Solutions: Solutes and Solvents

CHAPTER 9: THE AMAZING WORLD OF SOLUTES, SOLVENTS, AND SOLUTIONS

9.1 WHAT ARE SOLUTE, SOLVENT, AND SOLUTION?

**Definition of Solution:**

A **solution** is a uniform mixture formed when a solid is completely dissolved in a liquid, or when two liquids mix together uniformly. The components are evenly distributed throughout, so it appears as a single substance to the naked eye.

**Components of a Solution:**

When a solid dissolves in a liquid:

**Solute** = The solid substance being dissolved (example: salt, sugar)

**Solvent** = The liquid in which the solute dissolves (example: water)

**Solution** = The uniform mixture formed after dissolving

**Formula:**

Solute + Solvent = Solution

**Important Note for Liquid-Liquid Mixtures:**

When two liquids mix to form a solution, the substance present in the **smaller amount** is called the solute, and the substance present in the **larger amount** is called the solvent.

**Real-Life Example - Indian Context:**

The **Chashni (sugar syrup)** used to prepare **Gulab Jamuns** is made by dissolving a large amount of sugar (solute) in a small amount of water (solvent). Even though the water is the smaller quantity, it is still considered the solvent because it is the dissolving medium.

**Difference Between Solutions and Non-Uniform Mixtures:**

**Solutions** (uniform mixtures): Salt in water, sugar in water, salt + sugar in water - components are evenly distributed, cannot be seen separately

**Non-uniform mixtures**: Sand in water, chalk powder in water, sawdust in water - components are NOT evenly distributed, can be seen separately

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9.2 HOW MUCH SOLUTE CAN A FIXED AMOUNT OF SOLVENT DISSOLVE?

**Key Concept:**

A fixed amount of solvent (like 1 glass of water) can dissolve only a limited amount of solute (like salt or sugar). After this limit is reached, no more solute will dissolve.

**Activity 9.1 - Dissolution of Salt in Water:**

**Procedure:**

1. Take a clean glass tumbler and fill it half with water

2. Add one teaspoon of salt and stir well until it dissolves completely

3. Gradually keep adding salt one teaspoon at a time, stirring after each addition

4. Continue until you observe that salt no longer dissolves and settles at the bottom

5. Record observations in a table showing how many teaspoons dissolved before settling began

**Observations:**

Initially, salt dissolves completely in water

With each addition, more salt dissolves

After a certain point (say after 5-6 teaspoons), undissolved salt begins to settle at the bottom

This indicates that water has reached its maximum dissolving capacity

**Three Important Terms:**

1. **Unsaturated Solution:**

A solution in which **more solute can still be dissolved** at a given temperature. The solvent still has the capacity to dissolve additional solute.

Example: When you add 1 teaspoon of salt to water and it completely dissolves, this is an unsaturated solution because water can still dissolve more salt.

2. **Saturated Solution:**

A solution in which **no more solute can dissolve** at a given temperature. The solvent has reached its maximum dissolving capacity.

Example: When you add multiple teaspoons of salt and finally some salt does not dissolve and settles at the bottom, the solution becomes saturated at that temperature.

3. **Concentration of a Solution:**

**Concentration** is the **amount of solute present in a fixed quantity of solution (or solvent)**.

It can be classified as:

**Dilute Solution** = Small amount of solute dissolved in a fixed amount of solvent (weakly concentrated)

**Concentrated Solution** = Large amount of solute dissolved in a fixed amount of solvent (strongly concentrated)

**Important Note:** "Dilute" and "Concentrated" are **relative terms**. A solution that is dilute compared to one mixture can be concentrated compared to another mixture.

**Example:**

1 teaspoon salt in 100 mL water is more **dilute**

5 teaspoons salt in 100 mL water is more **concentrated**

2 teaspoons salt in 100 mL water is dilute compared to 4 teaspoons salt in 50 mL water

**Solubility - Definition:**

**Solubility** is the **maximum amount of solute that can dissolve in a fixed quantity of solvent at a particular temperature**, under standard conditions. It is a measure of how much of a substance can dissolve.

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9.2.1 HOW DOES TEMPERATURE AFFECT THE SOLUBILITY OF A SOLUTE?

**Key Concept:**

The solubility of most solid solutes **increases with an increase in temperature**.

