Chapter Notes

The Amazing World of Solutes, Solvents, and Solutions

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The Amazing World of Solutes, Solvents, and Solutions

What Are Solute, Solvent, and Solution?

A solution is a uniform mixture where two or more substances are mixed together. Think about making a glass of lemonade. You add sugar to water, and the sugar disappears, creating a uniform mixture.

  • Solute: This is the substance that dissolves. In the lemonade example, sugar is the solute. It's usually the solid component when a solid is mixed with a liquid.
  • Solvent: This is the substance that does the dissolving. In the lemonade example, water is the solvent. It's usually the liquid component.

The solute dissolves in the solvent to create the solution.

Example
Imagine you're making tea. The tea leaves are like the solute, and the hot water is like the solvent. When the tea leaves steep, they dissolve into the water, creating the tea solution.

Sometimes, it's not so clear-cut. When two liquids are mixed, the substance present in the smaller amount is considered the solute, and the one in the larger amount is the solvent.

Note
It's important to remember that in a solution, the solute is evenly distributed throughout the solvent, which is why your homemade Oral Rehydration Solution (ORS) tastes the same in every sip.

Air is also a solution, but it's a gaseous solution. Nitrogen is the solvent because it's present in the largest amount, and oxygen, argon, carbon dioxide, and other gases are the solutes.

Example
Even though sugar is a solid and water is a liquid, when making the sugar syrup called "Chashni" for Gulab jamun, water is still considered the solvent and sugar is the solute.

How Much Solute Can a Fixed Amount of Solvent Dissolve?

Think about adding sugar to your tea. At first, the sugar dissolves easily. But what happens if you keep adding more and more sugar? Eventually, the sugar will stop dissolving and settle at the bottom of the cup.

  • Unsaturated Solution: This is a solution where you can still dissolve more solute at a given temperature. Your tea is an unsaturated solution when you can still add more sugar and it will dissolve.
  • Saturated Solution: This is a solution where no more solute can be dissolved at a given temperature. Your tea becomes a saturated solution when the sugar starts settling at the bottom, no matter how much you stir.

The concentration of a solution refers to the amount of solute present in a fixed quantity of solution (or solvent).

  • Dilute Solution: This is a solution with a small amount of solute. One spoon of salt in water creates a dilute solution.
  • Concentrated Solution: This is a solution with a large amount of solute. Two or more spoons of salt in water create a concentrated solution compared to the one-spoon solution.
Note
"Dilute" and "concentrated" are relative terms. A solution with two spoons of salt is dilute compared to a solution with four spoons of salt.

Solubility is the maximum amount of solute that can dissolve in a fixed quantity of the solvent.

How does temperature affect the solubility of a solute?

Temperature plays a significant role in how much solute can dissolve in a solvent.

Example
Imagine you're trying to dissolve baking soda in water. At room temperature, you can only dissolve a certain amount before it starts settling at the bottom. But if you heat the water, you'll notice that the baking soda that was previously undissolved now dissolves.

For most substances, solubility increases with an increase in temperature. A saturated solution at a particular temperature can behave like an unsaturated solution if the temperature is increased.

Note
In Ayurveda, Siddha, and other traditional systems of medicine in India, water, hydro-alcoholic extracts, oils, ghee, and milk have been used as solvents for preparing medicinal formulations.

Solubility of Gases

Many gases, including oxygen, can dissolve in water, although oxygen does so to a small extent. This dissolved oxygen is crucial for aquatic life.

Example
Fish, plants, and other organisms in lakes and oceans rely on dissolved oxygen to survive.

The mixture of gases in water is a uniform mixture because the gases dissolve evenly to form a solution.

Temperature also affects the solubility of gases in liquids. The solubility of gases generally decreases as temperature increases.

Example
Cold water can hold more dissolved oxygen than warm water. This is why cold water is essential for aquatic life. When water warms up, oxygen solubility decreases, which can harm aquatic ecosystems.

Why Do Objects Float or Sink in Water?

