Chemistry 8. Chemical Analysis

In chemistry, what is the scientific definition of a 'pure' substance?

A single element or compound that is not mixed with any other substance.

How can melting and boiling point data be used to identify a pure substance?

Pure substances melt and boil at specific, fixed temperatures.

How does the presence of an impurity typically affect the melting point of a substance?

It lowers the melting point and causes the substance to melt over a range of temperatures.

What effect do impurities usually have on the boiling point of a substance?

They increase the boiling point.

In everyday language, such as on a food label, what does the term 'pure' usually imply?

That nothing has been added to the substance and it is in its natural state.

What is a 'formulation' in the context of chemical products?

A mixture designed as a useful product by mixing components in carefully measured quantities.

Why are the components of a formulation added in precise, measured amounts?

To ensure the product has the specific required properties for its intended use.

Give three examples of commercial products that are classified as formulations.

Any three from: medicines, paints, fuels, cleaning agents, alloys, or fertilisers.

What is the purpose of chromatography?

To separate mixtures and provide information to help identify substances.

What are the two phases involved in every chromatographic process?

A stationary phase and a mobile phase.

In paper chromatography, what material serves as the stationary phase?

The chromatography paper.

In paper chromatography, what serves as the mobile phase?

The solvent (for example, water or ethanol).

Upon what factor does the separation of substances in chromatography depend?

The distribution of substances between the stationary and mobile phases.

How many spots will a pure compound produce on a chromatogram, regardless of the solvent used?

A single spot.

How is the $R_{f}$ value of a substance calculated?

$R_{f} = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}$

What is the numerical range for any $R_{f}$ value?

Between 0 and 1.

How can $R_{f}$ values be used to identify unknown compounds?

By comparing the $R_{f}$ value of the unknown to known reference values in the same solvent.

Why is the origin line in paper chromatography drawn in pencil rather than ink?

Pencil is insoluble and will not dissolve in the solvent and travel up the paper.

In chromatography, why must the solvent level be below the pencil start line?

To prevent the spots from dissolving into the solvent in the beaker rather than travelling up the paper.

What does a high $R_{f}$ value indicate about a substance's relative attraction to the mobile phase?

The substance has a higher solubility in the solvent and a weaker attraction to the paper.

How is the test for Hydrogen gas performed?

Hold a burning splint at the open end of a test tube containing the gas.

What is the positive result for the Hydrogen gas test?

The gas burns rapidly with a 'squeaky pop' sound.

How is the test for Oxygen gas performed?

Insert a glowing splint into a test tube containing the gas.

What is the positive result for the Oxygen gas test?

The glowing splint relights.

How is the test for Carbon Dioxide gas performed?

Bubble the gas through an aqueous solution of calcium hydroxide (limewater).

What is the positive result for the Carbon Dioxide gas test?

The limewater turns milky or cloudy.

How is the test for Chlorine gas performed?

Place damp litmus paper into the gas.

What is the positive result for the Chlorine gas test?

The litmus paper is bleached and turns white.

Which metal ion produces a crimson flame in a flame test?

Lithium ($Li^{+}$).

Which metal ion produces a yellow flame in a flame test?

Sodium ($Na^{+}$).

Which metal ion produces a lilac flame in a flame test?

Potassium ($K^{+}$).

Which metal ion produces an orange-red flame in a flame test?

Calcium ($Ca^{2+}$).

Which metal ion produces a green flame in a flame test?

Copper ($Cu^{2+}$).

Why might it be difficult to identify a metal ion using a flame test if the sample contains a mixture of ions?

Some flame colours can be masked by others.

Which three metal ions form white precipitates when sodium hydroxide solution ($NaOH$) is added?

Aluminium ($Al^{3+}$), Calcium ($Ca^{2+}$), and Magnesium ($Mg^{2+}$).

How can you distinguish between Aluminium, Calcium, and Magnesium ions after they all form white precipitates with $NaOH$?

Only the Aluminium hydroxide precipitate dissolves in excess sodium hydroxide solution.

What colour precipitate is formed when sodium hydroxide is added to a solution containing Copper (II) ions ($Cu^{2+}$)?

Blue.

What colour precipitate is formed when sodium hydroxide is added to a solution containing Iron (II) ions ($Fe^{2+}$)?

Green.

What colour precipitate is formed when sodium hydroxide is added to a solution containing Iron (III) ions ($Fe^{3+}$)?

Brown.

How do you test for Carbonate ions ($CO_{3}^{2-}$)?

Add a dilute acid to the substance.

What is the positive result for a Carbonate ion test?

Fizzing or effervescence due to the production of Carbon Dioxide gas.

Which reagents are used to test for Halide ions in solution?

Dilute nitric acid ($HNO{3}$) followed by silver nitrate solution ($AgNO{3}$).

What colour precipitate is formed in a positive test for Chloride ions ($Cl^{-}$)?

White.

What colour precipitate is formed in a positive test for Bromide ions ($Br^{-}$)?

Cream.

What colour precipitate is formed in a positive test for Iodide ions ($I^{-}$)?

Yellow.

Why is nitric acid added before silver nitrate when testing for Halide ions?

To react with and remove any carbonate ions that would produce a false-positive white precipitate.

Which reagents are used to test for Sulfate ions ($SO_{4}^{2-}$)?

