Preparation of Soluble Salts
General Preparation: Soluble salts are made by carrying out chemical reactions that produce a salt able to dissolve easily in water. This means that after the reaction, the salt will mix with water to form a clear solution without any solid left.
Neutralisation Reaction: One common way to make soluble salts is through a neutralisation reaction. In this process, an acid reacts with an alkali (a soluble base) to produce a salt and water. For example, hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH) to make sodium chloride (NaCl) and water (H₂O).
Examples of Neutralisation: Another example is sulphuric acid (H₂SO₄) reacting with potassium hydroxide (KOH). This reaction produces potassium sulphate (K₂SO₄) and water.
Acid with Reactive Metal: Soluble salts can also be made by reacting acids with reactive metals. In this reaction, the metal reacts with the acid to form a salt and releases hydrogen gas. For example, hydrochloric acid (HCl) reacts with zinc (Zn) to form zinc chloride (ZnCl₂) and hydrogen gas (H₂).
Examples of Metal Reactions: Another example is sulphuric acid (H₂SO₄) reacting with magnesium (Mg) to form magnesium sulphate (MgSO₄) and hydrogen gas.
Acid with Metal Oxide: Acids can also react with metal oxides to make soluble salts. In these reactions, the products are a salt and water. For example, hydrochloric acid (HCl) reacts with copper(II) oxide (CuO) to form copper(II) chloride (CuCl₂) and water (H₂O).
Acid with Metal Carbonate: When acids react with metal carbonates, they produce a salt, water, and carbon dioxide gas. For instance, hydrochloric acid (HCl) reacting with calcium carbonate (CaCO₃) forms calcium chloride (CaCl₂), water (H₂O), and carbon dioxide gas (CO₂).
Preparation of Insoluble Salts
General Method: Insoluble salts are prepared through a method called precipitation, where two soluble salts react together and one of the products is an insoluble salt that falls out of solution.
Double Decomposition Reaction: This kind of reaction is called a double decomposition reaction. Two soluble salts swap parts to form a new insoluble salt and another soluble salt. For example, when barium chloride (BaCl₂) reacts with sodium sulphate (Na₂SO₄), they form barium sulphate (BaSO₄), which is insoluble, and sodium chloride (NaCl), which stays dissolved.
Example of Precipitation: Another example is lead(II) nitrate (Pb(NO₃)₂) reacting with sodium chloride (NaCl). This produces lead(II) chloride (PbCl₂), which is insoluble, and sodium nitrate (NaNO₃), which is soluble.
Mixing Solutions: To make an insoluble salt, you mix two solutions that contain the needed positive ion (cation) and negative ion (anion). When they meet, the desired insoluble salt forms and can be separated.
Example of Ion Mixing: For instance, mixing lead(II) nitrate (Pb(NO₃)₂) with potassium iodide (KI) will form yellow lead(II) iodide (PbI₂) as a solid and potassium nitrate (KNO₃) as a soluble product.
Crystallisation
Purpose of Crystallisation: Crystallisation is an important technique used to separate a solid salt from its solution. It also helps the salt to form beautiful, clearly shaped crystals that can be easily collected and studied.
Dissolving the Salt: First, the salt is completely dissolved in a suitable liquid, usually water, to create a solution. This ensures that all the salt particles are mixed evenly throughout the liquid.
Heating and Concentrating: The salt solution is then heated gently. This causes some of the water to evaporate, which makes the remaining solution richer in salt. The more water that evaporates, the more concentrated the solution becomes.
Cooling the Solution: Once enough water has evaporated and the solution is concentrated, it is allowed to cool down slowly. As the temperature drops, the salt cannot stay dissolved as easily, and it starts to come out of the solution, forming crystals.
Crystal Growth: If the solution cools slowly, the salt has enough time to arrange itself neatly into large, well-shaped crystals. Quick cooling usually results in small, uneven crystals, while slow cooling produces bigger and more perfect crystals.
Filtration: After crystals have formed, they are separated from the leftover liquid using filtration. During filtration, the crystals stay on the filter paper, and the remaining liquid (called the filtrate) passes through.
Washing the Crystals: To make sure the crystals are very pure, they are gently washed with a small amount of cold solvent, usually cold water. This helps to remove any leftover impurities that might be stuck to the crystals.
Drying the Crystals: Finally, the clean crystals are dried. This can be done by letting them air dry naturally or placing them carefully in a warm oven to remove any extra moisture.
Recrystallisation
Purpose of Recrystallisation: Recrystallisation is a special technique that is used when you have an impure salt and want to purify it. This method helps you get clean, pure crystals by dissolving and reforming them.
Process of Recrystallisation: In recrystallisation, the impure salt is first dissolved in a hot solvent. Then the solution is filtered to remove any undissolved dirt or other impurities. After filtering, the solution is allowed to cool slowly, so that pure salt crystals form, while the impurities remain dissolved in the liquid.
Solubility and Purification: Different substances have different solubilities at different temperatures. When the solution cools, the pure salt becomes less soluble and forms crystals, but most of the impurities stay dissolved. This difference in solubility is the key to making pure crystals.
Crystalline Structures
Crystal Shapes: When salts crystallise, they can form different crystal shapes, such as cubes, cuboids (rectangular box shapes), or rhombuses (diamond-like shapes). The shape depends on how the particles are arranged.
Factors Influencing Shape: The speed at which the solution cools and the type of salt being used greatly affect the final crystal shape. Slower cooling usually produces larger and better-shaped crystals.
Importance of Crystal Structure: Crystals have unique and beautiful shapes because their particles, such as ions, are arranged in a regular, repeating pattern called a lattice. This lattice structure is what gives crystals their neat edges and amazing geometrical designs.