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When we talk about solutions, simply stating "sugar in water" isn't enough to fully describe it. We often need to know how much sugar is actually present in that water. This brings us to the concept of concentration, which quantifies the amount of solute present in a given amount of solution.

Think back to Activity 2.2, where different groups prepared copper sulphate solutions with varying amounts of powder. Group A's solution appeared lighter in shade compared to Group B's. This visual difference directly relates to concentration. Group A's solution was dilute (less solute relative to solvent) compared to Group B's, which was more concentrated. "Dilute" and "concentrated" are comparative terms, indicating relative amounts of solute.

To gain a more precise understanding, we classify solutions based on their saturation level at a particular temperature:

• Saturated Solution: At any particular temperature, a solution that has dissolved as much solute as it is capable of dissolving is called a saturated solution. This means no more solute can be dissolved in it at that specific temperature. If you try to add more solute to a saturated solution, it will simply remain undissolved or precipitate out.
• Unsaturated Solution: If the amount of solute contained in a solution is less than the saturation level at a given temperature, it is called an unsaturated solution. In an unsaturated solution, more solute can still be dissolved.

The amount of solute present in a saturated solution at a specific temperature is known as its solubility. It's important to note that different substances have different solubilities in a given solvent at the same temperature. For example, in Activity 2.3, if you try dissolving salt in one beaker of water and sugar or barium chloride in another at the same temperature, you'll find that the amount that can be dissolved is often different. Also, increasing the temperature generally increases the solubility of most solid solutes in liquids. If you take a saturated solution at a certain temperature and cool it slowly, you might observe that some of the dissolved solute will crystallize out, as its solubility decreases at lower temperatures.

Expressing Concentration Quantitatively:

Beyond qualitative terms like "dilute" or "concentrated," chemists use specific quantitative methods to express concentration. Here are three common ways:

1. Mass by Mass Percentage of a Solution:
   This expresses the mass of the solute as a percentage of the total mass of the solution.
   Mass by mass percentage=(Mass of solute/Mass of solution)?×100
   Example: A solution contains 40 g of common salt in 320 g of water.
   Mass of solute (salt) = 40 g
   Mass of solvent (water) = 320 g
   Mass of solution = Mass of solute + Mass of solvent = 40 g + 320 g = 360 g
   Mass percentage of solution = (40 / 360) × 100 = 11.1% (approximately 11.1%)
2. Mass by Volume Percentage of a Solution:
   This expresses the mass of the solute as a percentage of the total volume of the solution.
   Mass by volume percentage=(Mass of solute?/Volume of solution)×100
3. Volume by Volume Percentage of a Solution:
   This expresses the volume of the solute as a percentage of the total volume of the solution. This is commonly used for liquid-liquid solutions.
   Volume by volume percentage=(Volume of solute?/Volume of solution)×100

Understanding how to calculate and interpret concentration is vital in various fields, from preparing chemical reagents in a lab to formulating beverages and medicines, ensuring the right amount of components for the desired properties.

Question for You: If you have 50g of sugar and you want to make a 20% mass by mass solution, how much water would you need?