6.10 - Effect of Heat on Salts

Gas Tests

Gas	Confirmatory observation
${\mathrm{O}}_2$	Rekindles a glowing wooden splinter
${\mathrm{H}}_2$	Burns with a pop near a lighted splinter
$\mathrm{C}{\mathrm{O}}_2$	Turns limewater cloudy
$\mathrm{N}{\mathrm{H}}_3$	Turns moist red litmus blue; forms white fumes with concentrated $\mathrm{HCl}$
$\mathrm{C}{\mathrm{l}}_2$	Bleaches moist blue litmus after first turning it red
$\mathrm{HCl}$	Forms white fumes of $\mathrm{N}{\mathrm{H}}_4\mathrm{Cl}$ with concentrated ammonia
$\mathrm{S}{\mathrm{O}}_2$	Decolourises acidified $\mathrm{KMn}{\mathrm{O}}_4$; changes acidified ${\mathrm{K}}_2\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7$ from orange to green
$\mathrm{N}{\mathrm{O}}_2$	Brown acidic gas; turns moist blue litmus red

• Precaution: carry out gas-generation tests in a fume chamber.
• Litmus must be moist so a gas can dissolve and produce the ions responsible for acidic or alkaline properties.

Heating Carbonate Salts

Experiment 6.7 — aim: investigate the effect of heat on carbonate salts.

• Hypothesis tested: all carbonate salts decompose on heating to release carbon dioxide.

• Variables: carbonate type (manipulated), decomposition products (responding), two spatulas of salt (fixed).

• Apparatus: test tubes, boiling tube, holder, Bunsen burner and stopper with delivery tube.

• Procedure:

  1. Put two spatulas of salt in a dry boiling tube and note its colour.
  2. Connect the delivery tube with its other end in limewater.
  3. Heat strongly and observe limewater and residue colours while hot and after cooling.
  4. Repeat for each carbonate salt.

• Materials: two spatulas each of $\mathrm{N}{\mathrm{a}}_2\mathrm{C}{\mathrm{O}}_3$, $\mathrm{CaC}{\mathrm{O}}_3$, $\mathrm{ZnC}{\mathrm{O}}_3$, $\mathrm{PbC}{\mathrm{O}}_3$ and $\mathrm{CuC}{\mathrm{O}}_3$, and limewater.

• Most metal carbonates decompose to a metal oxide and $\mathrm{C}{\mathrm{O}}_2$:

$\mathrm{ZnC}{\mathrm{O}}_3(\mathrm{s})\stackrel{\mathrm{\Delta }}{\to }\mathrm{ZnO}(\mathrm{s})+\mathrm{C}{\mathrm{O}}_2(\mathrm{g})$

• Conclusion: sodium and potassium carbonates do not decompose under these conditions, so the hypothesis that all carbonates decompose is rejected.

Heating Nitrate and Other Salts

Experiment 6.8: Heat sodium, magnesium, zinc, lead(II) and copper(II) nitrates; test gases using moist blue litmus and a glowing splinter.

• Aim and apparatus: investigate nitrate heating using a boiling tube, test tube holder, Bunsen burner and spatula.
• Compare salt colour before heating, residue colour hot and cold, gas colour, and the gas’s effect on moist blue litmus and a glowing wooden splinter.
• Most nitrates form metal oxide, brown $\mathrm{N}{\mathrm{O}}_2$ and ${\mathrm{O}}_2$:

$2Pb(\mathrm{N}{\mathrm{O}}_3{)}_2(\mathrm{s})\stackrel{\mathrm{\Delta }}{\to }2PbO(\mathrm{s})+4N{\mathrm{O}}_2(\mathrm{g})+{\mathrm{O}}_2(\mathrm{g})$

• Only some sulphates and chlorides decompose on heating; examples include $\mathrm{N}{\mathrm{H}}_4\mathrm{Cl}$, $\mathrm{ZnS}{\mathrm{O}}_4$ and $\mathrm{FeS}{\mathrm{O}}_4$.

$\mathrm{N}{\mathrm{H}}_4\mathrm{Cl}(\mathrm{s})\stackrel{\mathrm{\Delta }}{\to }\mathrm{N}{\mathrm{H}}_3(\mathrm{g})+\mathrm{HCl}(\mathrm{g})$

$\mathrm{ZnS}{\mathrm{O}}_4(\mathrm{s})\stackrel{\mathrm{\Delta }}{\to }\mathrm{ZnO}(\mathrm{s})+\mathrm{S}{\mathrm{O}}_3(\mathrm{g})$

$2FeS{\mathrm{O}}_4(\mathrm{s})\stackrel{\mathrm{\Delta }}{\to }\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3(\mathrm{s})+\mathrm{S}{\mathrm{O}}_2(\mathrm{g})+\mathrm{S}{\mathrm{O}}_3(\mathrm{g})$

Observation after heating	Inference
$\mathrm{C}{\mathrm{O}}_2$ released	Carbonate ion may be present
$\mathrm{N}{\mathrm{O}}_2$ and ${\mathrm{O}}_2$ released	Nitrate ion may be present
$\mathrm{N}{\mathrm{H}}_3$ released	Ammonium ion may be present
Residue yellow hot, white cold	$\mathrm{ZnO}$; $\mathrm{Z}{\mathrm{n}}^{2+}$ may be present
Residue brown hot, yellow cold	$\mathrm{PbO}$; $\mathrm{P}{\mathrm{b}}^{2+}$ may be present
Residue black hot and cold	$\mathrm{CuO}$; $\mathrm{C}{\mathrm{u}}^{2+}$ may be present