**Activity 9.2 - Effect of Temperature on Solubility of Baking Soda:**

**Apparatus Required:**

Glass beaker

Laboratory thermometer

Baking soda (sodium hydrogen carbonate/NaHCO₃)

Water

Heating device (spirit lamp, Bunsen burner)

Glass rod for stirring

Tripod stand

Wire gauze

**Procedure:**

1. **At Room Temperature (20°C):**

Take 50 mL of water in a beaker and measure temperature = 20°C

Add a spoonful of baking soda and stir

Keep adding baking soda in small amounts while stirring

Continue until solid baking soda remains undissolved at the bottom

2. **Heating to 50°C:**

Heat the beaker containing water and undissolved baking soda to 50°C while stirring

Observe that the undissolved baking soda dissolves as temperature increases

Add more baking soda at 50°C until some remains undissolved

3. **Heating to 70°C:**

Further heat the mixture to 70°C while stirring

Observe that more undissolved baking soda now dissolves

This shows that at 70°C, water can dissolve even more baking soda than at 50°C

**Observations:**

At 20°C: Small amount of baking soda dissolves (low solubility)

At 50°C: More baking soda dissolves (medium solubility)

At 70°C: Even more baking soda dissolves (high solubility)

**Inference:**

**For most solid solutes, as temperature increases, solubility increases.**

**Important Relationship:**

Saturated solution at one temperature → If temperature increases → Becomes unsaturated solution at the new temperature

This is because at higher temperature, the solvent can dissolve more solute

**Real-Life Example:**

Making **tea or coffee** at home - hot water dissolves more sugar than cold water. The same spoon of sugar that doesn't fully dissolve in cold water completely dissolves when hot water is poured over it.

**Scientific Heritage - Traditional Indian Medicine:**

Water has been the primary solvent in **Ayurveda**, **Siddha**, and other traditional Indian medicinal systems for preparing medicinal formulations. Additionally, **hydro-alcoholic extracts** (combinations of water and alcohol) were used to extract beneficial compounds from medicinal herbs. Other solvents like **oils, ghee, and milk** were also used in drug formulations to enhance therapeutic benefits.

**Notable Scientist - Asima Chatterjee:**

Pioneering Indian scientist who worked extensively on medicinal plants

Used solvents and solutions to extract and isolate important compounds from plants

Developed **anti-epileptic** and **anti-malarial** drugs

Became the **second Indian woman** to earn a Doctorate of Science (after Janaki Ammal)

First woman to receive the **Shanti Swarup Bhatnagar Award** in Chemical Science

Recipient of the **Padma Bhushan** honor

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9.3 SOLUBILITY OF GASES

**Do Gases Dissolve in Water?**

Yes, gases also dissolve in water, forming **uniform mixtures (solutions)**.

**Examples of Gases Dissolving in Water:**

Oxygen (O₂) dissolves in water

Carbon dioxide (CO₂) dissolves in water

Nitrogen (N₂) dissolves in water to a lesser extent

**Importance of Dissolved Oxygen:**

Oxygen dissolves in water in **very small amounts**

Even though present in **minute quantities**, this dissolved oxygen is crucial

Dissolved oxygen sustains **all aquatic life** including:

Fishes

Plants

Algae

Other aquatic organisms

**Nature of Gas-Water Mixture:**

The mixture of gases dissolved in water is a **uniform mixture** because the gases dissolve evenly and completely throughout the water.

**How Temperature Affects Gas Solubility:**

**Key Concept:** Unlike solids, the solubility of gases in liquids **DECREASES as temperature increases**.

**Relationship:**

↑ Temperature → ↓ Gas Solubility in water

↓ Temperature → ↑ Gas Solubility in water

**Explanation:**

**Cold water** can dissolve **more oxygen** (high gas solubility)

**Warm water** can dissolve **less oxygen** (low gas solubility)

**Real-Life Example:**

**Aquatic life thrives better in cold water** because cold water contains more dissolved oxygen

When water becomes warm, less oxygen dissolves, making it difficult for aquatic organisms to survive

Fish in rivers and ponds are more active in winter (cold water has more O₂) than in summer (warm water has less O₂)

**Practical Implication:**

This is why water aerators/fountains in warm regions bubble air through water - to increase oxygen levels by increasing the surface area for gas dissolution.

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9.4 WHY DO OBJECTS FLOAT OR SINK IN WATER?

**Observation:**

Some objects **float** on water (like cork, wood, oil, paper)

Some objects **sink** in water (like stone, iron, sand)

In washing rice, **husk floats** while **rice sinks**

**Common Misconception:**

People often think that objects float because they are "light" and objects sink because they are "heavy." However, this is not entirely accurate. The determining factor is a property called **density**.