You've probably noticed that some things float in water, while others sink. Husk particles float while washing rice, while the rice sinks. Oil floats on water.

Generally, it's believed that objects that float are lighter, and those that sink are heavier. However, it's more about a property called density.

Example
A wooden stick and an iron rod can be the same size, but the iron rod feels much heavier. This is because iron is denser than wood.
Note
Density isn't the only factor determining whether an object floats or sinks, but it's a major one.

What Is Density?

Imagine a crowded bus versus an empty bus. The crowded bus has a high density of people, while the empty bus has a low density. Similarly, a dense forest has trees growing close together, while a less dense forest has trees farther apart.

Density is defined as the mass present in a unit volume of a substance.

The formula for density is:

Density = Mass / Volume

Density is independent of shape or size but depends on temperature and pressure. Pressure primarily affects the density of gases.

The SI unit of density is kilogram per cubic meter (kg/m³). For liquids, gram per milliliter (g/mL) and gram per cubic centimeter (g/cm³) are also used.

Example
If an aluminum block has a mass of 27 g and a volume of 10 cm³, its density is 2.7 g/cm³. This means aluminum is 2.7 times denser than water.

Relative density compares the density of a substance to the density of water at the same temperature.

Relative density = Density of substance / Density of water

Relative density has no units.

Determination of density

To determine density, you need to measure mass and volume.

How to measure mass?

Mass is the quantity of matter in an object. A balance is used to measure mass. A digital weighing balance is used to measure mass.

Example
To measure the mass of a stone, place it on a digital weighing balance. The reading on the balance gives the mass of the stone.
Note
In science, mass and weight are different. Mass is the quantity of matter and is measured in grams (g) or kilograms (kg). Weight is the force of gravity on an object and is measured in newtons (N).

How to measure volume?

Volume is the space occupied by an object. The SI unit of volume is cubic meters (m³). Smaller volumes are expressed in decimeter cubes (dm³) or centimeter cubes (cm³). For liquids, volume is expressed in liters (L), which is equal to 1 dm³, or milliliters (mL), which is equal to 1 cm³.

A measuring cylinder is used to measure the volume of liquids. It's a narrow, transparent container with markings to indicate the volume of liquid.

Example
To measure 50 mL of water, pour water into a measuring cylinder up to the 50 mL mark.

When measuring volume in a measuring cylinder, the water forms a curved surface called the meniscus. For water and colorless liquids, read the mark at the bottom of the meniscus. For colored liquids, read the mark at the top of the meniscus.

Determining volume of solid objects with regular shapes

For objects with cuboid shapes, volume can be calculated using the formula:

Volume = length × width × height

Example
A notebook with a length of 25 cm, a width of 18 cm, and a height of 2 cm has a volume of 900 cm³.

Determining volume of objects with irregular shapes

For objects with irregular shapes, like a stone, volume can be determined by displacement.

  1. Fill a measuring cylinder with water and record the initial volume.
  2. Lower the object into the measuring cylinder.
  3. Record the final volume after the water level rises.
  4. Subtract the initial volume from the final volume to find the volume of the object.
Note
The density of an object or substance can be calculated using the formula: Density = Mass/Volume.

Effect of temperature on density

Generally, density decreases with heating and increases with cooling. As temperature increases, the particles of a substance move away and spread, increasing volume but not mass. Since Density = Mass/Volume, density decreases upon heating. This is why hot air rises, as it is less dense than cool air.

Example
Hot air balloons work because hot air is less dense than the surrounding cooler air, causing the balloon to rise.

Effect of pressure on density

Pressure affects density differently depending on the state of matter. For gases, increasing pressure causes particles to move closer, decreasing volume and increasing density. For liquids, pressure has a small effect because they are nearly incompressible. Solids are even less affected by pressure than liquids.

Example
Ice floats on water because it is less dense than liquid water. Water is densest at 4°C. As water freezes, it expands, taking up more space and decreasing its density.
Note
The fact that ice floats is crucial for aquatic life, as the layer of ice insulates the water below, allowing fish and other creatures to survive in cold weather.

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