Difference between revisions of "Mixture"
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In chemistry, a mixture is a material system made up by two or more different substance which are (mixed) together but are not combined chemically. Mixture refers to the physical combination of two or more substances the identities of which are retained. The molecules of two or more different substances are mixed in the form of alloys, solutions, suspensions, and colloids.
Mixtures are the product of a mechanical blending or mixing of chemical substances like elements and compounds, without chemical bonding or other chemical change, so that each ingredient substance retains its own chemical properties and makeup.[1] Nonetheless, despite there are no chemical changes to its constituents, the physical properties of a mixture, such as its melting point, may differ from those of the components. Some mixtures can be separated into their components by physical (mechanical or thermal) means. Azeotropes can be considered as a kind of mixtures which usually pose considerable difficulties regarding the separation processes required to obtain their constituents (physical or chemical processes or, even a blend of them).
Mixtures can be either homogeneous or heterogeneous. A homogeneous mixture is a type of mixture in which the composition is uniform. A heterogeneous mixture is a type of mixture in which the composition can easily be identified, as there are two or more phases present. Air is a homogeneous mixture of the gaseous substances nitrogen, oxygen, and smaller amounts of other substances. Salt, sugar, and many other substances dissolve in water to form homogeneous mixtures. A homogeneous mixture in which there is both a solute and solvent present is also a solution.
Types of mixtures
The following table shows the main properties of the three families of mixtures.
| Solution | Colloid | Dispersion | |
|---|---|---|---|
| Mixture homogeneity | Homogeneous | Visually homogeneous but microscopically heterogeneous | Heterogeneous |
| Particle size | < 1 nanometer | between 1 nanometer and 1 micrometer | > 1 micrometer |
| Physical stability | Yes | Yes | No: needs stabilizing agents |
| Tyndall effect | No | Yes | Yes |
| Separates by centrifugation | No | Yes | Yes |
| Separates by decantation | No | No | Yes |
The following table shows examples of the three families of mixtures.
| Dissolved or dispersed phase | Continuous medium | Solution | Colloid | Dispersion |
|---|---|---|---|---|
| Gas | Gas | Gas mixture: air (oxygen and other gases in nitrogen) | None | None |
| Liquid | Gas | None | Aerosol: fog, mist, vapor, hair sprays | Aerosol |
| Solid | Gas | None | Solid aerosol: smoke, cloud, air particulates | Solid aerosol: dust |
| Gas | Liquid | Solution: oxygen in water | Foam: whipped cream, shaving cream | Foam |
| Liquid | Liquid | Solution: alcoholic beverages | Emulsion: miniemulsion, microemulsion | Emulsion: milk, mayonnaise, hand cream |
| Solid | Liquid | Solution: sugar in water | Sol: pigmented ink, blood | Suspension: mud (soil, clay or silt particles are suspended in water), chalk powder suspended in water |
| Gas | Solid | Solution: hydrogen in metals | Solid foam: aerogel, styrofoam, pumice | Foam: dry sponge |
| Liquid | Solid | Solution: amalgam (mercury in gold), hexane in paraffin wax | Gel: agar, gelatin, silicagel, opal | Wet sponge |
| Solid | Solid | Solution: alloys, plasticizers in plastics | Solid sol: cranberry glass | Gravel, granite |
Physics and Chemistry
A heterogeneous mixture is a mixture of two or more compounds. Examples are: mixtures of sand and water or sand and iron filings, a conglomerate rock, water and oil, a salad, trail mix, and concrete (not cement). Gy's sampling theory [2] quantitatively defines the heterogeneity of a particle as:
- <math>h_i = \frac{(c_i - c_\text{batch})m_i}{c_\text{batch} m_\text{aver}} .</math>
where <math>h_i</math>, <math>c_i</math>, <math>c_\text{batch}</math>, <math>m_i</math>, and <math>m_\text{aver}</math> are respectively: the heterogeneity of the <math>i</math>th particle of the population, the mass concentration of the property of interest in the <math>i</math>th particle of the population, the mass concentration of the property of interest in the population, the mass of the <math>i</math>th particle in the population, and the average mass of a particle in the population.
During the sampling of heterogeneous mixtures of particles, the variance of the sampling error is generally non-zero.
Pierre Gy derived, from the Poisson sampling model, the following formula for the variance of the sampling error in the mass concentration in a sample:
- <math>V = \frac{1}{(\sum_{i=1}^N q_i m_i)^2} \sum_{i=1}^N q_i(1-q_i) m_{i}^{2} \left(a_i - \frac{\sum_{j=1}^N q_j a_j m_j}{\sum_{j=1}^N q_j m_j}\right)^2 .</math>
in which V is the variance of the sampling error, N is the number of particles in the population (before the sample was taken), q i is the probability of including the ith particle of the population in the sample (i.e. the first-order inclusion probability of the ith particle), m i is the mass of the ith particle of the population and a i is the mass concentration of the property of interest in the ith particle of the population.
It must be noted that the above equation for the variance of the sampling error is an approximation based on a linearization of the mass concentration in a sample.
In the theory of Gy, correct sampling is defined as a sampling scenario in which all particles have the same probability of being included in the sample. This implies that q i no longer depends on i, and can therefore be replaced by the symbol q. Gy's equation for the variance of the sampling error becomes:
- <math>V = \frac{1-q}{q M_\text{batch}^2} \sum_{i=1}^N m_{i}^{2} \left(a_i - a_\text{batch} \right)^2 .</math>
where abatch is the concentration of the property of interest in the population from which the sample is to be drawn and Mbatch is the mass of the population from which the sample is to be drawn.
References
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<references />, or <references group="..." />International Union of Pure and Applied Chemistry. "mixture". Compendium of Chemical Terminology Internet edition.ar:مزيج gn:Jehe'a bg:Смес ca:Mescla cs:Směs da:Kemisk blanding de:Gemisch el:Μίγμα es:Mezcla eo:Miksaĵo fa:مخلوط fr:Mélange gl:Mestura ko:혼합물 io:Mixuro id:Pencampuran it:Miscela (chimica) he:תערובת ht:Melanj etewojèn mk:Смеса ms:Campuran nl:Mengsel ja:混合物 pl:Mieszanina pt:Mistura qu:Chaqrusqa ru:Смесь (химия) simple:Mixture sk:Zmes sr:Смеше fi:Seos sv:Blandning tl:Halo th:การแบ่งประเภทของสารตามเนื้อสาร tr:Karışım ur:آمیزہ
zh:混合物- ↑ Atkins' Physical Chemistry, 7th Ed. by Julio De Paula, P.W. Atkins ISBN 0198792859
- ↑ Gy, P (1979) Sampling of Particulate Materials: Theory and Practice, Elsevier: Amsterdam, 431 pp.