**Note:** Density is not the ONLY factor deciding whether an object floats or sinks (other factors like shape and surface area can also matter), but it is the primary factor.

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9.5 WHAT IS DENSITY?

**Everyday Understanding of Density:**

1. **People in a Bus:**

Crowded bus with many people packed together = **High density**

Bus with only a few people = **Low density**

2. **Forest Example:**

Dense forest with trees close together = **High density** (Fig. 9.10a)

Forest with trees far apart = **Low density** (Fig. 9.10b)

**Scientific Definition of Density:**

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

It tells us how much matter (mass) is packed into a specific space (volume).

**Mathematical Formula:**

Density = Mass / Volume

Or: **ρ = m / V**

Where:

ρ (rho) = Density

m = Mass of the substance

V = Volume of the substance

**Units of Density:**

**SI Unit:** kilogram per cubic metre (kg/m³)

**Other Common Units for Liquids:**

gram per millilitre (g/mL)

gram per cubic centimetre (g/cm³)

gram per litre (g/L)

**Conversion Factor for Density:**

1 kg/m³ = 1000 g/m³ = 1 g/L = 1 g/1000 mL = 1 g/1000 cm³

**Density of Water (Reference):**

Mass of 1 mL of water ≈ **1 g** (at room temperature, ~4°C)

Therefore, density of water ≈ **1 g/mL** or **1 g/cm³**

This makes water a convenient reference for comparing densities of other substances

**Calculations with Water:**

10 mL of water = approximately 10 g

100 mL of water = approximately 100 g

1 L of water = approximately 1000 g = 1 kg

**Important Properties of Density:**

1. **Density is Independent of:** Shape and size of the object

A small piece of iron and a large piece of iron have the same density

Whether shaped as a rod, cube, or sphere, iron has the same density

2. **Density Depends On:** Temperature and Pressure

As temperature increases, density usually decreases (except for water around 4°C)

For solids and liquids, pressure has negligible effect

For gases, pressure has significant effect on density

**Example Calculation:**

If an aluminium block has:

Mass = 27 g

Volume = 10 cm³

Density of aluminium = 27 g / 10 cm³ = **2.7 g/cm³**

**Relative Density:**

**Definition:** Relative density (also called specific gravity) compares the density of a substance with the density of water.

**Formula:**

Relative Density = Density of that substance / Density of water (at same temperature)

**Example:** Since density of aluminium = 2.7 g/cm³ and density of water = 1 g/cm³

Relative Density of aluminium = 2.7 / 1 = **2.7** (no units)

This means aluminium is **2.7 times denser than water**.

**Real-Life Example - Think Like a Scientist:**

Oil and ghee packets are labelled as "1 litre" (volume) but weigh only about "910 grams" (mass). This tells us:

Density of oil = 910 g / 1000 mL = 0.91 g/mL

Since density of oil < density of water, oil is **less dense than water**

Therefore, oil floats on water (and this is why we see oil floating on top of water)

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9.5.1 DETERMINATION OF DENSITY

To find the density of an object, we need to:

1. **Measure the mass** of the object

2. **Measure the volume** of the object

3. **Apply the formula:** Density = Mass / Volume

HOW TO MEASURE MASS

**Definition of Mass:**

**Mass** is the **quantity of matter present in an object**.

**Instrument Used:** **Balance** (weighing device)

Types of balances:

Digital weighing balance

Pan balance

Spring balance

Two-pan balance (beam balance)

**Activity 9.3 - Measuring Mass Using Digital Weighing Balance:**

**Procedure:**

1. **Switch ON the digital weighing balance**

Ensure the balance is working properly

Check that display is visible

2. **Observe Initial Reading:**

Look at the digital display

Should show zero reading when balance is empty

3. **If Not Zero, Reset:**

Press the **Tare button** (also called Reset button) to bring reading to zero

This is important for accurate measurement

4. **Place Watch Glass on Pan:**

Take a clean, dry watch glass

Alternatively, use butter paper or filter paper

Place it on the weighing pan

5. **Note Reading:**

Read the mass displayed on the screen

6. **Reset Again:**

Press Tare/Reset button to bring reading back to zero

This subtracts the mass of the watch glass from future measurements

7. **Place Object to be Weighed:**

Carefully place the solid object (stone, or other object) on the watch glass

Do not drop it - place gently

8. **Note the Final Reading:**

Read the digital display

This gives the **mass of the object only** (watch glass mass is excluded)

