Topic 8 Flashcards

Purity & Formulations

Purity & Formulations: read all 4 questions and answers

1. In chemistry, what does 'pure' mean?
   A pure substance contains only ONE type of element or compound. This is different from everyday language where 'pure' means 'nothing harmful added' (e.g. Pure orange juice is actually a mixture).
2. How can you tell if a substance is pure using melting point?
   A PURE substance has a sharp, fixed melting point at an exact temperature. An IMPURE substance melts over a RANGE of temperatures and has a LOWER melting point than the pure substance.
3. What is a formulation?
   A mixture designed as a useful product. Each component is present in a MEASURED QUANTITY and contributes to the properties of the product. Examples: paints, medicines, fuels, alloys, cleaning agents.
4. Give the components of paint as a formulation.
   Pigment (colour), binder (holds pigment to the surface), solvent (makes it spreadable - evaporates when dry). Each is in a carefully controlled proportion.

Chromatography & R_f Values

Chromatography & Rf Values: read all 6 questions and answers

1. What is chromatography used for?
   Separating and identifying dissolved substances in a mixture. It works because different substances have different attractions to the MOBILE PHASE (solvent) and the STATIONARY PHASE (paper).
2. Describe how to carry out paper chromatography.
   1. Draw a pencil line near the bottom. 2. Place spots of samples on the line. 3. Stand paper in solvent (below the pencil line). 4. Wait - the solvent rises, carrying substances different distances.
3. Why must the pencil line be drawn in pencil, not pen?
   Pencil is INSOLUBLE - it won't dissolve in the solvent and move up the paper. Pen ink would dissolve and spread, contaminating the results.
4. Why must the solvent level be BELOW the pencil line?
   If the solvent is above the line, the sample spots would dissolve directly into the solvent rather than being carried up the paper by the mobile phase.
5. What is the Rf value formula?
   Rf = distance moved by substance ÷ distance moved by solvent front. Rf values are always between 0 and 1. Comparing Rf values under identical conditions helps identify substances, though different substances may share similar Rf values.
6. How does chromatography show if a substance is pure or a mixture?
   A PURE substance produces only ONE spot. A MIXTURE produces MULTIPLE spots - one for each substance in the mixture.

Flame Tests (Chemistry Only)

Flame Tests: read all 7 questions and answers

1. What colour does lithium (Li⁺) produce in a flame test?
   CRIMSON red.
2. What colour does sodium (Na⁺) produce in a flame test?
   YELLOW.
3. What colour does potassium (K⁺) produce in a flame test?
   LILAC.
4. What colour does calcium (Ca²⁺) produce in a flame test?
   ORANGE-RED.
5. What colour does copper (Cu²⁺) produce in a flame test?
   GREEN.
6. Describe the flame test method.
   1. Clean a nichrome wire by dipping in HCl and holding in a blue Bunsen flame (repeat until no colour). 2. Dip the clean wire in the sample. 3. Hold in the flame and observe the colour.
7. Why must the wire be cleaned between tests?
   To avoid CONTAMINATION - traces of the previous sample would give misleading colours in the next test.

Metal Hydroxide Precipitates (Chemistry Only)

Metal Hydroxide Precipitates: read all 5 questions and answers

1. What reagent do you add to test for metal ions using precipitates?
   Sodium hydroxide (NaOH) solution. Add a few drops to the solution being tested and observe the colour of the precipitate formed.
2. What precipitate does Cu²⁺ form with NaOH?
   BLUE precipitate - Cu(OH)₂.
3. What precipitate does Fe²⁺ form with NaOH?
   GREEN precipitate - Fe(OH)₂.
4. What precipitate does Fe³⁺ form with NaOH?
   BROWN precipitate - Fe(OH)₃.
5. Ca²⁺, Mg²⁺ and Al³⁺ all give white precipitates. How do you tell them apart?
   Add EXCESS NaOH: if the white precipitate DISSOLVES, it's aluminium (Al(OH)₃ is amphoteric). For Ca²⁺ vs Mg²⁺, use a FLAME TEST - calcium gives an orange-red flame, magnesium gives no flame colour.

Testing for Negative Ions & Gases

Testing for Negative Ions & Gases: read all 5 questions and answers

1. How do you test for carbonate ions (CO₃²⁻)?
   Add DILUTE HCl. Carbonates fizz (effervesce) and produce CO₂. Confirm CO₂ by passing gas through LIMEWATER - it turns milky (cloudy). Equation: CO₃²⁻ + 2H⁺ → H₂O + CO₂.
2. How do you test for halide ions? What must you add first?
   Add DILUTE NITRIC ACID first (to remove interfering carbonate or sulfite ions that would form precipitates with silver), then add SILVER NITRATE (AgNO₃) solution.
3. What precipitate colours identify chloride, bromide, and iodide?
   Chloride (Cl⁻): WHITE precipitate (AgCl). Bromide (Br⁻): CREAM precipitate (AgBr). Iodide (I⁻): YELLOW precipitate (AgI).
4. How do you test for sulfate ions (SO₄²⁻)?
   Add DILUTE HCl first, then BARIUM CHLORIDE (BaCl₂) solution. A WHITE precipitate of barium sulfate (BaSO₄) confirms sulfates.
5. What are the four gas tests you need to know?
   H₂: burning splint → squeaky pop. O₂: glowing splint → relights. CO₂: limewater → turns milky. Cl₂: damp litmus paper → bleaches white.

Instrumental Methods (HT Chemistry Only)

Instrumental Methods: read all 4 questions and answers

1. What is flame emission spectroscopy?
   A sample is heated in a flame. The light emitted passes through a spectroscope which produces a LINE SPECTRUM. Each element gives a unique spectral pattern - like a fingerprint for identification.
2. What can flame emission spectroscopy measure besides identity?
   The CONCENTRATION of metal ions. The intensity of the emitted light is compared with standard reference samples of known concentration.
3. Give four advantages of instrumental methods over traditional tests.
   1. More SENSITIVE - detect tiny quantities. 2. FASTER - rapid results. 3. More ACCURATE - numerical data, not subjective. 4. Require SMALLER samples.
4. What is the main disadvantage of instrumental methods?
   They are EXPENSIVE to buy and maintain, and require SPECIALIST TRAINING to operate and interpret the results.