Predicting and identifying reactions and products Revision Notes

Identifying Common Gases

Certain gases have specific tests that lead to visible changes. Hydrogen (H₂) gas is identified by holding a lit splint near it, producing a characteristic 'squeaky pop' sound. Oxygen (O₂) gas is tested with a glowing splint, which re-ignites in the presence of oxygen. Carbon dioxide (CO₂) gas turns limewater (calcium hydroxide solution) cloudy or milky when bubbled through it. Chlorine (Cl₂) gas is recognised by its ability to bleach damp blue litmus paper, turning it white. These simple tests are crucial for recognising products of various chemical reactions.

Identifying Positive Metal Ions (Cations)

Many common metal cations can be identified using sodium hydroxide solution. Copper(II) ions (Cu²⁺) form a blue precipitate of copper(II) hydroxide. Iron(II) ions (Fe²⁺) form a green precipitate of iron(II) hydroxide. Iron(III) ions (Fe³⁺) form a reddish-brown precipitate of iron(III) hydroxide. Aluminium ions (Al³⁺) produce a white precipitate of aluminium hydroxide, which uniquely redissolves in excess sodium hydroxide to form a colourless solution. Calcium ions (Ca²⁺) form a white precipitate of calcium hydroxide, but unlike aluminium hydroxide, this precipitate does not redissolve in excess sodium hydroxide solution. These colour changes and solubility characteristics are key for identification.

Identifying Negative Ions (Anions)

Carbonate ions (CO₃²⁻) are detected by adding a dilute acid, which causes fizzing due to the production of carbon dioxide gas. This gas is then bubbled through limewater, which turns cloudy. Sulfate ions (SO₄²⁻) are identified by first adding dilute hydrochloric acid (to remove any carbonate ions) then barium chloride solution, which forms a white precipitate of barium sulfate. Halide ions require adding dilute nitric acid (to remove carbonates) followed by silver nitrate solution. Chloride ions (Cl⁻) give a white precipitate of silver chloride, bromide ions (Br⁻) give a cream precipitate of silver bromide, and iodide ions (I⁻) give a yellow precipitate of silver iodide. These precipitation tests are very specific.

Identifying Water

Water can be identified using two common anhydrous tests. Anhydrous copper(II) sulfate is a white powder that turns blue when water is present. Anhydrous cobalt chloride paper is blue and turns pink in the presence of water. Both tests are based on the formation of hydrated salts, causing a distinct colour change. These methods are useful for confirming the presence of water in samples, especially when it's not visually obvious.

Testing for Cations with Sodium Hydroxide Solution

When sodium hydroxide solution is added to an unknown solution, different metal cations produce characteristic precipitates. Aluminium ions (Al³⁺) produce a white precipitate that redissolves in excess sodium hydroxide solution to form a colourless solution. Calcium ions (Ca²⁺) produce a white precipitate that does not redissolve in excess sodium hydroxide. Copper(II) ions (Cu²⁺) form a blue precipitate that does not redissolve in excess. Iron(II) ions (Fe²⁺) lead to a green precipitate that does not redissolve in excess. Iron(III) ions (Fe³⁺) form a red-brown precipitate that also does not redissolve in excess sodium hydroxide solution. These distinct observations allow for the identification of these specific metal cations.

Testing for Cations with Ammonia Solution

Ammonia solution can also be used to test for certain cations. When ammonia solution is added to a solution containing copper(II) ions (Cu²⁺), a blue precipitate forms, which then redissolves in excess ammonia solution to give a deep blue solution. This is a key distinguishing feature for copper. For iron(II) ions (Fe²⁺), a green precipitate forms that does not redissolve in excess ammonia. Iron(III) ions (Fe³⁺) produce a red-brown precipitate that also does not redissolve in excess. Aluminium ions (Al³⁺) form a white precipitate that does not redissolve in excess ammonia. These reactions complement the sodium hydroxide tests and help confirm the presence of ions.

