Questions and Answers
What is formed when a few drops of NH3(aq) are added to Cu2+?
Light blue precipitate
What happens when excess NH3(aq) is added to the light blue precipitate formed from Cu2+?
It dissolves to form a deep blue solution.
Which ion forms a white precipitate when NaOH(aq) is added?
The precipitate formed by Zn2+ can dissolve in excess NaOH.
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What color precipitate does Fe2+ form when NaOH(aq) is added?
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What is the observation for ammonium ion, NH4+, when tested with NaOH(aq)?
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What is observed when a carbonate ion (CO32-) is tested with dilute hydrochloric acid?
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What precipitate forms when sulfates (SO42-) are tested with barium nitrate solution?
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Oxygen can rekindle a glowing splint.
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What test is used to identify ammonia gas?
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What color change indicates the presence of water when cobalt(II) chloride is tested?
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What is a salt?
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Which of the following ions make salts soluble in water? (Select all that apply)
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All carbonates are soluble.
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What is the product of reacting hydrochloric acid and magnesium oxide?
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Water is present in the crystals of certain compounds, giving a compound its crystalline properties. This is known as ______.
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What is the example of a hydrated salt mentioned?
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Barium sulfate is soluble in water.
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Which method is suitable for preparing soluble salts when starting materials are both soluble? (Select the correct answer)
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What ion is liberated when an acid reacts with a metal?
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The process used by a chemist to identify the cations and anions in an unknown solution is known as ______.
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Which of the following results indicates that a cation can be identified? (Select all that apply)
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What precipitate is formed from a solution containing Cu2+ ions when NaOH is added?
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Study Notes
Salts Overview
- Salts are compounds formed when hydrogen ions in acids are replaced by metallic ions or ammonium ions, e.g., Na⁺, K⁺, Mg²⁺.
- Salts consist of two parts:
- Cation (metallic or ammonium ion).
- Anion (derived from the acid, such as Cl⁻, NO₃⁻, SO₄²⁻).
Reactions Producing Salts
- Salts can be created through reactions between acids and:
- Insoluble metals (e.g., ZnSO₄ from Zn(OH)₂ and H₂SO₄).
- Insoluble bases (e.g., NaCl from NaOH and HCl).
- Insoluble carbonates (e.g., MgCl₂ from HCl and MgCO₃).
Water of Crystallization
- Some salts contain water molecules (hydrated salts) that contribute to their crystalline structure.
- Heating can remove this water, transitioning from hydrated salt to anhydrous salt.
Solubility of Salts
- All salts containing Na⁺, K⁺, and NH₄⁺ are soluble in water.
- All nitrates are soluble; important examples include Pb(NO₃)₂ and AgNO₃.
- Chlorides are generally soluble except for AgCl and PbCl₂.
- Sulfates are soluble except for BaSO₄, CaSO₄, and PbSO₄.
- Most carbonates are insoluble, with exceptions for Na₂CO₃, K₂CO₃, and (NH₄)₂CO₃.
Methods for Preparing Salts
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Method 1: Acid with Insoluble Substances
- Suitable for preparing soluble salts using acids with insoluble metals, bases, or carbonates.
- Use an excess of insoluble reactants to ensure complete reaction and filtration.
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Method 2: Acid with Soluble Base (Titration)
- Suitable for sodium, potassium, and ammonium salts; requires precise measurement using titration.
- An indicator is used to identify the endpoint of the reaction.
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Method 3: Precipitation Method
- Involves mixing two solutions to form an insoluble salt as a precipitate.
- Example: Barium sulfate preparation using Ba(NO₃)₂ and Na₂SO₄.
Qualitative Analysis
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A method for identifying cations and anions in unknown solutions.
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Identifying Cations:
- Add NaOH or NH₃(aq); precipitates indicate the presence of specific cations.
- Precipitate colors and solubility in excess reagents help determine cation identity.
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Example Observations:
- Cu²⁺ forms a light blue precipitate with NaOH, which dissolves in excess NH₃ to form a deep blue solution.
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Summary of Cation Tests with NaOH:
- Zn²⁺, Pb²⁺, and Al³⁺ produce white precipitates that dissolve in excess to form colorless solutions.
- Cu²⁺ yields a light blue precipitate, which is insoluble in excess.
Conclusion
- Understanding salts, their formation, and their solubility rules is crucial for studying various chemical reactions and processes in chemistry.### Identifying Cations
- Iron(II) ion (Fe2+): Forms a green precipitate, insoluble in excess.
- Iron(III) ion (Fe3+): Forms a reddish-brown precipitate, insoluble in excess.
- Ammonium ion (NH4+): No precipitate; ammonia gas is released upon heating, turning moist red litmus paper blue.
Tests with Aqueous Ammonia (NH3(aq))
- Zinc ion (Zn2+): Produces a white precipitate that dissolves in excess ammonia to form a colorless solution.
- Aluminium ion (Al3+): Forms a white precipitate, insoluble in excess.
- Lead(II) ion (Pb2+): Produces a white precipitate, insoluble in excess.
- Calcium ion (Ca2+): No precipitate observed.
Tests with Sodium Hydroxide (NaOH(aq))
- Copper(II) ion (Cu2+): Forms a light blue precipitate that dissolves in excess to create a deep blue solution.
- Iron(II) ion (Fe2+): Produces a green precipitate that is insoluble in excess.
- Iron(III) ion (Fe3+): Forms a reddish-brown precipitate, insoluble in excess.
Identifying Anions
- Carbonate ion (CO32–): Effervescence observed when dilute hydrochloric acid is added. The released gas forms a white precipitate with limewater.
- Nitrate ion (NO3–): Effervescence occurs when sodium hydroxide and aluminum foil are warmed; ammonia gas is produced, turning moist red litmus paper blue.
- Sulfate ion (SO42–): Addition of dilute nitric acid and barium nitrate yields a white precipitate of barium sulfate.
- Chloride ion (Cl–): Mixing dilute nitric acid with silver nitrate results in a white precipitate of silver chloride.
- Iodide ion (I–): Similar process with a yellow precipitate of silver iodide formed.
Identifying Gases
- Hydrogen (H2): Colorless, odorless gas. A lighted splint extinguishes with a 'pop' sound.
- Oxygen (O2): Colorless, odorless gas. Rekindles a glowing splint.
- Carbon Dioxide (CO2): Colorless, odorless. Bubbles through limewater to form and dissolve a white precipitate.
- Chlorine (Cl2): Greenish-yellow gas with a pungent odor. Turns moist blue litmus paper red, then bleaches it.
- Sulfur Dioxide (SO2): Colorless gas with a pungent smell. Acidified potassium manganate(VII) turns colorless when exposed to it.
- Ammonia (NH3): Colorless gas with a pungent smell. Turns moist red litmus paper blue.
Tests for the Presence of Water
- Water released when heating hydrated salts, e.g., CuSO4.7H2O yields CuSO4 and water.
- Anhydrous Cobalt(II) Chloride Test: Changes from blue to pink in the presence of water.
- Anhydrous Copper(II) Sulfate Test: Changes from white to blue when water is present. These tests confirm the presence but not the purity of water.
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Explore the essentials of salts in Chapter 12. This quiz will help you define what salts are, understand their solubility in water, and learn the basics of preparing and analyzing them. Test your knowledge on qualitative analysis and practical applications of salts.
