Experimental techniques and chemical analysis - Sciences - Co-ordinated (Double) (0654) - Cambridge IGCSE

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🔬 C12 Experimental Techniques and Chemical Analysis 🧪

Welcome to the laboratory! This chapter is your essential guide to the tools and methods chemists use every day. Think of it as learning the standard operating procedures for scientific "detective work." You will learn how to accurately measure substances, separate complex mixtures, and identify unknown chemicals. Mastering these techniques is crucial, not just for practical exams, but for understanding the foundation of all chemical investigation!

C12.1 Experimental Design: Measuring and Understanding Mixtures

Accurate results depend on using the right tool for the job. Here are the key pieces of apparatus you must know for measuring time, temperature, mass, and volume:

Essential Measuring Apparatus

Time: Use stop-watches or digital timers. When measuring short intervals, we often measure many repeats (a multiple) and then calculate an average time to increase accuracy.
Temperature: Use thermometers. (Ensure you know whether to read a mercury or alcohol thermometer at eye level).
Mass: Use balances (digital or mechanical).
Volume (High Accuracy):
- Burettes: Used for dispensing variable, precise volumes of liquid (crucial for titrations).
- Volumetric Pipettes: Used for measuring and transferring a single, highly accurate fixed volume (e.g., 25.0 cm³).
Volume (Lower Accuracy / Larger Volumes):
- Measuring Cylinders: Used for measuring approximate volumes quickly.
- Gas Syringes: Used for collecting and measuring the volume of gases produced in a reaction.

Quick Tip: Always read the volume of a liquid from the bottom of the meniscus (the curved surface) at eye level to prevent parallax error!

Key Definitions for Solutions and Separation

Understanding how mixtures are formed is the first step to separating them:

Solvent: The substance that dissolves the solute (e.g., water is the universal solvent).
Solute: The substance that dissolves in the solvent (e.g., sugar or salt).
Solution: The mixture formed when one or more solutes are dissolved in a solvent (e.g., salt water).
Saturated Solution: A solution containing the maximum concentration of solute dissolved in the solvent at a specified temperature. If you add any more solute, it just settles at the bottom!

When separating mixtures, we get two key products:

Residue: The solid substance that remains behind after a process like evaporation, distillation, or filtration.
Filtrate: The liquid or solution that has successfully passed through a filter (like coffee passing through filter paper).

C12.4 Separation and Purification Techniques

Separation techniques exploit differences in physical properties (like boiling point or solubility) to isolate components of a mixture.

1. Separating Soluble Solids from Solvents

If you have salt dissolved in water, you use techniques that remove the solvent:

Evaporation (Simple): Used to recover the solute only (the solid). The solvent is lost to the atmosphere (e.g., leaving sea salt behind).
Crystallisation: Used to recover the pure solid in crystal form.
1. Heat the solution to evaporate most of the solvent, creating a hot saturated solution.
2. Allow the solution to cool slowly. The solubility decreases, and the solute forms crystals.
3. Filter the crystals and dry them (e.g., between filter paper or in a low-temperature oven).

2. Separating Insoluble Solids from Liquids

Filtration: Used to separate a solid (residue) from a liquid (filtrate). The mixture is poured through filter paper set in a funnel. (E.g., filtering sand out of water).
Using a Suitable Solvent: Used first in the case of separating two solids (e.g., salt and sand). Add a solvent (like water) that dissolves only one solid (the salt), then filter the insoluble solid (the sand).

3. Separating Miscible Liquids (Distillation)

These methods separate liquids based on differences in their boiling points. The substance with the lower boiling point boils first, turns into gas, and is then cooled back into a liquid (condensed) and collected.

Simple Distillation: Used to separate a solvent (liquid) from a soluble solute (solid), and we want to recover the pure solvent. (E.g., getting pure water from salt water).
Fractional Distillation: Used to separate two or more miscible liquids with different boiling points (e.g., ethanol and water, or components of petroleum). A fractionating column helps ensure better separation.

4. Assessing Purity

How do we know if our separation worked perfectly? We check the substance's physical properties:

A pure substance melts or boils at a specific, fixed temperature.
An impure substance (mixture) will melt over a range of temperatures (a lower range than the pure substance) and boil over a range of temperatures (a higher range than the pure substance).

Key Takeaway: Separation techniques rely on using different physical properties—solubility for filtration/crystallisation, and boiling points for distillation—to recover pure components.

C12.3 Chromatography

Chromatography is a powerful technique used to separate mixtures of soluble coloured substances (like inks or dyes) using a suitable solvent.

How Paper Chromatography Works (The Race Analogy)

Imagine a race where different runners (the coloured substances) are moving across a muddy field (the paper) while being pushed by a stream (the solvent).

A spot of the mixture is placed on the baseline of special paper (stationary phase).
The paper is dipped into a suitable solvent (mobile phase).
The solvent moves up the paper, carrying the substances in the mixture with it.
Different substances travel at different speeds depending on their solubility in the solvent and their attraction to the paper.
The mixture separates into different coloured spots (a chromatogram).

Interpreting Chromatograms

Identifying Unknowns: By running known samples alongside the unknown mixture, we can identify components. If a spot from the unknown travels the exact same distance as a spot from a known substance, they are likely the same substance.
Purity: A pure substance shows only one spot on the chromatogram (assuming it is coloured). An impure substance (mixture) shows multiple spots.

Supplement: The R_f Value

The R_f (retardation factor) value is used to precisely identify a substance, regardless of how far the solvent front moves, as long as the same conditions (solvent, paper, temperature) are used.

$$R_f = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent}}$$

The R_f value is always less than 1.0. This calculated value can be compared to known R_f values to confirm the identity of a substance.

