Learning Objective

In this lesson we will learn about the properties of solutions and look at examples of different types of solutions. We will also learn how solubility is related to the components of a solution and how it is affected by temperature and pressure.

Learning Outcomes

By the end of this lesson you will be able to:

  • Define solute, solvent and solution.

  • Describe the process of dissolving.

  • Distinguish between the terms soluble and insoluble, with examples.

  • Distinguish between concentrated and dilute solutions.

  • Define solubility and saturation point.

  • Describe saturated and supersaturated solutions.

  • Explain how temperature and pressure affect the solubility of solids and gases in liquids.

 

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Dissolution

  • If salt is added to a cup of water and stirred, it seems to disappear. We know that it is still there, because the water now tastes salty, but we can no longer see the salt.
  • The salt hasn’t disappeared, it has just changed its form.
    The particles that make up the salt are still there, but they have been separated and interspersed with the water. Individually, they are too small to see.
    The salt is said to have dissolved in the water.
    This process is also known as dissolution.

 
dissolving salt in water

Dissolving salt in water

(Image: Chris 73, Wikimedia Commons)

 

Forming Solutions

  • When the salt and the water were combined, a type of mixture called a solution was formed.
    A solution is a mixture that is formed when a substance, known as a solute, is dissolved in another substance, known as a solvent.
  • A solution is a homogeneous mixture.
    This means that it has a uniform composition throughout.
    The solute particles have completely separated and dispersed evenly throughout the solvent.

 
formation of a solution dissolving solute dissolution

A solution is a homogeneous mixture that is formed when a solute dissolves in a solvent.

 

  • If the solute and solvent were weighed separately before the solution was made, we would find that the sum of their masses equals the mass of the solution.

 
mass of solution mixture

The mass of a solution equals the combined masses of the solvent and solute.

 

Coloured Solutions

  • Salt doesn’t impart any colour to a solution.
    However, if we dissolved copper sulfate crystals into a beaker of water, the solution would be blue, just like the crystals.
    The copper sulfate crystals are no longer visible, but they are still there and impart a blue colour to the solution, just as the salt particles impart a salty taste to the salt water solution.
  • Coloured solutions – like colourless solutions – are transparent, which means they are clear and light easily passes through them.

 

 copper sulfate powder  copper sulfate solution

Solutions may be colourless or coloured, but they are always transparent (clear).

(Images: Benjah-bmm27, Wikimedia Commons; LHcheM, Wikimedia Commons)

 

Soluble and Insoluble Substances

  • The salt and copper sulfate are able to dissolve in water because they are both soluble in water.
    Being soluble means that the water (solvent) is able to completely separate the salt and copper sulfate (solute) particles from each other.
  • However, not all substance are soluble in water.
    For example, if chalk powder or some sand was added to a beaker of water, no amount of stirring will get them to dissolve.
    The chalk and sand are both insoluble in water.

 

 magnesium sulfate epsom salts soluble  silicon dioxide sand insoluble

Left: Epsom salts (magnesium sulfate) are soluble in water.
Right: Sand (silicon dioxide) is insoluble in water.

(Image: Chemicalinterest, Wikimedia Commons; PDPics, Pixabay)

 

  • When an insoluble substance is added to a solvent, a heterogeneous mixture is formed.
    This means that it has a non-uniform composition.
    For example, when chalk powder is added to water, as soon as the stirring is stopped, it would settle to the bottom.

 

 calcium carbonate powder  calcium carbonate insoluble

Chalk powder (calcium carbonate) is insoluble in water.
It settles to the bottom of the tube, forming a heterogeneous mixture.

(Images: Walkerma, Wikimedia Commons; Danny S, Wikimedia Commons)

 

Effect of a Solute on Melting and Boiling Point

  • The formation of a solution results in changes in physical properties of the solvent, such as melting point and boiling point.
    For example, the addition of antifreeze to water results in both a lower freezing point and higher boiling point than pure water. This reduces the chance of water in car radiators freezing during winter and boiling when the engine gets hot.
    Similarly, the addition of salt to water lowers the freezing point of the water. By putting salt on icy roads and footpaths, the ice melts quicker, reducing the slip hazard.

 

 antifreeze melting boiling point  salt on icy road melt

Left: Radiator coolant contains antifreeze to prevent both freezing and boiling.
Right: Putting salt on ice causes the ice to melt faster.

