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I hold a Ph.D. in Chemistry and a B.Sc. in Biochemistry from the University of Sussex, England. I am a professional stay-at-home mom to three kids. I love the wonderful world of Chemistry and its practicality. Prior experiences in research and industry will be the stimulus for helping others in their understanding of Chemistry.

Enzyme Catalysis in Chemical Transformations

Acid-Base Catalysis

Screengrab of animation of Acid-Base catalysis from the Physical Chemistry book

Enzymes are the catalysts of biological systems. They are remarkable molecular designs that determine the pattern of chemical transformations.

The most striking characteristics of enzymes are their catalytic power and specificity. By using the full gamut of intermolecular forces, enzymes bring substrates together in an optimum orientation. Continue reading

An Overview of Colligative Properties

Phase diagram of a solvent non-volatile solute system

Click to enlarge

The thermodynamics needed to treat the behaviour of solutions is explored in Chapter 5.8 of the Physical Chemistry book. The aim of this post is to use real life examples to explain the property of dilute solutions labelled colligative properties.

Colligative properties are an interesting scientific occurrence that can be applied in controlled environments (e.g. laboratory) as well as in day-to-day life.  Examples of colligative properties include freezing point, boiling point, vapour pressure and osmotic pressure.

What is a colligative property?

We can divide the properties of solutions into two main groups: colligative and non-colligative properties. A colligative property is a characteristic of a substance (such as its freezing point, boiling point, vapour pressure or osmotic pressure) that can be changed when a solute is added to it.

It is dependent only on the amount of dissolved particles in solution and it does not take into account their identity, structure or mass. In other words, irrespective of whether the substance is salt, sugar or gasoline – it will behave the same way.

Non-colligative properties on the other hand, depend on the identity of the dissolved species and the solvent.

Freezing Point Depression

It is common knowledge that salt applied dry is not efficient at melting ice in the winter until it is dissolved into brine. This process can be explained in terms of colligative properties. Once salt is dissolved in water, the solute (in this case salt brine) will determine its freeze-point lowering potential or freezing point depression.

Any substance that dissolves in water has this effect. In other words, adding another compound or substance to a liquid result in a phenomenon called freezing point depression, whereby the solution has a lower freezing point than that of the pure solvent. Similarly, this occurrence can be applied to explain why seawater has a lower freezing point than that of pure water.

Boiling Point Elevation

A common example of boiling point elevation can be observed when salt is added to water; thus increasing the boiling point of water. Boiling point elevation occurs when the boiling point of a solution becomes higher than the boiling point of a pure solvent. The temperature at which the solvent boils is increased by adding any non-volatile solute.

Vapour Pressure Lowering

Again we can explain vapour pressure lowering as a colligative property of matter because the vapour pressure of a liquid will decrease when another substance is dissolved in it. The decrease will be proportional to the quantity of the added substance.

We can analyse why this occurs. By adding substance to a liquid, the number of liquid molecules at the surface to escape and produce pressure will be fewer, since the surface now contains a mix of liquid and added substance. Therefore the solvent molecules will have a lower probability to escape the solution than the pure solvent. This fact is reflected in the lower vapour pressure for a solution relative to the pure solvent.

Osmotic Pressure

Measuring Osmotic Pressure

Osmotic pressure arises when there is a concentration difference between two solutions separated by a semi-permeable membrane. The (osmotic) pressure is the force of a liquid against this special membrane. The pressure required to stop osmosis is called the osmotic pressure (illustrated above in Figure 5.16).

For instance, when a cell is added to an aqueous solution with a much higher concentration of ions than the liquid within the cell, water leaves the cell by flowing through the cell membrane until the cell shrinks and ultimately breaks the membrane. Alternatively, when a cell is placed in a solution that has a much lower ionic strength, water pours into the cell, and the cell expands until the cell membrane bursts.

These are examples of colligative properties of matter which means that it depends on the number of particles in a solution (its concentration) and not on the structure of the particles or their mass. These properties ideally depend on changes in the entropy of the solution on dissolving the solute. The rationale for these colligative properties is the increase in entropy on mixing solutes with the water.

Digging into Phase Diagrams & Cooling Curves

Thermal Analysis

Thermal Analysis to determine a phase diagram (click to enlarge)

Condensed Binary Systems and  Thermal Analysis  are described in sections 6.4 and 6.5 of the Physical Chemistry textbook.

While most of us may not realise it, phase and phase transformations are ubiquitous in everyday life. The most simplistic illustration is a saucepan filled with boiling water – here the bubbles of steam formed by the water changes from liquid to vapour phase. Continue reading

A look into Gibbs Energy

Conditions for chemical equilibrium
The Gibbs Energy, G is a function of state that provides useful information for deciding whether or not a change of any kind will tend to occur. It is defined as G = HTS where H, T and S are the enthalpy, temperature and entropy. This ubiquitous subject is reviewed in Chapters 3.6 and 3.7 of the Physical Chemistry book. Continue reading

Entropy and the Second Law of Thermodynamics

Spontaneous Process

So far we have learnt from Chapter 2 of the Physical Chemistry book that the First Law of Thermodynamics is essentially the law of conservation of energy, i.e. energy can neither be created nor destroyed but it may change from one form to another. Continue reading

Try our bestselling undergraduate Physical Chemistry courseware

Thermodynamics Module - Physical Chemistry
Thermodynamics module
Chapters 1 to 6 of Physical Chemistry - Laidler, Meiser, Sanctuary

Includes multimedia that opens on relevant pages and allows the student to visualize many of the concepts by varying parameters and plotting different graphs. Things students often have difficulty with, such as isothermal, isobaric, isochoric and adiabatic process, are clearly visualized.

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Physical Chemistry - Laidler, Meiser, Sanctuary
Physical Chemistry textbook
by Laidler, Meiser, Sanctuary

This popular Physical Chemistry text book is now available in electronic format. We have preserved much of the material of the former hard copy editions, making changes to improve understanding of the concepts in addition to including some of the recent discoveries in physical chemistry. Many chapters have new sections and the coverage of several chapters has been greatly expanded.

Get it from: Physical Chemistry - Laidler, Meiser, Sanctuary

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