Example: If display shows 16.400 g, the mass of stone = 16.400 g

**To Measure Mass of Liquids:**

Replace the watch glass with a beaker

Pour the desired amount of liquid into the beaker

Follow the same procedure

Subtract the mass of the beaker (by taring) to get mass of liquid only

**Important Note - Mass vs Weight:**

Many people use the terms "mass" and "weight" interchangeably in everyday language, but in science they have different meanings:

| Property | Mass | Weight |

|----------|------|--------|

| **Definition** | Quantity of matter in an object | Force with which Earth attracts an object |

| **Unit** | Gram (g), Kilogram (kg) | Newton (N) |

| **Changes with location** | Always same (constant everywhere) | Changes with location (different on Earth and Moon) |

| **Measured by** | Balance | Spring scale |

| **Type of quantity** | Scalar (has only magnitude) | Vector (has magnitude and direction) |

**Note on Balances:** Most digital balances actually measure weight, but their scales are calibrated and marked in mass units (grams or kilograms), so the display shows mass values.

**Two-Pan Balance:** This is the most accurate type of balance because it directly compares masses. Weights are placed on one pan and object on the other.

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HOW TO MEASURE VOLUME

**Definition of Volume:**

**Volume** is the **space occupied by an object** or the **amount of space a substance takes up**.

**Units of Volume:**

| Unit | Abbreviation | Equivalent |

|------|--------------|-----------|

| Cubic metre | m³ | SI unit of volume |

| Decimetre cube | dm³ | 1 dm³ = 1 Litre |

| Centimetre cube | cm³ | 1 cm³ = 1 mL |

| Centimetre cube | cc | Same as cm³ |

| Litre | L | 1 L = 1 dm³ = 1000 mL |

| Millilitre | mL | 1 mL = 1 cm³ |

**Conversion Chart for Volume:**

1 m³ = 1000 dm³ = 1000 L

1 L = 1 dm³ = 1000 cm³ = 1000 mL

1 cm³ = 1 mL

**Real-Life Example:**

A **tetra pack of buttermilk (chach)** is labelled as "200 mL" - this means the volume of buttermilk in that pack is 200 millilitres.

**Measuring Cylinder - Apparatus for Measuring Volume:**

**Description:**

A measuring cylinder is a narrow, transparent cylindrical glass container used to measure the volume of liquids.

**Features:**

Shape: Tall, narrow, cylindrical

One side is **open at the top**

Other side is **closed at the bottom**

**Transparent body** allows visibility of liquid level

**Marked graduations/scale** on the side indicating volume in mL

Flat bottom so it stands stable on table

**Available Sizes:**

Measuring cylinders come in various capacities:

5 mL, 10 mL, 25 mL, 50 mL, 100 mL, 250 mL, 500 mL, 1000 mL

**Smallest Reading (Least Count):**

Different measuring cylinders can measure different smallest volumes:

10 mL cylinder: Least count = 0.1 mL

25 mL cylinder: Least count = 0.1 mL

100 mL cylinder: Least count = **1 mL**

250 mL cylinder: Least count = 2 mL

500 mL cylinder: Least count = 5 mL

**Activity 9.4 - Determining Smallest Reading of a Measuring Cylinder:**

**Procedure to Find Least Count:**

1. **Observe the measuring cylinder carefully**

2. **Maximum Capacity:**

Read the marking at the top

Example: 100 mL cylinder can measure up to 100 mL

3. **Find Volume Between Two Major Marks:**

Look at two consecutive big marked lines

Example: Between 10 mL and 20 mL mark

Volume difference = 20 - 10 = 10 mL

4. **Count Smaller Divisions:**

Count how many smaller division lines exist between the two major marks

Example: Between 10 mL and 20 mL, there are 10 small divisions

5. **Calculate One Division Value:**

Smallest volume readable = Total volume difference / Number of divisions

= 10 mL / 10 divisions

= 1 mL per small division

**Therefore:** For the 100 mL measuring cylinder, the **least count = 1 mL**

**Choosing the Right Measuring Cylinder:**

For measuring 70 mL of water:

| Cylinder Type | Can Measure 70 mL in One Step? | Accuracy | Reason |

|---------------|--------------------------------|----------|--------|

| 50 mL | No | - | Maximum capacity is only 50 mL; need two steps (50 mL + 20 mL) |

| 100 mL | Yes | High | Perfect capacity; least count = 1 mL |

| 250 mL | Yes | Lower | Least count = 2 mL; less accurate than 100 mL |

| 500 mL | Yes | Lowest | Least count = 5 mL; least accurate |

**Best Choice:** **100 mL measuring cylinder** because:

Can measure 70 mL in one step

Has smallest least count (1 mL) = highest accuracy

Not too large (which would reduce accuracy)