(Images: EvelynGiggles, Wikimedia Commons; Michael Pereckas, Wikimedia Commons)

 

Concentrated and Dilute Solutions

  • A solution that has a high proportion of dissolved solute particles is said to be a concentrated solution.
    A solution that has a low proportion of dissolved solute particles is said to be a dilute solution.
  • For coloured solutions like copper sulfate solution, the more concentrated the solution, the darker it appears.
    For colourless solutions like salt water, concentrated and dilute solutions appear the same. (However, the more concentrated the salt water solution, the saltier would taste.)

 
concentrated and dilute solutions

A concentrated solution contains more dissolved solute than a dilute solution.

 

Saturated Solutions

  • Even if a substance is soluble, there is a limit to how much of it can be dissolved, for a given set of conditions.
    For example, if salt was continually added to a beaker of salt water, eventually there would be a point where no additional salt will dissolve, regardless of how much the solution is stirred.
    This point is known as the saturation point, and the solution is said to be saturated.
  • If any water was to evaporate from a saturated solution containing a dissolved solid, solute particles would start to come out of solution and form crystals.
    For example, this is how limestone cave formations, such as stalactites and stalagmites, are created. Water evaporates from solutions that are saturated in calcium carbonate and other minerals, resulting in the formation of elaborate solid limestone crystals.

 
limestone cave formations

Limestone cave formations crystalise from saturated solutions.

(Image: smadalsl, Pixabay)

 

Solubility

  • The amount of a solute that can dissolve in a particular solvent, before a solution becomes saturated, is referred to as its solubility.
  • Solubility depends on both the solute and the solvent.
    Therefore, they both need to be mentioned when giving a measure of solubilty.
    For example, a maximum of 360 g of salt can dissolve in 1 L of water, therefore we say that the solubility of salt in water is 360 g/L.

 
nail polish remover

Nail polish remover contains solvents such as acetone, which can dissolve nail polish.

(Image: Leticia Wilson, Adobe Stock)

 

Effect of Temperature on the Solubility of Solids

  • The amount of a solid solute than can dissolve is affected by temperature.
    Therefore, solubility values should also include specific temperatures.
    In the example above, the solubilty quoted for salt in water is for a temperature of 20 °C.
  • Generally, the solubility of solids increases with temperature.
    For example, the solubility of copper sulfate in water is 320 g/L at 20 °C, but increases to 620 g/L at 60 °C.

 
sugar dissolve hot cold water

Sugar will dissolve more readily in hot water than cold water.

(Image: rawpixel, Pixabay)

 

Effect of Temperature on the Solubility of Gases

  • Solutions can also be formed by dissolving gases in liquids.
    For example, oceans contain dissolved oxygen that allows fish and other organisms to survive under water.
  • The amount of a gas solute that can dissolve is also affected by temperature, but in the opposite way to solids.
    Generally, the solubility of gases decreases with temperature.
    For example, the solubility of carbon dioxide in water is 1.7 g/L at 20 °C, but decreases to 0.6 g/L at 60 °C.

 
warm ocean water

Warmer ocean water contains less dissolved gases, such as oxygen and carbon dioxide.

(Image: 12019, Pixabay)

 

Effect of Pressure on the Solubility of Gases

  • The solubility of gases (but not solids) is also affected by pressure.
    Therefore, in addition to temperatures, solubility values for gases should include specific pressures.
    In the example above, the solubilty quoted for carbon dioxide in water is for a pressure of 1 atm (normal atmospheric pressure).
  • Generally, the solubility of gases increases with pressure.
    For example, at 20 °C, the solubility of carbon dioxide in water increases from 1.7 g/L at 1 atm to 15 g/L at 2 atm.

 
carbonated water pressure carbon dioxide gas

Carbonated drinks, such as soda water, contain carbon dioxide gas that is forced into them under high pressure.

(Image: Eirik Newth, Pixabay)

 

Supersaturated Solutions

  • As described above, more solute can be dissolved in a saturated solution by increasing temperature (for solid solutes) or pressure (for gas solutes).
  • When a solution contains more solute than it can normally dissolve, it is said to be supersaturated.
  • However, as the temperature or pressure decreases, the extra solute will come out of solution.
    For a solution containing a solid solute, such as a salt solution, this will result in the formation of crystals.
    For a solution containing a gas solute, such as carbonated water, this will result in the gas escaping into the atmosphere.

 
soft drink soda bottle

Soft drink is supersaturated with carbon dioxide gas; it will eventually go flat if the lid is left off.

