Class 8 Science · Chapter 9 NotesThe Amazing World of Solutes, Solvents, and Solutions

A solution is a uniform mixture where the components are evenly distributed and cannot be seen separately. When a solid dissolves in a liquid, the solid is called the solute and the liquid is the solvent. For example, in salt water, salt is the solute and water is the solvent. When two liquids mix, the one present in smaller amount is the solute and the larger amount is the solvent. Air is a uniform mixture of gases, so it is also a solution. In contrast, mixtures like sand in water or sawdust in water are non-uniform because the particles are not evenly spread and can be seen.

• Solutionuniform mixture of two or more substances.
• Solutethe substance that dissolves.
• Solventthe substance in which the solute dissolves.
• Examplesalt + water → salt solution (salt is solute, water is solvent).

When you keep adding salt to water and stirring, a point comes when no more salt dissolves and some remains undissolved at the bottom. Such a solution is called a saturated solution at that temperature. A solution in which more solute can still be dissolved is unsaturated. The amount of solute present in a fixed quantity of solution is its concentration. A dilute solution has less solute, while a concentrated solution has more solute. These terms are relative. For example, a solution with one spoon of salt in a glass of water is dilute compared to one with four spoons in the same amount of water.

• Saturated solutionno more solute can dissolve at a given temperature.
• Unsaturated solutionmore solute can be dissolved.
• Concentrationamount of solute in a fixed quantity of solution.
• Diluteless solute; concentrated: more solute.

Solubility is the maximum amount of solute that can dissolve in a fixed quantity of solvent at a particular temperature. For most solids, solubility increases with temperature. For example, more baking soda dissolves in hot water than in cold water. A saturated solution at a lower temperature becomes unsaturated when heated, because the solvent can then dissolve more solute. However, the solubility of gases in liquids decreases as temperature increases. That is why cold water can hold more dissolved oxygen, which is essential for aquatic life. When water warms up, oxygen solubility drops, which can stress fish and other organisms.

• Solubilitymaximum solute that dissolves in a fixed amount of solvent at a given temperature.
• Solidssolubility generally increases with temperature.
• Gasessolubility generally decreases with temperature.
• Aquatic life depends on dissolved oxygen, which is higher in cold water.

Density is the mass per unit volume of a substance. It is calculated as Density = Mass / Volume. The SI unit of density is kilogram per cubic metre (kg/m³), but for convenience, grams per cubic centimetre (g/cm³) or grams per millilitre (g/mL) are often used. Density is independent of the shape or size of an object but depends on temperature and pressure. Objects with density less than that of the liquid float, while those with greater density sink. For example, oil floats on water because it is less dense. Ice floats because its density is lower than that of liquid water. The relative density of a substance is its density compared to water at the same temperature, and it has no units.

• Density = Mass / Volume.
• SI unitkg/m³; common units: g/cm³, g/mL.
• Relative density = Density of substance / Density of water.
• Objects float if their density is less than the liquid's density.

Mass is the quantity of matter in an object and is measured using a balance. A digital balance can be used by first taring it with an empty container, then placing the object and reading the mass. Volume is the space occupied by an object. For liquids, a measuring cylinder is used; the reading is taken at the bottom of the meniscus (the curved surface) for colourless liquids, and at the top for coloured liquids. For regular solids, volume is calculated using length × width × height. For irregular solids, volume is found by displacement: lower the object into water in a measuring cylinder and note the rise in water level. The volume of the object equals the volume of water displaced.

• Mass measured with a balance; volume of liquid with a measuring cylinder.
• Meniscuscurved surface of liquid; read at the bottom for water.
• Volume of regular solid = length × width × height.
• Volume of irregular solid = final water level – initial water level.

Generally, when a substance is heated, its particles move apart, so its volume increases while its mass remains the same. Since density = mass/volume, the density decreases. Cooling has the opposite effect. This explains why hot air rises: it is less dense than cool air. Pressure affects gases significantly: increasing pressure compresses the gas, reducing its volume and increasing its density. Liquids and solids are much less affected by pressure because their particles are already close together. Water has a special property: its density is highest at 4 °C. As water cools below 4 °C and freezes, it expands, so ice is less dense than liquid water and floats.

• Heating generally decreases density; cooling increases it.
• Pressure increases the density of gases; effect on liquids and solids is negligible.
• Water has maximum density at 4 °C.
• Ice floats because it is less dense than liquid water.