Testing for Anions

Different tests are used for identifying common anions. To test for halide ions: add dilute nitric acid, followed by silver nitrate solution. Chloride ions (Cl⁻) give a white precipitate of silver chloride (AgCl). Bromide ions (Br⁻) give a cream precipitate of silver bromide (AgBr). Iodide ions (I⁻) give a yellow precipitate of silver iodide (AgI). For sulfate ions (SO₄²⁻): add dilute hydrochloric acid, followed by barium chloride solution. A white precipitate of barium sulfate (BaSO₄) forms. For carbonate ions (CO₃²⁻): add a dilute acid (e.g., hydrochloric acid). Effervescence (bubbling) indicates the production of carbon dioxide. To confirm carbon dioxide, bubble the gas through limewater (calcium hydroxide solution), which will turn cloudy if CO₂ is present.

PAG C5: Identifying Ions

PAG C5 involves carrying out practical tests to identify different ions. For cations, take small samples of the solution in test tubes. Add drops of sodium hydroxide solution to one sample, observing precipitate colour and solubility in excess. Repeat with ammonia solution for other samples. For anions, for halides, add dilute nitric acid then silver nitrate solution and observe precipitate colour. For sulfate, add dilute hydrochloric acid then barium chloride solution and observe for a white precipitate. For carbonate, add dilute acid and test any gas evolved with limewater. Safety precautions include wearing eye protection, handling reagents carefully, and disposing of chemical waste appropriately. Ensure results are recorded systematically.

General Physical Properties of Transition Metals

Transition metals are typically hard, strong, and have high densities and high melting points. These properties distinguish them from the Group 1 alkali metals, which are soft and have low melting points and densities. Iron, copper, and nickel are good examples; they are all tough, dense metals that require high temperatures to melt. They are also excellent conductors of heat and electricity, making them suitable for many industrial applications, such as electrical wiring (copper) and cooking utensils.

Chemical Properties: Coloured Compounds and Catalytic Activity

A key chemical characteristic of transition metals is their ability to form coloured compounds when in solution or solid state. For example, copper(II) compounds often appear blue, while iron(II) compounds are typically pale green and iron(III) compounds are reddish-brown. This colour variation is due to their electronic structure. Another important chemical property is their catalytic activity. Many transition metals and their compounds can act as catalysts, speeding up chemical reactions without being consumed. Iron powder is a catalyst in the Haber process (synthesis of ammonia), and nickel is used as a catalyst in the hydrogenation of vegetable oils.

Examples of Transition Metals and Their Uses

Iron is an abundant transition metal primarily used in the manufacture of steel, an alloy essential for construction, vehicles, and machinery due to its strength. Copper is valued for its high electrical conductivity, making it ideal for electrical wiring, and its resistance to corrosion, used in plumbing. Nickel is often used in alloys, such as stainless steel, to enhance corrosion resistance and strength. All three are good examples of the general properties described, being dense, having high melting points, and forming coloured compounds.

What are Flame Tests?

Flame tests are a method used in chemistry to detect the presence of certain metal ions. They are a qualitative test, meaning they tell you which metal ion is present, not how much. When metal ions are heated in a hot flame, their electrons absorb energy and jump to higher energy levels. These higher energy levels are unstable. As the electrons fall back to their original energy levels, they emit light of specific wavelengths, which we see as distinct colours. Each metal produces a unique colour, like a fingerprint, allowing us to identify it.

How to Perform a Flame Test

To conduct a flame test accurately, a clean nichrome wire loop is essential. First, dip the wire into concentrated hydrochloric acid to clean it and make it ready to pick up the sample. The acid helps convert the metal compound into its chloride, which is often more volatile, producing a brighter and more distinct flame colour. Then, dip the loop into the solid sample or a solution of the compound. Finally, hold the wire loop with the sample in the hot, non-luminous flame of a Bunsen burner. Observe and record the colour of the flame produced. It's important to clean the wire thoroughly between tests to prevent contamination, which could lead to incorrect results.

Characteristic Flame Colours of Common Metal Ions

Different metal ions produce characteristic colours in a flame test, which allows for their identification:

• Lithium (Li+): Crimson (red)
• Sodium (Na+): Orange-yellow
• Potassium (K+): Lilac (pale purple)
• Calcium (Ca2+): Orange-red (brick-red)
• Barium (Ba2+): Green (apple-green)
• Copper (Cu2+): Blue-green

It is important to be aware that the bright yellow flame of sodium can sometimes mask other colours, especially in mixtures. Observing through cobalt blue glass can help filter out the yellow sodium flame, making it easier to see potassium's lilac colour if sodium is also present.