Thermal Decomposition of Salts
General Concept: Thermal decomposition is a chemical process where many salts break apart into simpler substances when they are heated to high temperatures. The exact substances that are produced after heating depend mainly on the type of negative ion (anion) that was originally part of the salt.
TitleThermal Decomposition of Carbonates
General Reaction: When metal carbonates are heated strongly, they undergo a chemical change called decomposition, where they break down into two products: a metal oxide and carbon dioxide gas. This reaction is a common example of thermal decomposition studied in chemistry.
Examples of Carbonate Decomposition:
- When calcium carbonate (CaCO₃) is heated, it breaks down into calcium oxide (CaO), a white solid, and carbon dioxide gas (CO₂).
- When zinc carbonate (ZnCO₃) is heated, it changes into zinc oxide (ZnO) and releases carbon dioxide gas (CO₂).
- Copper(II) carbonate (CuCO₃) decomposes when heated to form copper(II) oxide (CuO) and carbon dioxide gas (CO₂).
- Lead(II) carbonate (PbCO₃) breaks apart into lead(II) oxide (PbO) and carbon dioxide gas (CO₂) upon heating.
Stable Carbonates: Some carbonates, like sodium carbonate (Na₂CO₃) and potassium carbonate (K₂CO₃), are very stable. This means that they do not decompose easily and need extremely high temperatures before they start breaking down.
Observations: During thermal decomposition, many carbonates show a change in colour as they are transformed into metal oxides. Also, the production of carbon dioxide gas can be confirmed by bubbling the gas through limewater; the limewater will turn cloudy because of the formation of calcium carbonate.
Thermal Decomposition of Nitrates
General Reaction: Most metal nitrates, when heated, decompose into three products: a metal oxide, nitrogen dioxide gas, and oxygen gas.
Examples of Nitrate Decomposition:
- Lead(II) nitrate (2Pb(NO₃)₂) decomposes to form lead(II) oxide (2PbO), nitrogen dioxide gas (4NO₂), and oxygen gas (O₂).
- Copper(II) nitrate (2Cu(NO₃)₂) decomposes into copper(II) oxide (2CuO), nitrogen dioxide gas (4NO₂), and oxygen gas (O₂).
- Zinc nitrate (2Zn(NO₃)₂) breaks down into zinc oxide (2ZnO), nitrogen dioxide gas (4NO₂), and oxygen gas (O₂).
Exceptions: Some nitrates like sodium nitrate (NaNO₃) and potassium nitrate (KNO₃) behave differently. Instead of giving nitrogen dioxide gas, they decompose to form metal nitrites (NaNO₂ and KNO₂) and oxygen gas.
Observations: During decomposition, nitrogen dioxide gas is released, which is easy to spot because it has a reddish-brown colour. Oxygen gas is also produced and can be tested by inserting a glowing splint into the gas — if the splint relights, it proves that oxygen is present.
Thermal Decomposition of Sulfates
General Reaction: Some metal sulfates, when heated, decompose into a metal oxide and sulfur trioxide gas. This kind of reaction usually needs a strong heating source.
Examples of Sulfate Decomposition:
- Copper(II) sulfate (CuSO₄) decomposes to produce copper(II) oxide (CuO) and sulfur trioxide gas (SO₃).
- Iron(II) sulfate (2FeSO₄) decomposes into iron(III) oxide (Fe₂O₃), sulfur dioxide gas (SO₂), and sulfur trioxide gas (SO₃).
Observations: Sulfur trioxide gas is colourless and has a very strong, sharp smell that makes it easy to detect when working with heated sulfates in experiments.
Identifying Gases
Carbon Dioxide (CO₂): Carbon dioxide gas can be detected by bubbling it through limewater. If the gas is carbon dioxide, the limewater will turn cloudy because calcium carbonate will form inside it.
Oxygen (O₂): Oxygen gas can be identified using a simple test: a glowing splint is inserted into the gas, and if it bursts back into flames, oxygen is definitely present.
Nitrogen Dioxide (NO₂): Nitrogen dioxide can be recognized easily because it is a reddish-brown gas. Also, when it comes into contact with moist blue litmus paper, the paper turns red, showing that the gas is acidic.
Summary of Thermal Decomposition
Carbonates: When carbonates are heated, they break down to form a metal oxide and carbon dioxide gas.
Nitrates: When nitrates are heated, they decompose into a metal oxide, nitrogen dioxide gas, and oxygen gas.
Sulfates: When sulfates are heated, they break apart into a metal oxide and sulfur trioxide gas.
Qualitative Analysis
Purpose of Analysis: Qualitative analysis is the process scientists use to find out what types of ions are inside a salt. They do this by looking at the salt’s properties and how it reacts with other chemicals.
Thermal Decomposition Clues: When heating a salt, important clues such as the colour change of the residue and the types of gases produced help scientists figure out the identities of the ions in the salt.
Anion Tests:
- Carbonates react with acids to produce carbon dioxide gas.
- Chlorides react with silver nitrate to make a white solid called silver chloride.
- Sulfates react with barium chloride to make a white solid called barium sulfate.
- Nitrates, when tested with sulfuric acid and iron(II) sulfate, form a brown ring, proving the presence of nitrate ions.
Cation Tests:
- Metal cations react with sodium hydroxide (NaOH) or ammonia to form solids with specific colours called precipitates.
- Sometimes additional tests, like using iodide ions, are needed to confirm special cations like lead(II) ions (Pb²⁺).
Flow Diagram Usage: Scientists often use flow diagrams as a guide that shows them step-by-step which tests to do to identify unknown salts. By following these diagrams, they can logically figure out the salt’s identity by observing its reactions and appearances.