Solutions

A solution is a homogeneous mixture composed of a solute dissolved in a solvent. Solutions are uniform throughout and appear as a single substance, even though they contain two or more components.

Components of Solutions

Solute

The component that is dissolved in the solution. Usually present in smaller amount.

• Examples: Salt in saltwater, sugar in water, oxygen dissolved in water
• Can be: Solid, liquid, or gas

Solvent

The component that dissolves the solute. Usually the major component of the solution.

• Most common solvent: Water
• Other solvents: Alcohol, acetone, turpentine, cooking oil
• The medium in which solute dissolves

Solution

The homogeneous mixture of solute and solvent.

• Appearance: Uniform, single phase
• Properties: Uniform throughout
• Composition: Variable (the ratio of solute to solvent can change)

Characteristics of Solutions

• Homogeneous - Uniform appearance and composition
• Transparent - Usually clear, allowing light to pass through
• No Tyndall effect - Light passes through without scattering
• Do not settle - Components remain mixed indefinitely
• Cannot be separated by filtration - Particles are too small
• Single phase - Appears as one substance
• Stable - Components do not separate over time

Types of Solutions

By Physical State

Solid Solutions (Alloys)

• Solute and solvent are both solids
• Examples: Bronze (Cu in Sn), brass (Cu in Zn), steel (C in Fe)

Liquid Solutions

• Solute can be solid, liquid, or gas; solvent is liquid
• Examples: Saltwater, sugar solution, alcohol in water, oxygen in water

Gaseous Solutions

• Solute and solvent are gases
• Example: Air (mixture of N₂, O₂, Ar, CO₂, etc.)

By Concentration

Dilute Solution

• Contains a small amount of solute relative to solvent
• Example: Very weak tea or coffee

Concentrated Solution

• Contains a large amount of solute relative to solvent
• Example: Strong, dark tea or coffee, syrup

Saturated Solution

• Contains the maximum amount of solute that can dissolve at a given temperature
• No more solute can dissolve
• Any additional solute will not dissolve and settles at the bottom

Unsaturated Solution

• Contains less solute than the maximum amount that can dissolve
• More solute could be dissolved

Supersaturated Solution

• Contains more solute than normally possible at a given temperature
• Unstable and can crystallize if disturbed
• Created by cooling a hot saturated solution carefully

Solubility

Solubility is the maximum amount of a substance that can dissolve in a given amount of solvent at a specific temperature.

Factors Affecting Solubility:

Temperature

• For most solid solutes in water: Solubility increases with temperature
• Heating increases molecular motion, allowing more solute to dissolve
• Example: Sugar dissolves better in hot water than cold water
• Exception: For some gases in water, solubility decreases with temperature

Pressure

• Affects solubility of gases in liquids
• Increasing pressure increases gas solubility
• Example: CO₂ is more soluble in carbonated drinks under pressure

Nature of Solute and Solvent

• "Like dissolves like" - polar solvents dissolve polar solutes, non-polar solvents dissolve non-polar solutes
• Examples:

- Water (polar) dissolves salt and sugar (polar) - Oil (non-polar) dissolves in gasoline (non-polar) - Oil and water don't mix (one polar, one non-polar) Particle Size

• Finely ground solutes dissolve faster (but not more)
• Affects rate of dissolution, not solubility

Expressing Concentration

See the Concentration page for detailed information about expressing concentration using various methods.

Solubility Curves

Solubility Curves

A solubility curve is a graph showing how solubility changes with temperature.

Characteristics:

• X-axis: Temperature (°C)
• Y-axis: Solubility (g of solute per 100 g water)
• Most curves slope upward - Solubility increases with temperature for most solid solutes
• Used to predict solubility - At any temperature, you can read the solubility from the curve

Solubility Chart Visualization

Practice Questions

Example 1: NaCl (Salt)

• At 20°C, the solubility of NaCl is 36.0 g per 100 g water
• At 100°C, the solubility increases to 39.1 g per 100 g water
• Observation: Salt shows a slight increase in solubility with temperature

Example 2: KNO₃ (Potassium Nitrate)

• At 20°C, the solubility of KNO₃ is 37.0 g per 100 g water
• At 100°C, the solubility dramatically increases to 246 g per 100 g water
• Observation: Potassium nitrate shows a dramatic increase in solubility with temperature

Comparison Questions:

