| Characteristics | Compounds | Mixtures |
|---|---|---|
| Composition | Made up of atoms of elements in a fixed proportion | Made up of elements, or compounds, or both in any proportion |
| Nature | Particles are of the same kind | Particles are of different kinds |
| Structure | Always homogeneous | May or may not be homogeneous |
| Appearance | Components cannot be seen separately | Components may or may not be seen separately |
| Preparation | Always involves a chemical change | Involves only physical change |
| Properties | Entirely different from those of the constituents | No property of their own Show the average properties of all the constituents |
| Separation | Components can be separated only by chemical means | Components can be separated by physical means |
| Energy changes | Energy is always evolved or absorbed | Generally no energy is evolved or absorbed |
The following example of iron, sulphur and iron sulphide will make us understand the difference between mixtures and compounds.
| Properties | Iron | Sulphur |
|---|---|---|
| Colour | Greyish black | Yellow |
| Action of magnet | Attracted | Not attracted |
| On stirring the mixture with water | Sinks, and forms the lower layer of iron | Sinks and forms a layer over the iron |
| Action of dilute acids | Dissolves, producing hydrogen | Does not dissolve |
| Action with carbon disulphide | Does not dissolve | Dissolves |
The following experiment shows the difference in properties between the elements iron and sulphur (Fig.4.1).
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All matter can be broadly divided into two major groups “Pure” and “Impure”. The term ‘purity’ has quite a different meaning in chemistry than in our day-to-day life. Normally when we refer to pure water, pure milk, etc., what is implied is that the water, milk etc., are free from harmful substances such as bacteria, fungi, viruses, etc. ‘Purity’ in chemistry is entirely of a different nature. When we say a substance is pure, it means that the substance is made of only one type of constituent particles.
Example: In chemical terms, pure water means that it is made of only one type of molecules i.e., H2O.
As mentioned above matter can be divided into pure and impure substances. The pure substances can be further divided into “Elements” and “Compounds”. The impure substances, commonly called “Mixtures” can also be divided further into ‘Homogeneous’ and ‘Heterogeneous’ mixtures.
Learn what catalysts are and how they affect the activation energy and reaction rate of a chemical reaction.
A catalyst is a chemical substance that affects the rate of a chemical reaction by altering the activation energy required for the reaction to proceed. This is called catalysis. A catalyst is not consumed by the reaction and it may participate in multiple reactions at a time. The only difference between a catalyzed reaction and an uncatalyzed reaction is that the activation energy is different. There is no effect on the energy of the reactants or the products. The ΔH for the reactions is the same.
Usually when someone refers to a catalyst, they mean a positive catalyst, which is a catalyst which speeds up the rate of a chemical reaction by lowering its activation energy. There are also negative catalysts or inhibitors, which slow the rate of a chemical reaction or make it less likely to occur.
A promoter is a substance that increases the activity of catalyst. A catalytic poison is a substance that inactivates a catalyst.
Catalysts permit an alternate mechanism for the reactants to become products, with a lower activation energy and different transition state. A catalyst may allow a reaction to proceed at a lower temperature or increase the reaction rate or selectivity. Catalysts often react with reactants to form intermediates that eventually yield the same reaction products and regenerate the catalyst. Note that the catalyst may be consumed during one of the intermediate steps, but it will be created again before the reaction is completed.
I’m teaching general chemistry this semester. Acids and bases are currently on our agenda, in particular how to assess the strength of an acid based on its molecular structure. When dissolved in water, strong acids, such as hydrochloric acid (HCl) or sulfuric acid (H2SO4) always transfer their protons (H) to water. For example: HCl + H2O → Cl– + H3O+. Weak acids result when only some acid molecules transfer their protons to water. Organic acids, containing only carbon, oxygen, hydrogen and nitrogen, are generally weak acids. The archetypical weak organic acid is acetic acid, better known as vinegar: CH3COOH. It’s not the simplest organic acid, that would be formic acid: HCOOH.
Formic acid was first characterized in the late 17th century. Naturalists had observed that the vapors emitted by ant hills were acidic (using the equivalent of litmus paper), and in 1671 John Ray extracted the pure acid by distilling the crushed remains of red ants. Formica is Latin for ant, hence the name translates pretty literally as “ant acid”. Formic acid is at least partially responsible for the sting in bee stings, ant bites and stinging nettles.
Even though chemists call formic acid weak, a 0.10 M solution has a pH of 2.4 (for comparison’s sake, the same concentration of HCl has a pH of 1.0).
http://cultureofchemistry.blogspot.com/2008/02/ant-acids.html
What does the p in pH stand for?
The term pH has been in use for more than a century. It is a logarithmic measure of the hydrogen ion concentration ([H+]): pH = -log10[H+]. (Technically, there aren’t bare protons (H+) floating around in solutions, but that wasn’t known when pH was introduced!) The original symbol used by Sorensen was pH+.
Theories vary as to the origin of the p – most agree it means power but whether in Latin, French or German, seems in dispute. Thinking it would be either French or Latin as the original paper was published in French, I was surprised to find that it’s neither, though the legend is both old and persistent. By 1920, many authors were assuming that it meant “power”, but Jens Norby returned to the original sources and points out that it was the arbitrary choice of the letters p and q for two variables in the work-up of the experimental data. The variable p eventually ends up in the formula arrived at for the concentration of the hydrogen ion.
The modern form pH was introduced in 1920, “as a matter of typographical convenience”.
For the full explanation, see Jens G. Norby, The origin and the meaning of the little p in pH, Trends in Biochemical Sciences 25, 36-37 (2000). The illustration is a selection from the original paper: Sorensen, Compt. redn. du Lab. de Carlsberg 8 1-168 (1909).
