Identifying Pure Substances, Mixtures and Solutions: Properties and Concentration
Identifying Characteristic Properties of Pure Substances
Pure substances can be identified by their qualitative properties, such as odor, color, physical state (solid, liquid, gas), crystalline form and hardness.
Their quantitative properties include density, melting temperature, boiling temperature, solubility and electrical conductivity.
Melting Point
Ice melts at 0 °C. A pure substance melts at a determined temperature, and while it is melting its temperature remains constant. If the substance is not pure, its melting occurs over a range of temperatures and the melting point decreases as impurities are added.
Boiling Point
A pure substance boils at a determined temperature that remains constant during boiling. If the pressure increases, the boiling temperature increases as well.
A substance is known to be pure if its melting and boiling temperatures remain constant during the change of state. Impurities cause the melting temperature to decrease and the boiling temperature of a liquid to increase, since a host of dissolved substances raises the boiling point.
Currently, chemists manufacture a large number of synthetic substances. For example, vanillin reproduces the aroma of natural vanilla.
Mixtures
A mixture consists of two or more substances combined together. The substances that form mixtures are called components. Mixtures are classified as heterogeneous and homogeneous.
Heterogeneous Mixtures
A mixture is heterogeneous when its components can be distinguished with the naked eye or with a magnifying lens or microscope. For example, granite rock lets us distinguish three minerals, and sand and water or oil and water are heterogeneous mixtures.
Heterogeneous dispersions, such as milk or mayonnaise, are mixtures that do not require a microscope to appreciate their heterogeneity. Others include milk, salsa, vinaigrette and shaving cream.
Homogeneous Mixtures
A mixture is homogeneous, or a solution, when its components cannot be distinguished, even with a microscope. For example, vinegar, water solutions of acid, and coffee with milk are homogeneous.
Solutions are formed when one or more substances are dissolved in another substance. The substance that dissolves is the solute, and the substance that does the dissolving is the solvent. When substances are miscible with each other (such as tincture of iodine, carbonated beverages in which carbon dioxide is dissolved in water, or gasoline, a solution of hydrocarbons), they form a solution. A substance is soluble if it can dissolve in a solvent, such as water.
Solubility
Solubility is the maximum amount of solute that can dissolve in 100 g of solvent. The solubility of a substance in a solvent generally increases with liquid temperature.
The points of the curve correspond to saturated solutions under the given conditions. Points above the curve correspond to supersaturated solutions, which occur when a solution is cooled slowly.
Most gases become less soluble in liquids as temperature increases. For liquids, increasing the pressure of the gas in contact with the liquid increases gas solubility, which is directly proportional to the gas pressure.
Diluted and Concentrated Solutions
Solutions can be diluted or concentrated. A diluted solution contains a small amount of solute relative to the amount of solvent, while a concentrated solution contains a large amount of solute in relation to the amount of solvent. The concentration, or composition of a solution, can be expressed in the following ways:
Percentage by mass: indicates the grams of solute in 100 g of solution. It is used when the solute is solid.
% mass = mass of solute / (mass of solute + mass of solvent) × 100
Percentage by volume: expresses the volume of solute in 100 volumes of solution. It is used when the solute and solvent are liquids or the solute is gaseous.
% volume = volume of solute / volume of solution × 100
Mass concentration: expresses the mass of solute dissolved in a unit volume of solution.
Mass concentration = mass of solute / volume of solution
ppm (parts per million): expresses the milligrams of solute per kilogram of solution. It is used for diluted solutions.