The Amazing World of Solutes Solvents and Solutions Class 8 Notes Science Chapter 9

Students can rely on these Class 8 Science Notes Chapter 9 The Amazing World of Solutes Solvents and Solutions Class 8 Notes for complete exam preparation.

Class 8 Science Chapter 9 The Amazing World of Solutes Solvents and Solutions Notes

Class 8 Science Curiosity Chapter 9 Notes

Class 8 Science Chapter 9 Notes – The Amazing World of Solutes Solvents and Solutions Notes Class 8

→ Solution A uniform mixture formed when a solute dissolves completely in a solvent.

→ Solute The substance that dissolves in a solvent to form a solution, (e.g., Salt in water).

→ Solvent The substance (usually a liquid) that dissolves the solute, (e.g., Water in salt water).

→ Saturated Solution A solution in which no more solute can dissolve at a given temperature.

→ Unsaturated Solution A solution that can dissolve more solute at a given temperature.

→ Solubility The maximum amount of solute that can dissolve in 100 mL of solvent at a specific temperature.

→ Concentration The amount of solute present in a given quantity of solution (dilute = less solute, concentrated = more solute).

→ Density Mass per unit volume of a substance.
(Density = mass/volume).

→ Relative Density A unit less ratio comparing a substance’s density to water’s density.

→ Meniscus The curved surface of a liquid in a measuring cylinder and read at the bottom for accuracy.

→ Expansion Increase in volume of a substance when heated and causing a decrease in density.

→ What is the difference between a solute and solvent in a solution?

→ Give an example of a solid-liquid solution and identify its solute and solvent.

→ In a solution of lemon juice and water, which substance is the solute?

→ Why is nitrogen considered the solvent in air?

→ What is the effect of temperature on solubility of solids?

→ What happens to solubility of solids on increasing temperature?

→ Explain what is the relative density?

→ State why do aquatic life struggle to survive in warm water?

→ What is the effect of pressure on density?

→ Differentiate between dilute and concentrated solutions.

→ What is the effect of temperature and pressure on density?

→ Why does ice float on water?

→ What is the formula of volume of a cuboid?

→ What happens to the density of air inside a hot air balloon when it’s heated?

→ Why is a measuring cylinder used for volume measurements?

Solute, Solvent and Solution

• Uniform mixture is referred to as mixture in which the components are evenly distributed throughout and their components are not visible separately, e.g., Mixture of sugar, salt and water.
• Non-uniform mixture is referred to as a mixture in which the components are not evenly distributed and their components can be seen either with the naked eye or with a magnifying device, e.g., Mixture of sand and water, sawdust and water, etc.
• A solution is a uniform mixture formed when two or more substances mix evenly.
• Solution is made from mixing solute (solid component) and solvent (liquid component).
• In liquid solutions, the substance present in a smaller amount is called the solute, and the one in larger quantity is the solvent.

• Air is a uniform gaseous solution, where nitrogen (largest amount) is solvent and other gases (lesser amount) are considered as solutes.

In Indian sweets like Gulab Jamun, chashni (sugar syrup) is prepared by dissolving a large amount of sugar (solute) in a small quantity of water (solvent). Even though sugar is present in greater proportion, water remains the solvent because it dissolves the sugar to form a uniform solution. This example shows that the solvent isn’t always the majority component—it’s the substance that dissolves the other.

Concentration and Solubility of a Solution

• Unsaturated solution is the solution in which more solute can be dissolved at a given temperature.
• Saturated solution is the solution in which no more solute can be dissolved or when the solute stops dissolving and begins to settle at the bottom at a particular temperature.

• The concentration of a solution refers to the amount of solute present in a fixed quantity of solution (or solvent).
• A solution with less amount of solute is called a dilute solution, while one with more amount of solute is called a concentrated solution.
• The solubility of a solute is the maximum amount of solute that can dissolve in a fixed quantity of solvent (usually 100 mL) at a specific temperature.

Effect of Temperature on Solubility

• For most substances, solubility increases with an increase in temperature. For example, more baking soda dissolves in hot water than in cold water.
• A saturated solution at a particular temperature behaves as an unsaturated solution if the temperature is increased.
• Flowever, gases do not follow the general trend of solubility.

Solubility of Gases

• Many gases, including oxygen, dissolve in water. The dissolved oxygen is essential for aquatic life because it 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 with rising temperature, which is why cold water holds more oxygen than warm water.

Why Do Objects Float or Sink in Water?

• The ability of objects to float or sink in water is determined by their density relative to water.
• For instance, wood floats-because its density is lower than water’s, while iron rod sinks due to its higher density.
• A ship made of steel floats because of hollow spaces present in it.

Density

• Density is a special property which describes the heaviness of an object or measures how tightly the matter is packed.
• A crowded bus, a thick forest with closely spaced trees is dense, whereas widely spaced trees are less dense.
• Density depends on how much mass occupies a given space.
• Density is defined as the mass per unit volume of a substance. It is calculated by using the formula
  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.
• The SI unit of density is kg/m³ (kilogram per cubic metre), but for liquids, g/mL (gram per millilitre) or g/cm³ (gram per cubic centimetre) is commonly used.
• For example, the density of water is approximately 1 g/mL at room temperature.

Conversion Factor for Density

• Density can be expressed in different units depending on the measurement system used. These units are interrelated through simple conversions.
• 1 kg/m³ = 1000 g/m³ = 1000 g/1000L
  = 1 g/L = 1 g/1000 mL = 1 g/1000 cm³

Relative Density

It compares the density of a substance to that of water. For instance, if aluminium has a density of 2.7 g/cm³, its relative density is 2.7, meaning it is 2.7 times denser than water.

Relative density of any subtance with respect to water = \(\frac{\text { Density of that substance }}{\text { Density of water at that temperature }}\)

Determination of Density

The density of an object can be determined by measuring its mass and volume.

Measurement of Mass

• Mass represents the amount of matter present in an object or substance. Its units are gram (g) and kilogram (kg).

• Mass is measured by using an instrument called balance.
• These days digital weighing balances are in use.
• In everyday use, mass and weight are often confused. Scientifically, mass is the amount of matter (in grams or kilograms) present in an object, while weight is the force of gravity acting on that mass (in newtons). Most balances (except two pan balances) actually measure weight, but their scales are marked in mass units, so they show values in grams or kilograms.

Measurement of Volume

• Volume refers to the amount of space an object occupies.
• The SI unit of volume is the cubic meter (m³). It is the volume of a cube whose each side is one metre in length. Volume of smaller objects is conveniently expressed in a decimetre cube (dm³) or centimetre cube (cm³).
• A measuring cylinder is one of the most commonly used apparatus for accurately determining liquid volumes. It is a narrow transparent, cylindrical container with the following features
  1. Graduated markings along its side for precise measurement.
  2. An open top for pouring liquids.
  3. A closed base for stability.
• The smallest volume that a measuring cylinder can measure depends on the capacity of the measuring cylinder.

• Usually, it is 0.1 mL in smaller measuring cylinders with a capacity of 10 mL or 25 mL, it is 1 mL in a 100 mL measuring cylinder, 2 mL in a 250 mL measuring cylinder, and 5 mL in a 500 mL measuring cylinder.
• The narrow design enhances measurement precision by creating a clear meniscus (the curved liquid surface) that helps in reading volumes accurately.
• The water inside the measuring cylinder forms a curved surface. This curved surface is called the meniscus.
• The measurement at the bottom of the meniscus at eye level is done for colourless liquids and above the meniscus for coloured liquids.

Volume of Regular Shaped Object
• The volume of an object having cuboid shape and whose length is ‘l’, width is ‘b’ and height is ‘h’ can be calculated as Volume = l × w × h

Volume of Irregular Shaped Object

• The volume of such objects is also measured using a measuring cylinder.
• For such volume calculations, we put the object in a measuring cylinder containing water and the difference in volume level before and after putting the object in the measuring cylinder gives us the volume.

Some Key Conversions and Units

• Cubic decimeter (dm³)
• Cubic centimeter (cm³ or cc)
• Litre (L), where 1 L = 1 dm³
• Millilitre (mL), where 1 mL = 1 cm³
• 1 m³ = 1000 dm³ (or litre)
• 1 L = 1000 mL
• 1 cm³ = 1 mL

Density of Layers of the Earth

• Earth is composed of several layers, such as crust, upper mantle, lower mantle, outer core, and inner core, each with its particular range of density.

• The crust (outermost layer) is the lightest of all and the density gradually increases as we move towards the centre of the earth.
• As we move deeper into the Earth, both the pressure and the temperature rise significantly, making the materials heavier and more compact.

Long before modern ships, people built simple rafts and boats using bamboo and wooden logs. Bamboo, being light and hollow, was ideal for floating on water. These handmade crafts were used for fishing, travel, and trade—and in some places, such traditional boats are still used today.

Effect of Temperature on Density

• The density of a substance decreases with heating and increases with cooling.
• When things get hotter, their particles start moving faster and spread out – whether it’s a solid, liquid or gas. This spreading makes the substance take up more space (volume increases) while the actual amount of material (mass) remains the same.
• Since density is calculated as mass divided by volume (Density = Mass/Volume), the density decreases when something heats up. This explains why hot air moves up as it is less dense than the cool air around it.
• Hot air balloons also work in this similar way. The heated air inside the balloon becomes less dense than the surrounding air, creating the lift that makes the balloon float upwards.

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.
• For liquids, pressure has a small effect because they are nearly incompressible.
• Solids are even less affected by pressure than liquids, and changes in their density are usually negligible.

Ice floats on water because it is less dense than liquid water. Water is heaviest at 4 °C. As it cools to 0 °C and freezes, its particles arrange in a way that takes up more space—this is called expansion. The increased volume lowers its density, making ice lighter than water. Floating ice forms an insulating layer that keeps water below from freezing, helping aquatic life survive in cold weather.