(Image: StockSnap, Pixabay)

 

  • Supersaturated sugar solutions are often used in making confectionary, such as toffee and caramel. The mixtures are heated to dissolve excess sugar, then cooled, resulting in crystallised sugar.
    This process can also be used to ‘grow’ copper sulfate crystals in the laboratory.

 

 toffee crystals  copper sulfate crystals

Toffee and copper sulfate crystals both result from the slow cooling of supersaturated solution.

(Images: Crystal Titan, Wikimedia Commons; Stratford490, Wikimedia Commons)

 

Types of Solutions

  • You may think of solutions as being liquids, but they can also be gases or solids.
    Similarly, solutes and solvent can be solids, liquids or gases.
    Some examples of solutions that aren’t formed from a solid dissolved in a liquid are listed below.
  • A gas dissolved in a gas.
    Example: Entonox.
    Entonox is a mixture of nitrous oxide (laughing gas) and oxygen, which is used for pain relief.

 
entonox laughing gas

Entonox

(Image: Owain.davies, Wikimedia Commons)

 

  • A gas dissolved in a liquid.
    Example: hydrochloric acid.
    Hydrochloric acid is formed by dissolving hydrogen chloride gas in water.

 
hydrochloric acid solution

Hydrochloric acid

(Image: Walkerma, Wikimedia Commons)

 

  • A liquid dissolved in a liquid.
    Example: hydrogen peroxide solution.
    Hydrogen peroxide solution is formed by dissolving hydrogen peroxide in water.

 
hydrogen peroxide solution

Hydrogen peroxide solution

(Image: Patcat88, Wikimedia Commons)

 

  • A solid dissolved in a solid.
    Example: brass.
    Metal alloys, such as brass, are formed by combining two metals, in this case copper and zinc; the metals are melted, then mixed and cooled back to a solid.

 
brass metal alloy

Brass (metal alloy)

(Image: schuetz-mediendesign, Pixabay)

 

  • A liquid dissolved in a solid.
    Example: amalgam.
    Amalgam is a type of metal alloy where mercury, which is a liquid, is combined with other metals, including silver, tin and copper.

 
dental amalgam filling

Dental amalgam

(Image: Kauzio, Wikimedia Commons)

 

  • Solutions may have more than one solute, and these may be a mixture of solid, liquid and gas.
    For example, soft drink has many ingredients, including sugar (solid), food colouring (liquid) and carbon dioxide (gas), all dissolved in water.

 
soft drink soda

Soft drink

(Image: brooklynphotoshoot, Pixabay)

 

Summary

  • A solution is a homogeneous mixture containing a solute dissolved in a solvent.
  • A solution has a different melting and boiling point to the original solvent.
  • When a substance (solute) dissolves, its particles separate and disperse evenly throughout the dissolving medium (solvent).
  • Only substances that are soluble in a particular solvent can dissolve and form solutions.
  • Insoluble substances cannot dissolve and result in heterogeneous mixtures when added to a solvent.
  • A concentrated solution has a high proportion of dissolved solute, whereas a dilute solution has a low proportion of dissolved solute.
    For coloured solutions, the colour intensity will reflect how concentrated or dilute the solution is.
  • Solubility refers to how much of a particular solute can dissolve in a given solvent.
  • Solubility depends on both the solute and the solvent.
  • Solubility is also affected by environmental factors such as temperature and pressure.
    Increased temperature results in increased solubility of solids in liquids.
    Increased temperature results in decreased solubility of gases in liquids.
    Increased pressure results in increased solubility of gases in liquids.
  • A saturated solution contains the maximum amount of solute dissolved in a solvent, at a particular temperature and pressure.
  • A supersaturated solution contains more solute than can normally be dissolved. These result from dissolving solids at increased temperature or gases at increased pressure.
  • Solutions can be gases liquids or solids.
    Similarly, solutes be can gases liquids or solids.
  • Solutions can contain multiple solutes.

 
chemical solutions laboratory glassware

(Image: bdyczewski, Pixabay)

 

(Header image: Petr Kratochvil, Public Domain Pictures)

 

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