Did you know? Paper chromatography is used in forensic science to analyse trace amounts of dyes in clothing or ink in ransom notes!

C12.2 Acid-Base Titrations

A titration is a quantitative technique used to find the exact volume of a solution of known concentration (often an acid or an alkali) that exactly reacts with a solution of unknown concentration.

The Process of Titration

Titration involves a neutralisation reaction between an acid and an alkali:

A volumetric pipette is used to accurately measure a fixed volume of the unknown solution (or the known solution) into a conical flask.
A few drops of a suitable indicator (like methyl orange or phenolphthalein) are added to the flask.
A burette is filled with the other solution (the titrant) and the initial reading is recorded.
The titrant is slowly added from the burette to the solution in the flask, swirling constantly.
The addition continues until the end-point is reached—when the indicator just changes colour permanently.
The final burette reading is recorded. The difference between the initial and final readings is the volume of titrant used.

The End-Point: This is identified by the sharp, permanent colour change of the indicator. This colour change tells us that the acid and alkali have perfectly neutralised each other.

Quick Review: Pipette (measures exact volume into the flask). Burette (dispenses variable volume of solution). Indicator (shows the moment of neutralisation).

C12.5 Identification of Ions and Gases (Chemical Analysis)

This section involves learning specific chemical "spot tests" to identify unknown ions (cations and anions) and gases. Think of these as chemical fingerprints.

1. Testing for Anions (Negative Ions)

Anion	Test Procedure	Observation/Result
Carbonate ($\text{CO}_3^{2-}$)	Add dilute acid.	Fizzy effervescence (gas produced). Test the gas with limewater. Limewater turns cloudy (confirms $\text{CO}_2$).
Halides ($\text{Cl}^-$, $\text{Br}^-$, $\text{I}^-$)	Acidify with dilute nitric acid, then add aqueous silver nitrate solution.	$\text{Cl}^-$: White precipitate formed. $\text{Br}^-$: Cream precipitate formed. $\text{I}^-$: Yellow precipitate formed.
Sulfate ($\text{SO}_4^{2-}$)	Acidify with dilute nitric acid, then add aqueous barium nitrate solution.	White precipitate formed.
Nitrate ($\text{NO}_3^-$)	Warm the substance with aluminium foil and aqueous sodium hydroxide solution.	Ammonia gas produced (smell and damp red litmus turns blue).

Analogy: To test for Chloride, Bromide, or Iodide, think of a silver lining: you always add Silver Nitrate. The precipitate colour tells you which halogen is present.

2. Testing for Aqueous Cations (Positive Ions)

We typically use aqueous Sodium Hydroxide ($\text{NaOH}$) and aqueous Ammonia ($\text{NH}_3$) to differentiate metal cations based on the precipitate they form and whether this precipitate redissolves in excess reagent.

Cation	Aqueous NaOH	Aqueous $\text{NH}_3$
Ammonium ($\text{NH}_4^+$)	Ammonia gas produced on warming (damp red litmus turns blue).	N/A (no precipitate formed)
Calcium ($\text{Ca}^{2+}$)	White precipitate, insoluble in excess.	No precipitate or slight white precipitate.
Copper(II) ($\text{Cu}^{2+}$)	Light blue precipitate, insoluble in excess.	Light blue precipitate, soluble in excess to give deep blue solution.
Iron(II) ($\text{Fe}^{2+}$)	Green precipitate, insoluble in excess.	Green precipitate, insoluble in excess.
Iron(III) ($\text{Fe}^{3+}$)	Red-brown precipitate, insoluble in excess.	Red-brown precipitate, insoluble in excess.
Zinc ($\text{Zn}^{2+}$)	White precipitate, soluble in excess.	White precipitate, soluble in excess.

Mnemonics for Precipitates:

Fe(II) is Fresh, so it's Green. ($\text{Fe}^{2+}$ = green ppt.)
Fe(III) is Fire, so it's Red-Brown. ($\text{Fe}^{3+}$ = red-brown ppt.)

3. Flame Tests for Cations (Metal Ions)

These tests use a clean wire dipped in the solid or concentrated solution, then held in a hot Bunsen flame. The metal ions emit specific, vibrant colours.

Cation	Flame Colour
Lithium ($\text{Li}^+$)	Red
Sodium ($\text{Na}^+$)	Yellow/Orange
Potassium ($\text{K}^+$)	Lilac
Copper(II) ($\text{Cu}^{2+}$)	Blue-green

Memory Aid: Lithium is Red like a Ferrari. Sodium is Orange like a streetlamp. Potassium is Lilac like a soft flower.

4. Testing for Gases

Gas	Test	Observation/Result
Ammonia ($\text{NH}_3$)	Hold damp red litmus paper near the gas.	Paper turns blue (ammonia is alkaline).
Carbon Dioxide ($\text{CO}_2$)	Bubble the gas through limewater (aqueous calcium hydroxide).	Limewater turns cloudy/milky.
Chlorine ($\text{Cl}_2$)	Hold damp litmus paper (usually blue or red) near the gas.	Paper is bleached white (chlorine is acidic and a bleaching agent).
Hydrogen ($\text{H}_2$)	Hold a lighted splint near the gas.	A "pop" sound is heard (explosive reaction).
Oxygen ($\text{O}_2$)	Hold a glowing splint near the gas.	The splint relights (oxygen supports combustion).

Key Takeaway: Identifying unknown chemicals requires precise, standardized tests using specific reagents and observing characteristic changes like colour shifts, precipitate formation, or gas production.