1.6. Exercises

A solution is a homogeneous mixture of two or more substances. Solutions are classified as solid, liquid and gaseous solutions. The concentration of a solution is expressed in terms of mole fraction, molarity, molality and in percentages.

A solution is a kind of mixture where two or more substances combine evenly. This means that every part of the solution has the same composition. For example, in a saltwater solution, the salt is uniformly distributed throughout the water, so every sip is salty. Solutions can be of different states, primarily solid (like alloys), liquid (like sugar in water), and gas (like air). The concentration of a solution indicates how much solute (the dissolved substance) is in a given amount of solvent (the substance doing the dissolving) and can be expressed in various ways—such as by mole fraction, which compares the amount of solute to the total amount in the solution; molarity, which is the number of moles of solute per liter of solution; and molality, which measures moles per kilogram of solvent.

Think of a glass of lemonade. The lemonade mix is the solute, and the water is the solvent. When you mix them, you make a solution where the lemonade mix is evenly mixed throughout the water, no matter where you take a sip. The concentration tells you how strong that lemonade flavor is.

The dissolution of a gas in a liquid is governed by Henry’s law, according to which, at a given temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas.

Henry's Law states that the amount of gas that can dissolve in a liquid depends on the pressure of the gas above the liquid. If you increase the pressure of the gas, more of it can dissolve in the liquid. For example, in carbonated drinks, carbon dioxide is forced into the liquid under high pressure. When you open the bottle, the pressure drops, and some of the carbon dioxide escapes as gas, which is what causes the fizz.

Imagine a soda can. Before you open it, the carbon dioxide is under high pressure. When you open the can, the pressure decreases, allowing some of the carbon dioxide to escape. That's why you hear that 'pop' sound, and why soda can go flat if you leave it open too long—a lot of gas is escaping, which means less fizz in your drink.

The properties of solutions which depend on the number of solute particles and are independent of their chemical identity are called colligative properties. These are lowering of vapour pressure, elevation of boiling point, depression of freezing point and osmotic pressure.

Colligative properties are unique characteristics of solutions that depend on how much solute is present rather than what kind of solute it is. For example, when you add salt to ice, it depresses the freezing point, meaning the ice melts at lower temperatures, which is why salt is used on roads in winter. Similarly, adding solute raises boiling point and lowers vapor pressure. All these changes occur because the solute particles disrupt the ability of solvent molecules to evaporate or freeze.

Think about cooking pasta. When you add salt to boiling water, it raises the boiling point slightly, allowing your pasta to cook faster. Conversely, when you put salt on ice, it causes the ice to melt. In both cases, the salt's effects demonstrated how solute impacts the physical properties of the solution.

When there is dissociation of solute into ions, the experimentally determined molar mass is always lower than the true value. This brings into light the rule that, when there is dissociation of solute into ions, the experimentally determined molar mass is always lower than the true value.

The van’t Hoff factor (i) accounts for the effect of solute dissociation in solutions. When an ionic substance like salt dissolves, it breaks into separate ions. For instance, table salt (NaCl) dissociates into Na+ and Cl- in solution, effectively doubling the number of particles. This results in properties of the solution being greater than predicted based on the original amount of salt alone, leading to an apparent molar mass that is lower than expected.

Consider a group of friends at a party; if they all arrive together, they show as one group, but if they split off into individuals, there are now many more interactions happening, just like how dissolved ions increase interactions in a solution. Hence, if you were to estimate just how many people were there based on groups alone, you'd say fewer than actual attendees.

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