**Activity 9.5 - Measuring 50 mL of Water:**

**Procedure:**

1. **Prepare the Measuring Cylinder:**

Place the clean, dry measuring cylinder on a **flat, level surface**

Ensure it is standing straight and not tilted

2. **Pour Water Slowly:**

Pour water slowly into the cylinder

Continue until water level reaches near the 50 mL mark

Slow pouring prevents splashing and allows precise measurement

3. **Fine Adjustment:**

Use a dropper to add water drop by drop

Adjust water level to reach exactly the 50 mL mark

Or remove water if too much is added

4. **Observe the Meniscus:**

Look carefully at the water surface inside the cylinder

Water forms a **curved surface** at the top

This curved surface is called the **meniscus**

5. **Reading the Meniscus:**

For water (colorless liquid): Read at the **BOTTOM of the meniscus** (lowest point)

For colored/opaque liquids: Read at the **TOP of the meniscus**

This is the correct volume measurement

**Why Meniscus Forms:**

Water molecules are attracted to glass walls

Water climbs up the walls slightly, creating a curved surface

The bottom of the curve (meniscus) is the true liquid level

**Diagram to Draw - Measuring Cylinder with Meniscus:**

[Draw a tall, narrow cylinder with:

Scale markings on the side (0, 10, 20, 30, 40, 50 mL, etc.)

Water inside filling to about 50 mL

A curved surface (meniscus) at the top of water

Arrow pointing to "Bottom of meniscus" with text "Read here for water"

Label: "Measuring Cylinder"]

**Why are Measuring Cylinders Designed Tall and Narrow Instead of Short and Wide?**

Measuring cylinders are specifically designed to be **tall and narrow (cylindrical)** instead of short and wide for important reasons:

1. **Increased Accuracy:**

In a tall, narrow cylinder, the same volume change causes a **larger change in height**

Larger height change is easier to see and measure accurately

In a short, wide cylinder, the height change would be small and hard to read precisely

2. **Better Reading Precision:**

The tall shape allows small volume differences to be visible as noticeable height changes

Smallest divisions can be clearly marked and read

Reduces measurement error

3. **Stability:**

Tall, narrow design provides better stability

Less likely to tip over due to low center of gravity relative to height

Flat bottom base provides firm support

4. **Prevents Splashing:**

Narrow opening at top reduces splashing when pouring

Liquid stays contained within the cylinder

**Real-Life Example:**

The same principle is used in **water bottles in India** - often they are tall and cylindrical (like water dispensers, thermoses, or cooler bottles) rather than wide and short. This design:

Makes them easier to store in narrow kitchen spaces

Shows water level changes more visibly

Easier to pour water without spilling

More stable when standing

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COMPLETE SUMMARY OF KEY CONCEPTS

**Solutions and Mixtures:**

Solutions are uniform mixtures where solute dissolves completely in solvent

Non-uniform mixtures like sand in water are NOT solutions

Solutions appear as single substances but are actually homogeneous mixtures

**Saturation and Concentration:**

**Unsaturated:** More solute can still dissolve

**Saturated:** No more solute can dissolve at that temperature

**Concentration:** Amount of solute in fixed solvent/solution

**Dilute vs Concentrated:** Relative terms comparing amounts of solute

**Temperature Effects:**

Solid solute solubility: **INCREASES** with temperature rise

Gas solubility: **DECREASES** with temperature rise

A saturated solution becomes unsaturated if temperature increases

**Density Concept:**

Density = Mass / Volume

Independent of shape/size

Dependent on temperature and pressure

Relative density compares substance to water

Objects less dense than water float; more dense sink

Oil floats on water because it has lower density

**Measurement:**

Mass measured using balance in grams (g) or kilograms (kg)

Volume measured using measuring cylinder in mL or cm³

Least count of measuring cylinder depends on its size

Always read meniscus at bottom for water

Tall, narrow measuring cylinders give more accurate readings than wide ones

**Aquatic Life:**

Depends on dissolved oxygen in water

Cold water has more dissolved oxygen than warm water

Warm water, less oxygen → difficult for aquatic life

**Important Note:** All the definitions, formulas, procedures, and concepts described in this chapter should be understood clearly as they form the basis for understanding solutions, solubility, and density in higher classes.