Mixture
In chemistry, a mixture is a material made up of two or more different substances which are physically combined. A mixture is the physical combination of two or more substances in which the identities are retained and are mixed in the form of solutions, suspensions and colloids.
Mixtures are one product of mechanically blending or mixing chemical substances such as elements and compounds, without chemical bonding or other chemical change, so that each ingredient substance retains its own chemical properties and makeup. Despite the fact that 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 using physical means. Azeotropes are one kind of mixture that usually poses considerable difficulties regarding the separation processes required to obtain their constituents.
Mixture characteristics =Characteristics of mixtures
Mixtures can be either homogeneous or heterogeneous. A mixture in which its constituents are distributed uniformly is called homogeneous mixture, such as salt in water. A mixture in which its constituents are not distributed uniformly is called heterogeneous mixture, such as sand in water.
One example of a mixture is air. 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. Mixtures can have any amounts of ingredients.
Mixtures are unlike chemical compounds, because:
- The substances in a mixture can be separated using physical methods such as filtration, freezing, and distillation.
- There is little, see Enthalpy of mixing, or no energy change when a mixture forms.
- Mixtures have variable compositions, while compounds have a fixed, definite formula.
- When mixed, individual substances keep their properties in a mixture, while if they form a compound their properties can change.
| Dispersion medium | Dissolved or dispersed phase | Solution | Colloid | Suspension |
| Gas | Gas | Gas mixture: air | ||
| Gas | Liquid | Liquid aerosol: fog, mist, vapor, hair sprays | Spray | |
| Gas | Solid | Solid aerosol: smoke, ice cloud, air particulates | Dust | |
| Liquid | Gas | Solution: oxygen in water | Liquid foam: whipped cream, shaving cream | Sea foam, beer head |
| Liquid | Liquid | Solution: alcoholic beverages | Emulsion: milk, mayonnaise, hand cream | Vinaigrette |
| Liquid | Solid | Solution: sugar in water | Liquid sol: pigmented ink, blood | Suspension: mud, chalk powder suspended in water |
| Solid | Gas | Solution: hydrogen in metals | Solid foam: aerogel, styrofoam, pumice | Foam: dry sponge |
| Solid | Liquid | Solution: amalgam, hexane in paraffin wax | Gel: agar, gelatin, silicagel, opal | Wet sponge |
| Solid | Solid | Solution: alloys, plasticizers in plastics | Solid sol: cranberry glass | Clay, silt, sand, gravel, granite |
Physics and chemistry
A heterogeneous mixture is a mixture of two or more chemical substancess. Examples are: mixtures of sand and water or sand and iron filings, a conglomerate rock, water and oil, a portion salad, trail mix, and concrete. A mixture of powdered silver metal and powdered gold metal would represent a heterogeneous mixture of two elements.Making a distinction between homogeneous and heterogeneous mixtures is a matter of the scale of sampling. On a coarse enough scale, any mixture can be said to be homogeneous, if the entire article is allowed to count as a "sample" of it. On a fine enough scale, any mixture can be said to be heterogeneous, because a sample could be as small as a single molecule. In practical terms, if the property of interest of the mixture is the same regardless of which sample of it is taken for the examination used, the mixture is homogeneous.
Gy's sampling theory quantitavely defines the heterogeneity of a particle as:
where,,,, and are respectively: the heterogeneity of the th particle of the population, the mass concentration of the property of interest in the th particle of the population, the mass concentration of the property of interest in the population, the mass of the th particle in the population, and the average mass of a particle in the population.
During 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:
in which V is the variance of the sampling error, N is the number of particles in the population, q i is the probability of including the ith particle of the population in the sample, 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.
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:
where abatch is that 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.