of silver chloride. A photo fixative solution containing thiosulfate ions removes undeveloped grains of silver bromide from the negative and creates a soluble silver thiosulfate complex, leaving a negative that retains the silver image after bathing. Referred to as negatives, the darkest areas (most elemental silver) of the developed film correspond to portions exposed to the most light. A positive print is made by exposing print paper also coated with a silver halide emulsion to light, which passes through the superimposed negative. The exposed print paper is then developed and fixed. Around 1900, before the advent of color photography, "silver prints" based on silver bromide constituted high quality black and white photography. In radiography, after exposure to X-rays, an image is developed by treating the emulsion film with a solution of alkali and metol (a developer), where the alkali penetrates the protective coating of gelatin and then the metol reduces the exposed silver bromide to black metallic oxide before it is fixed.
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Overview of Today’s Lesson What are Salts? Uses of Salts Methods of Preparing Salts
What is a Salt?Recap: Reactions of Acids✽ Acids + Metal ✽ Acids + Carbonate ✽ Acids + Alkalis ?????????
What is a Salt?Recap: Reactions of Acids✽ Acids + Metal ✽ Acids + Carbonate ✽ Acids + Alkalis Salt + H2 (g)2HCl + Zn ZnCl2 + H22HCl + CaCO3 CaCl2 + CO2 + H2OSalt + CO2 (g) + H2O (l)Salt + H2O(l)HCl + NaOH NaCl + H2O
What is a Salt? A salt is a compound formed when the hydrogen ofan acid is replaced by a metal. They are made when acids react with a metal, analkali or a metal carbonate.Mg + 2HCl → MgCl2 + H2acidmetalNaOH + HNO3 → NaNO3 + H2Oacidmetal
What is a Salt?Different acids form different salts.The salts of sulfuric acid are known assulfates.The salts of hydrochloric acid are known aschlorides.The salts of nitric acid are known as nitrates.
Some Acids and their Salts Hydrochloric acid HClSodium chlorideMagnesium chlorideCopper(II) chlorideAluminium chlorideNaClCuCl2AlCl3MgCl2
Some Acids and their Salts Sulfuric acid H2SO4Sodium sulfateCopper(II) sulfateAluminium sulfateNa2SO4CuSO4Al2(SO4 )3
Some Acids and their Salts Nitric acid HNO3Sodium nitratePotassium nitrateCopper(II) nitrateAluminium nitrateNaNO3Cu(NO3)2Al(NO3)3KNO3
Importance of Salts Salts are important to our bodies. They helpus maintain the concentration of our bodyfluids. We lose salt from our bodies through sweatand urine. Salt must be replaced through food anddrinks.
Uses of some salts1) Silver salts in photography eg. silver bromide (AgBr) A component on photographic film. A pale-yellow, insoluble, very sensitive to light. It turns black on exposure to light.
Uses of some salts2) Food preservatives eg. sodium sulphite, sodium nitrite, sodiumcitrate
Uses of some salts3) Food flavourings eg. sodium chloride (salt),monosodium glutamate (MSG)4) Medical uses eg. calcium sulfate in plaster of Paris
Uses of some salts5) Fertilisers in agriculture eg. ammonium sulfate6) Chemicals in industry eg. sodium chloride
Test your understanding1. Identify the acids needed to prepare the followingsalts.2. State the alkali that can react with the acid youmentioned in Question 1 to produce the three salts.SALTS ACID NEEDED ALKALIPotassiumnitrateSodiumsulfateAluminiumethanoate
Test your understanding1. Identify the acids needed to prepare the followingsalts.2. State the alkali that can react with the acid youmentioned in Question 1 to produce the three salts.SALTS ACID NEEDED ALKALIPotassiumnitrateNitric acidHNO3Potassium hydroxideKOHSodiumsulfateSulfuric acidH2SO4Sodium hydroxideNaOHAluminiumethanoateEthanoic acidCH3COOHAluminium hydroxideAl(OH)3
Methods of Preparing SaltsDepends on: Solubility of Salts in water; Solubility of the chemicals used toprepare the salts.
Insoluble MetalsInsoluble metals that react with acid-magnesium, zinc, iron, aluminium(Copper, silver and gold are insoluble metalsthat do not react with acid)
Solubility of Carbonates Soluble- carbonates of Group 1 elementse.g : sodium carbonate,potassium carbonate etcInsoluble – All other carbonatese.g : calcium carbonate,copper carbonatezinc carbonate etc
PREPARATION OF SALTMethod 1- Acid and Metal reaction
Acid + Metal → Salt + Hydrogen Salt: ZnSO4Zn (s) + H2SO4 (aq) → ???excessSubstances leftin the beaker?Sulfuricacid
Acid + Metal → Salt + Hydrogen Salt: ZnSO4Zn (s) + H2SO4 (aq) → ZnSO4 (aq) + H2(g)excessSubstances left in the beaker?Zn (s) , ZnSO4 solutionall used up escapes
How to remove the zinc from thezinc sulfate solution?●Excess zinc can be removed by filtration.●The zinc sulfate solution will be collectedas the filtrateZinc sulfatesolution
How to obtain the zinc sulfate saltfrom its solution? Carry out evaporation of the zinc sulfatesolution until it is saturated.(not to dryness as the salt might decompose ) Allow the saturated zinc sulfatesolution to cool so that the crystals ofzinc sulfate will appear. Filter to obtain the zinc sulfate crystalsand dry them between sheets of filterpaper.
Crystallization- involves evaporationand cooling processes.Solution of zincsulfateEvaporatingdishCrystals of zincsulfate
Reviewing the whole process ofpreparing zinc sulfate salt
Is this method suitable for preparing allsalts?Recall:Under the topic on Acids,which metals cannot be used to react withacids?• Metals that are too reactive, eg: Na, K and Ca• Metals that are unreactive, eg: Cu, Ag and AuAcid + Metal → Salt + Hydrogen
Method 2 Acid and insoluble carbonate reaction
Acid + Insoluble CarbonateCuCO3(s) + 2HCl(aq) → CuCl2(aq) + H2O (l) + CO2(g)Substances left in the beaker?CuCO3 (s) , CuCl2 solution• Salt : CuCl2escapesexcess all used up
copper(II) carbonatepowderresidue:CuCO3filter paperfiltrate:CuCl2solutionevaporatingdishcopper(II) chloridecrystalsfilterpaper142536PREPARATION OF SALTSCrystals ofcopper(II) chloridediluteHClaciddiluteHClacid
STEPS WHY?1) Warm / Heat some acid in a beaker. More metal/carbonate candissolve in the acid & tospeed up reaction.2) Add excess metal/carbonate to theacid.Stir until no more solid can dissolve.To ensure that all acid is usedup.3) Filter the mixture to remove theresidue and collect filtrate of saltsolution.To remove excessmetal/carbonate as residue.4) Heat the filtrate until half or one-third of the original volume.To obtain a saturatedsolution.5) Allow the solution to cool. For crystallization to takeplace.6) Filter to collect the crystals.Dry the crystals between filter paper.Filter to separate crystalsfrom solution.Dry to collect dry crystals.