Dilute hydrochloric acid ($HCl$) followed by barium chloride solution ($BaCl_{2}$).

What is the positive result for a Sulfate ion test?

The formation of a white precipitate (barium sulfate).

State three advantages of using instrumental methods for chemical analysis rather than manual chemical tests.

Instrumental methods are faster, more sensitive, and more accurate.

How does Flame Emission Spectroscopy work?

A sample is placed in a flame, and the emitted light is passed through a spectroscope to produce a line spectrum.

What two pieces of information can be determined using Flame Emission Spectroscopy?

The identity of metal ions and their concentrations in a solution.

What defines the 'solvent front' in chromatography?

The furthest point reached by the solvent as it travels up the paper.

In the context of chromatography, what determines the $R_{f}$ value of a particular substance?

The relative solubility of the substance in the solvent versus its attraction to the stationary phase.

Why should you avoid using Hydrochloric acid when acidified silver nitrate is used to test for Halides?

Hydrochloric acid contains chloride ions, which would create a false-positive white precipitate.

What is the chemical formula for the white precipitate formed in a positive Sulfate test?

$BaSO_{4}$ (Barium sulfate).

Which ion is being tested for when a crimson flame is observed?

Lithium ($Li^{+}$).

Which specific metal ion forms a precipitate that dissolves in excess sodium hydroxide to form a colourless solution?

Aluminium ($Al^{3+}$).

If a sample produces a lilac flame and a white precipitate with Barium Chloride, what is the compound?

Potassium sulfate ($K{2}SO{4}$).

What does it mean if an instrumental method is 'sensitive'?

It can detect very small amounts or low concentrations of a substance.

Why is it important to use a 'clean' nichrome wire handle for flame tests?

To prevent contaminants (like sodium) from masking the true flame colour of the sample.

How should distances be measured on a chromatogram to ensure accuracy?

From the pencil start line to the centre of the coloured spot.

What does a line spectrum from Flame Emission Spectroscopy represent?

A unique 'fingerprint' for a specific metal ion.

When describing the Carbon Dioxide test, what precise language should be used for the result?

The limewater turns 'cloudy' or 'milky'.

Which ions are present in a solution that produces a green flame and a yellow precipitate with silver nitrate?

Copper ($Cu^{2+}$) and Iodide ($I^{-}$).

Which ions are present in a solution that produces a brown precipitate with $NaOH$ and bubbles with dilute acid?

Iron (III) ($Fe^{3+}$) and Carbonate ($CO_{3}^{2-}$).

In chromatography, what happens if the chromatography paper touches the sides of the beaker?

Capillary rise at the edges will distort the solvent front.

Why is distilled water used in ion testing rather than tap water?

Tap water contains dissolved ions that could interfere with the results of the tests.

What is the primary risk associated with the test for Chlorine gas?

Chlorine gas is toxic and should be handled in a well-ventilated area.

Which metal ion is likely present if a solution forms a white precipitate with $NaOH$ that remains after adding excess $NaOH$?

Either Calcium ($Ca^{2+}$) or Magnesium ($Mg^{2+}$).

How can you identify the presence of sulfate ions in potash alum ($KAl(SO{4}){2}$)?

Add dilute $HCl$ and barium chloride solution to observe a white precipitate.

In the halide test, what is the chemical name of the white precipitate formed by chloride ions?

Silver chloride ($AgCl$).

Which instrumental method can show a line spectrum to identify metal ions?

Flame Emission Spectroscopy.

What is the purpose of adding dilute hydrochloric acid before barium chloride in the sulfate test?

To remove any carbonate ions that would otherwise form a white precipitate with barium.

Which two metal cations produce coloured precipitates with sodium hydroxide?

Copper (II) and Iron (II or III).

What property of a substance does its $R_{f}$ value depend on?

Its relative affinity for the stationary phase versus the mobile phase.

Why is it important to mark the 'solvent front' immediately after removing chromatography paper from the solvent?

The solvent front will disappear as the paper dries, making it impossible to calculate $R_{f}$ later.

A student finds an $R_{f}$ value of 1.2. Why must this result be incorrect?

The $R_{f}$ value cannot exceed 1 as the solute cannot travel further than the solvent.

Which metal ion produces a green flame but a blue precipitate with sodium hydroxide?

Copper (II) ($Cu^{2+}$).

What is the common name for an aqueous solution of calcium hydroxide used to test for $CO_{2}$?

Limewater.

In chromatography, what determines how far a substance travels up the paper?

Its solubility in the mobile phase and its attraction to the stationary phase.

How do you confirm the presence of $Al^{3+}$ specifically among the three ions that form white precipitates with $NaOH$?

Keep adding sodium hydroxide; the Aluminium hydroxide precipitate will redissolve.

In a mixture of ions, why might Sodium ions mask the presence of Potassium ions in a flame test?

The intense yellow flame of Sodium is much brighter than the lilac flame of Potassium.

What is the benefit of using a capillary tube to place spots on chromatography paper?

It allows for the creation of small, concentrated, and reproducible spots.

What does it mean for an instrumental method to be 'accurate'?

The values it provides are very close to the true value.

Which non-metal ion is identified by the formation of a cream precipitate with silver nitrate?

Bromide ($Br^{-}$).