To separate a mixture of iodine (I₂) and potassium iodide (KI), the student can use a technique called solvent extraction. This involves dissolving the mixture in a suitable solvent and then using selective solubility to separate the components.

Procedure:

1. Dissolve the mixture in hot water. Iodine is slightly soluble in hot water, while potassium iodide is very soluble.
2. Allow the hot water to cool. As the water cools, the solubility of iodine decreases.
3. The iodine will crystallize out of the solution as the water cools, forming purple crystals.
4. The potassium iodide will remain dissolved in the water.
5. The iodine crystals can be separated from the solution by filtration.
6. The iodine crystals can then be further purified by recrystallization from hot water.

Principle: The separation is based on the principle of difference in solubility. Iodine has a lower solubility in cold water than potassium iodide. When the hot solution cools, the solubility of iodine decreases significantly, causing it to precipitate out of the solution as solid crystals. Potassium iodide remains dissolved in the water because it is highly soluble. This difference in solubility allows for the physical separation of the two components of the mixture.

The student can distinguish between copper sulfate (a compound) and pure copper (an element) by observing their chemical properties. Copper sulfate is a compound formed from copper and sulfate ions. Pure copper is an element and is characterized by its metallic properties.

Here's a procedure the student could follow:

1. Reaction with dilute acid: Copper sulfate will react with dilute acid (e.g., hydrochloric acid, HCl) to produce a blue solution and copper ions. The reaction is: CuSO₄(aq) + 2HCl(aq) → CuCl₂(aq) + H₂SO₄(aq). Pure copper will not react with dilute acid. If the student adds dilute acid to the copper, nothing will happen.
2. Electrical Conductivity: Copper is an excellent conductor of electricity, while copper sulfate in solution is not. The electrical conductivity test will clearly differentiate between the two. A wire of pure copper will conduct electricity, while a solution of copper sulfate will not.
3. Colour: Copper sulfate is a blue solid, while pure copper is a reddish-brown solid. This is a simple visual distinction.

Therefore, the student can conclude that the blue, reactive substance is copper sulfate, and the reddish-brown, non-reactive substance is pure copper.

Element: An element is a pure substance that consists of only one type of atom. It cannot be broken down into simpler substances by chemical means. Examples include hydrogen (H), oxygen (O), and gold (Au). Elements are defined by their atomic number, which indicates the number of protons in the nucleus of an atom. They are separated using physical methods such as distillation or electrolysis, depending on their physical state and properties.

Compound: A compound is a pure substance formed when two or more different elements are chemically bonded together in a fixed ratio. The constituent elements are chemically combined, meaning they form new substances with different properties than the original elements. Water (H₂O) and carbon dioxide (CO₂) are examples of compounds. Compounds can only be separated into their constituent elements by chemical reactions. Common separation techniques include electrolysis, decomposition, and chemical reactions.

Mixture: A mixture is a combination of two or more substances that are physically combined but not chemically bonded. The components of a mixture retain their individual properties. Mixtures can be separated by physical methods such as filtration, evaporation, decantation, magnetism, and chromatography. The composition of a mixture can vary.

Here's a table summarizing the differences: