The Amazing World of Solutes, Solvents, and Solutions
Science Class 8 — Comprehensive Chapter Notes
Have you ever wondered why sugar completely disappears when you stir it in water, but sand does not?
Welcome to The Amazing World of Solutes, Solvents, and Solutions! In this chapter, you will learn what happens when substances mix, why some mixtures become clear while others remain cloudy, and how solutions such as fruit juices and seawater are part of everyday life. We shall study the meaning of solutes, solvents and solutions, types of solutions, solubility, the effect of temperature, the behaviour of gases in liquids, and the ideas of density and buoyancy through practical examples and simple experiments.
1. What Are Solute, Solvent and Solution?
What Is a Solution?
Solution: A solution is a uniform or homogeneous mixture in which the components are evenly distributed and cannot be seen separately.
Example: When salt or sugar is mixed with water and dissolves completely, the result is a clear solution.
Non-example: When sand or sawdust is mixed with water, the solid particles do not dissolve. They either float or settle at the bottom. Therefore, the mixture is not a solution.
Key Definitions
Solute: The substance that dissolves in the solvent. In many solid-liquid solutions, the solid, such as salt or sugar, is the solute and is usually present in a smaller amount.
Solvent: The substance that dissolves the solute. The solvent is usually present in a larger amount and determines the physical state of the solution. For example, water is the solvent in saltwater.
Solution Formation: A solution forms when solute particles spread uniformly among solvent particles, so that the individual components cannot be seen or separated by simple mechanical methods.
2. Types of Solutions and Examples
Solid in Liquid
Solute: Solid, such as salt or sugar.
Solvent: Liquid, such as water.
Example: Saltwater or sugar water is a clear and homogeneous solution.
Liquid in Liquid
When two liquids mix to form a homogeneous mixture, the liquid present in the smaller quantity is called the solute and the liquid present in the larger quantity is called the solvent.
Example: Vinegar is a solution of acetic acid in water. Acetic acid is the solute and water is the solvent.
Gas in Gas: Gaseous Solutions
Gases can form solutions. Air is a common example of a gaseous solution.
Solvent: Nitrogen, which forms about 78% of air.
Solutes: Oxygen, argon, carbon dioxide and other gases, which are present in smaller amounts.
Special Cases and Interesting Facts
Excess Solute: Even if the amount of solute is greater than the amount of solvent, the liquid is still treated as the solvent because it is the medium in which the solute dissolves. For example, in chashni, or sugar syrup, a large amount of sugar dissolves in a small amount of water. Water remains the solvent.
Factors Affecting Solubility: Solubility depends on the nature of both the solute and the solvent. It also depends on conditions such as temperature and pressure.
3. How Much Solute Can a Fixed Amount of Solvent Dissolve?
A simple salt-in-water experiment shows that a solvent has a finite capacity to dissolve a solute. This experiment also introduces the terms unsaturated, saturated, concentrated and dilute.
Experiment Steps
Step 1: Take a clean glass tumbler half-filled with water.
Step 2: Add a spoonful of salt and stir until it dissolves.
Step 3: Keep adding spoonfuls of salt and stir each time until some of the added salt no longer dissolves and settles at the bottom.
Observations and Inferences
1. At first, the salt dissolves completely and the solution remains clear.
2. After a certain number of spoonfuls, further salt does not dissolve and settles at the bottom. The exact number depends on the amount of water and its temperature.
3. This shows that the solvent has reached its dissolving limit for that temperature and amount. The solution is then called saturated.
Important Concepts
Unsaturated solution: A solution that can still dissolve more solute at the given temperature. For example, a solution in which only one or two spoonfuls of salt have dissolved may be unsaturated.
Saturated solution: A solution that cannot dissolve any more solute at the given temperature. Any excess solute remains undissolved and settles at the bottom.
Concentration: The amount of solute present in a given quantity of solution or solvent. It tells us how strong the solution is.
Dilute solution: A solution containing a relatively small amount of solute. For example, one spoon of salt in a large volume of water forms a dilute solution.
Concentrated solution: A solution containing a relatively large amount of solute. For example, many spoons of salt in a small volume of water form a concentrated solution.
Solubility: The maximum amount of solute that can dissolve in a fixed amount of solvent at a given temperature. It is often expressed as grams of solute per 100 mL of solvent at a specified temperature.
4. Effect of Temperature on Solubility
Temperature often affects how much solute a solvent can dissolve. For most solid solutes in liquids, solubility increases with temperature. For gases dissolved in liquids, solubility usually decreases as temperature rises.
Experiment: Heating Increases the Solubility of Many Solids
• Take 50 mL of water in a beaker and note its temperature, for example, 20°C.
• Add small amounts of baking soda, stirring until each amount dissolves, until some baking soda remains undissolved.
• Heat the mixture to about 50°C while stirring. Observe that some of the previously undissolved baking soda dissolves.
• Heat it further, for example, to 70°C, and observe that even more baking soda dissolves before saturation occurs.
Observations and Inferences
1. At higher temperatures, the same amount of solvent can dissolve more of many solid solutes.
2. Heating a saturated solution can often make it unsaturated, allowing more solute to dissolve.
3. This practical fact is used in processes such as recrystallisation in the laboratory and in preparing concentrated sugar syrups.
5. Solvents in Traditional Indian Medicine
Water as Solvent: Water is the principal solvent used to prepare many medicinal formulations in Ayurveda, Siddha and other traditional systems.
Hydro-alcoholic Extracts: These are made by using mixtures of water and alcohol to extract active compounds from herbs.
Other Solvents: Other solvents used in traditional formulations include oils, ghee and milk. They help extract and deliver fat-soluble compounds and may enhance therapeutic action.
6. Be a Scientist: Asima Chatterjee
Inspiration: Asima Chatterjee carried out research on medicinal plants, isolating useful compounds by using solvents and solution techniques.
Key Achievements
• Developed plant-based anti-epileptic and anti-malarial drugs.
• Was awarded a Doctorate of Science and was one of the earliest Indian women to receive it.
• Was the first woman recipient in chemical sciences of the Shanti Swarup Bhatnagar Award.
• Was honoured with the Padma Bhushan.
7. Solubility of Gases
Dissolved Oxygen: Many gases, including oxygen, dissolve in water, although usually only in small amounts. Dissolved oxygen is vital for aquatic life.
Homogeneous Mixture: A mixture of a gas dissolved in a liquid is homogeneous because gas molecules are distributed uniformly throughout the solvent.
Effect of Temperature on Gas Solubility
General Rule: For most gases, solubility in a liquid decreases with increasing temperature.
Example: Cold water can hold more dissolved oxygen than warm water. This is important for fish and other aquatic organisms.
8. Why Do Objects Float or Sink in Water?
Role of Density: Objects float or sink mainly according to their density, which is a measure of how much mass is present in a given volume.
Material Differences: Two objects of the same size may behave differently in water because their densities differ. For example, a wooden log and an iron rod of the same size have different densities. The iron rod is much denser and sinks, while the wooden log floats.
Buoyancy: Other factors, such as shape and the way forces act on an object, also affect floating or sinking. The upward force exerted by a liquid is called buoyancy. However, density is the primary property used to predict this behaviour.
9. What Is Density?
Definition: Density is a measure of how much mass is contained in a unit volume of a substance.
Independence from Size/Shape: Density does not depend on the shape or size of a sample. It depends on the nature of the material and on temperature and pressure.
Temperature Effect: Temperature changes affect density because heating generally increases volume while mass remains the same. Therefore, density usually decreases on heating.
Pressure Effect: Pressure affects gases strongly. Increasing pressure increases the density of a gas by reducing its volume. Liquids and solids are much less compressible, so pressure has a negligible effect on their density under ordinary conditions.
Oil Packet Example
Observation: Many 1-litre oil or ghee packets have a mass of about 910 g. This indicates that the density of the oil is less than 1 g/cm³, the approximate density of water.
Calculation: 910 g in 1000 cm³ gives a density of 0.91 g/cm³. This explains why oil floats on water.
10. Units of Density
SI Unit: The SI unit of density is kilogram per cubic metre, written as kg/m³.
Laboratory Units: For laboratory and school work, densities of solids and liquids are often expressed in gram per cubic centimetre, written as g/cm³, or gram per millilitre, written as g/mL.
Equivalence: 1 mL = 1 cm³.
At room temperature, 1 mL of water has a mass close to 1 g. Therefore, 10 mL of water has a mass close to 10 g, and 100 mL has a mass close to 100 g, to a good approximation.
11. Relative Density or Specific Gravity
Definition: Relative density of a substance with respect to water is the ratio of its density to the density of water at the same temperature. It is a pure number and has no units.
Example: If an aluminium block has a mass of 27 g and a volume of 10 cm³, its density is 2.7 g/cm³. Thus, it is 2.7 times as dense as water. Its relative density with respect to water is 2.7.
12. Determination of Density: Measuring Mass and Volume
Mass: How to Measure It
Mass Definition: Mass is the quantity of matter in an object. The instrument used to measure mass is a balance.
Digital Balance: Digital balances give readings directly in grams or kilograms. Before measuring, ensure that the balance shows zero. Use the tare function to subtract the mass of a container.
Measuring a Solid: Place a watch glass or small container on the pan, press tare to reset the reading to zero, then place the solid on it and read the mass directly.