1. Which substance has greater solubility at room temperature (20°C)? (Salt and Potassium nitrate are nearly equal)
2. Which substance is more affected by temperature changes? (Potassium nitrate shows a much steeper curve)
3. How much more soluble is KNO₃ compared to NaCl at 100°C? (246 g vs 39.1 g - a difference of 206.9 g)

How to Read the Chart:

• Find the temperature at the top
• Find the substance on the left
• The number at the intersection is the solubility in grams per 100 g of water

Observations:

• NaCl shows relatively small change with temperature (nearly flat curve)
• KNO₃ shows dramatic increase with temperature (steep curve)
• Sugar is highly soluble and increases substantially with heat
• CuSO₄ shows a sudden large increase at high temperatures

Temperature Effects on Different Types of Substances

Most substances follow different solubility patterns with temperature:

Solids (Most Solutes) - INCREASE with Temperature

• Examples: KNO₃, sugar, CuSO₄, KI
• As temperature increases, solute molecules dissolve more easily
• Reason: Higher temperatures provide more molecular motion energy
• NaCl is an exception - shows almost no change with temperature

NaCl (Sodium Chloride) - NEARLY CONSTANT

• Shows almost no change from 0°C to 100°C
• Solubility remains around 35-39 g per 100g water
• This makes salt solutions useful across a wide temperature range
• Unusual behavior - most other ionic solids are temperature-dependent

Gases (Dissolved in Water) - DECREASE with Temperature

• Examples: NH₃, HCl, SO₂, O₂, CO₂
• As temperature increases, gases become LESS soluble
• Reason: Higher temperatures cause gas molecules to escape more easily
• Cold water can dissolve more gas than hot water
• This is why fish need cooler water (more dissolved oxygen)

Solubility Curve for Binary Compounds (Showing All Three Patterns)

Key Observations:

1. Solid Solutes (KI - red line) - Curves slope UPWARD

- Solubility increases significantly with temperature - More dramatic the increase, the more temperature-dependent

1. NaCl (blue line) - Almost FLAT

- Unique behavior - nearly independent of temperature - Solubility barely changes from 0°C to 100°C - Makes salt commercially important

1. Gaseous Solutes (NH₃, HCl, SO₂ - green, purple, yellow lines) - Curves slope DOWNWARD

- Solubility DECREASES as temperature increases - Cold water holds more dissolved gas than hot water - This is why carbonated drinks lose fizz when heated

Solving Solubility Problems Using the Chart

Ejemplo 1: Determining if a solution is saturated

At 20°C, is a solution containing 50 g of KNO₃ in 100 g of water saturated or unsaturated? Solution:

• From the chart: At 20°C, KNO₃ solubility = 37.0 g per 100 g water
• Actual amount dissolved: 50 g
• Since 50 g > 37.0 g, the solution is SUPERSATURATED
• The excess KNO₃ (50 - 37 = 13 g) cannot remain dissolved and will crystallize

Ejemplo 2: Predicting solubility changes with temperature

A saturated solution of NaNO₃ is made at 20°C with 96 g in 100 g water. What happens if we cool it to 0°C? Solution:

• At 20°C: 96 g is saturated (matches the chart value exactly)
• At 0°C: Maximum solubility = 87 g per 100 g water
• Since we have 96 g but can only dissolve 87 g at 0°C
• Amount that crystallizes: 96 - 87 = 9 g of crystals will form

Ejemplo 3: Finding temperature from solubility chart

At what temperature will 60 g of KCl dissolve in 100 g of water? Solution:

• Looking at the KCl row, find where solubility = 60 g
• This falls between 40°C (solubility 40.1 g) and 60°C (solubility 45.8 g)...
• Actually, 60 g exceeds even 100°C solubility (56.3 g)
• Therefore, 60 g of KCl cannot fully dissolve at any normal temperature
• We would need to heat above 100°C for this amount

Ejemplo 4: Comparing solubilities

Which substance is more soluble at 60°C: CuSO₄ or Sugar? Solution:

• At 60°C: CuSO₄ solubility = 40.8 g per 100 g water
• At 60°C: Sugar solubility = 288 g per 100 g water
• Since 288 > 40.8, Sugar is much more soluble (about 7 times more soluble)
• This explains why we can make very sweet solutions by dissolving sugar in hot water

why water disolves almos anything

The "Like Dissolves Like" Principle

Understanding how polarity determines what dissolves in what solvent:

Volume Additivity in Solutions

When mixing solutions, volumes don't always add up perfectly. Learn why: