The Amazing World of Solutes, Solvents, and Solutions
Class 08 ScienceWhen salt and sugar are mixed with water, a uniform mixture is formed, whereas when chalk powder or sand, or sawdust is mixed with water, the components are not evenly distributed. Such mixtures are known as non-uniform mixtures.
Solute, Solvent, and Solution
A uniform mixture, such as that of salt or sugar, and water, is called a solution. Whenever a solid is mixed with a liquid to form a solution, the solid component is called the solute, and the liquid component is called the solvent. The solute dissolves in the solvent to form a solution.
Solute + Solvent → Solution
When a solution is formed by mixing two liquids, it is not always clear which substance is dissolving the other. In such cases, the substance present in smaller amount is called the solute, while the one in larger amount is called the solvent.
Air is a gaseous solution. Since nitrogen is present in the largest amount in the air, it is considered as the solvent, while oxygen, argon, carbon dioxide, and other gases are considered as solutes.
Saturated Solution
Initially, the salt completely dissolves in the water, forming a solution. After adding a few more spoons of salt, a stage comes when the added salt does not dissolve completely and the undissolved salt settles at the bottom. This indicates that the water can no longer dissolve any more salt because it has reached its limit.
The solution in which more solute can be dissolved at a given temperature, is called an unsaturated solution. However, when the solute stops dissolving and begins to settle at the bottom, the solution is called a saturated solution at that particular temperature.
The amount of solute present in a fixed quantity of solution (or solvent) is termed as its concentration. Depending upon the amount of solute present in a fixed quantity of solution, it can be called a dilute solution (less amount of solute) or a concentrated solution (more amount of solute). Dilute and concentrated are relative terms.
The maximum amount of solute that dissolves in a fixed quantity of the solvent is called its solubility.
Effect of Temperature
For most of the substances, the solubility increases with an increase in temperature. A saturated solution at a particular temperature behaves as an unsaturated solution if the temperature is increased.
Solubility of Gases
Many gases, including oxygen, dissolve in water. Oxygen dissolves in water only to a small extent. Even though present in minute quantities, it is this dissolved oxygen that sustains all aquatic life, including plants, fishes, and other organisms.
The mixture of gases in water is a uniform mixture because the gases dissolve evenly in water to form a solution.
The solubility of gases generally decreases as temperature increases. More oxygen can dissolve in cold water, ensuring sufficient oxygen for aquatic life. On the other hand, when water warms up, the solubility of oxygen decreases.
Density
Matter is anything that possesses mass and occupies space (volume). Density is defined as the mass present in a unit volume of that substance.
$$ \text{Density} = \frac{\text{Mass}}{\text{Volume}} $$
The density of a substance is independent of its shape or size. However, it is dependent on temperature and pressure. Pressure primarily affects the density of gases, while its effect on solids and liquids is negligible.
Units
The units in which density is expressed will depend upon the units of mass and volume taken. The SI units of mass and volume are kilogram (kg) and cubic metre (m3), respectively. Therefore, the SI unit of density is kilogram per cubic metre, abbreviated as kg/m3.
In case of liquids, other units of density are also used for convenience, such as gram per millilitre, abbreviated as g/mL and gram per cubic centimetre, abbreviated as g/cm3.
1 kg/m3 = 1000 g/m3 = 1000 g/1000 L = 1 g/L = 1 g/1000 mL = 1 g/1000 cm3
The mass of 1 mL of water is close to 1 g at room temperature. For the measurement of the mass of water, we generally consider the volume in mL and its mass in g. Hence, 10 mL of water would be approximately 10 g. Similarly, 100 mL of water would be approximately 100 g.
For example, suppose the mass of an aluminium block is 27 g and its volume is 10 cm3, its density is 2.7 g/cm3. So, aluminium is 2.7 times denser than water. It means that the relative density of aluminium with respect to water is 2.7. It is a number without any units.
$$ \text{Relative Density} = \frac{\text{Density of substance}}{\text{Density of water}} $$
Effect of Temperature on Density
Generally, the density of a substance decreases with heating and increases with cooling. As temperature increases, the particles of a substance whether, solid, liquid, or gas, tend to move away and spread. This results in an increase in volume but there is no change in mass.
This explains why hot air moves up as it is less dense than the cool air around it. The hot air balloon works on the same principle.
Effect of Pressure on Density
Pressure affects density differently depending on the state of matter. For gases, increasing pressure causes the particles to move closer together. As a result, the volume of the gas decreases and its density increases.
In the case of liquids, pressure has a small effect because they are nearly incompressible. The particles in solids are very close to each other. So, solids are even less affected by pressure than liquids, and changes in their density are usually negligible.