Measuring a Liquid: Tare the empty beaker, pour in the liquid and read the mass shown by the balance.
Mass Versus Weight
Mass: The amount of matter in an object. It is expressed in grams or kilograms.
Weight: The force of gravity acting on that mass and is measured in newtons. A balance responds to weight but is calibrated to display mass units.
Volume: How to Measure It
Volume Definition: Volume is the space occupied by a substance. The SI unit is cubic metre (m³). In the laboratory, litre, millilitre and cubic centimetre are also commonly used.
Measuring Cylinder: The volume of liquids is commonly measured using a measuring cylinder, which is a clear cylindrical tube marked with volume graduations.
Choosing Cylinder Size: Choose a measuring cylinder that is closest in size to the required volume for better accuracy. For example, use a 100 mL cylinder to measure 70 mL, rather than using a much larger cylinder.
Meniscus Reading: Read the bottom of the meniscus, or curved surface of the liquid, at eye level for transparent liquids. For coloured or opaque liquids, read the appropriate visible level as instructed for the liquid.
Determining the Volume of Regular Solids
For regular objects such as cuboids, measure their dimensions with a scale or ruler and apply the formula:
Example: A notebook with length = 25 cm, width = 18 cm and height = 2 cm has a volume of 25 × 18 × 2 = 900 cm³.
Determining the Volume of Irregular Solids
The water displacement method can be used to find the volume of an irregular solid that does not dissolve in water in a measuring cylinder.
Step 1: Fill the measuring cylinder with a known volume of water and note the reading.
Step 2: Lower the irregular object, such as a stone tied to a thread, fully into the water and note the new water level.
Step 3: The volume of the object is the difference between the final and initial water levels:
13. Calculating Density: Worked Example
14. Some Interesting Facts and Examples
Earth's Layers: Earth is made of layers: the crust, mantle, outer core and inner core. These layers have different densities. Density generally increases with depth because of higher pressure and differences in composition.
Ancient Rafts: Bamboo and wooden logs were used in ancient times to make rafts and simple boats because they are light and float easily.
Hot-Air Balloons: Hot air rises because heating decreases the density of air. Hot-air balloons operate using this principle.
15. Why Does Ice Float on Water?
Maximum Density of Water: Water has the unusual property that its density is greatest at about 4°C. When water freezes at 0°C, its molecules form a crystalline structure that occupies more volume than liquid water.
Density Difference: Because the same mass occupies a larger volume as ice, the density of ice is less than the density of liquid water. Therefore, ice floats.
Ecological Importance: This behaviour is important for life in cold climates because a floating layer of ice insulates the water below and allows fish and other organisms to survive.
16. Egg Experiment: A Practical Observation
Fresh Water: Place a fresh raw egg in a glass tumbler filled with tap water. The egg sinks to the bottom because its density is greater than that of fresh water.
Salt Water: Add salt to the water and stir until it dissolves. The density of the water increases, and the egg may begin to float when the density of the water becomes equal to or greater than the density of the egg.
17. Effect of Temperature on Density
In general, when a substance is heated, its particles move farther apart and its volume increases. Since its mass remains unchanged, its density decreases on heating.
Rising Warm Air: This explains why warm air rises: it is less dense than the surrounding cooler air. Objects generally expand slightly on heating.
18. Effect of Pressure on Density
Gases: Increasing pressure compresses gases, reducing their volume and increasing their density.
Liquids and Solids: They are nearly incompressible under ordinary conditions, so pressure changes have only a very small effect on their densities.
19. Why Ice Floating Matters and Final Notes
Insulating Layer: Because ice floats, it forms an insulating layer above liquid water during winter. This helps aquatic life survive in cold seasons.
Everyday & Scientific Value: Understanding solutes, solvents and solutions is useful in chemistry, biology, environmental science and everyday life, from cooking to medicine.
20. Key Terms to Remember
Solution: A uniform mixture formed when two or more substances mix evenly.
Solute: The substance that dissolves. In solid-liquid solutions, it is usually the solid.
Solvent: The substance that dissolves the solute. In solid-liquid solutions, it is usually the liquid.
Saturated solution: A solution that cannot dissolve more solute at a given temperature.
Unsaturated solution: A solution that can dissolve more solute at a given temperature.
Solubility: The maximum amount of solute that dissolves in a fixed amount of solvent at a specific temperature.
Concentration: The amount of solute present in a given quantity of solution or solvent.
Density: Mass per unit volume, calculated using Density = Mass / Volume.
Relative density: The ratio of the density of a substance to the density of water at the same temperature. It has no units.
Measurement Devices: A balance measures mass and a measuring cylinder measures the volume of a liquid.
Temperature Effects: The solubility of most solids in liquids increases with temperature, whereas the solubility of gases in liquids usually decreases with temperature.
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