Views: 328

MATTER

1. State four differences between a  physical and  a chemical change

Chemical change is a process that causes a substance to change into a new substance with a new chemical formula.

 chemical reaction is  a process involving the breaking or making of interatomic bonds and the transformation of a substance (or substances) into another. Chemical changes cause a substance to change into an entirely substance with a new chemical formula. Chemical changes are also known as chemical reactions. The “ingredients” of a reaction are called reactants, and the end results are called products.

physical change is a process that does not cause a substance to become a fundamentally different substance. Physical changes only change the appearance of a substance, not its chemical composition.

2. Name the physical properties represented in the following changes (the symbol o represents a particle )

3. Differences Between Physical and Chemical Change

4. The diagram below  shows the relationship between the physical states of matter

 Identify the processes labeled a,b and c

5. Discuss the criteria for determining purity of a substance
6. Physical Comparison With a Pure Standard

One of the simplest ways to check the purity of any substance is to compare the substance with a certified pure sample. Even physical comparisons can reveal a lot about the purity of a sample. Visual comparison can reveal the presence of any large impurities, such as dirt or other differently colored impurities. If the substance is nontoxic, a smell test can be used to compare it with the pure sample. Any dissimilar odors indicate the presence of at least one impurity. If the substance is edible, a taste test can be conducted. A difference between the taste of the substance and the taste of the pure sample hints at the presence of impurities.

1. Melting and Boiling Point Determination

The physical properties of a substance can be used to establish its purity. These properties include the melting point and boiling point. Different substances tend to have different melting and boiling points, and any pure substance will have a specific melting and boiling point. However, the presence of impurities will cause a lower melting point as well as a change in boiling point.

1. Colorimetric Methods

There are many colorimetric methods for determining whether a substance is pure or if there are impurities present. These usually involve the use of a chemical for detecting the presence of common impurities, which will turn the chemical a certain color. These methods are simple and are usually designed to determine the presence of impurities, not to determine the amount or the percent purity of the substance. One common use of such colorimetric methods is in forensics, where color tests are frequently used to identify illegal drugs as well as to determine their purity.

1. Color is one of the more useful properties for identifying substances without doing any chemical or physical tests. A violet vapor, for example, is characteristic of iodine. A red/brown gas could well be bromine or nitrogen dioxide (NO2 ) A characteristic yellow/brown color in water may be indicative of organically bound iron.

The human eye responds to colors of electromagnetic radiation ranging in wavelength from about 400 nanometers (nm) to somewhat over 700 nm. Within this wavelength range humans see light; immediately below 400 nm is ultraviolet radiation, and somewhat above 700 nm is infrared radiation. Light with a mixture of wavelengths throughout the visible region, such as sunlight, appears white to the eye. Light over narrower wavelength regions has the following colors:  400–450 nm, blue; 490– 550 nm, green; 550–580 nm, yellow; 580–650 nm, orange, 650 nm–upper limit of visible region, red. Solutions are colored because of the light they absorb.

Red, orange, and yellow solutions absorb violet and blue light; purple solutions absorb green and yellow light; and blue and green solutions absorb orange and red light. Solutions that do not absorb light are colorless (clear); solids that do not absorb light are white

1. Density

The density is a much used criterion of the purity of liquid substances. It can be measured relatively easily and with high precision. If the density differs by more than 0.02 per cent from reliable published data , the substance cannot be considered to be pure.

1. Refractive Index

The refractive index is the most simply and quickly measurable constant of liquid substances ,and a very small amount of the substance is sufficient for its determination. This criterion of purity is therefore used whenever possible. For precise work in the field of phase equilibria we require that the refractive index should not differ from the literature values for the substance by more than ± 1 in the fourth place.

1. Specific Gravity

Often densities are expressed by means of specific gravity defined as the ratio of the density of a substance to that of a standard substance. For solids and liquids, the standard substance is usually water; for gases it is usually air. For example, the density of ethanol (ethyl alcohol) at 20˚C is 0.7895 g/ml.

The specific gravity of ethanol at 20˚C referred to water at 4˚C is given by

Specific gravity = density of ethanol   = 0.7895 g/ml    = 0.7895

1. Density of water         1.000 g/ml

For an exact value of specific gravity, the temperatures of the substances should be specified. In this case the notation of specific gravity of ethanol at 20˚/4˚ C shows that the specific gravity is the ratio of the density of ethanol at 20˚C to that of water at 4˚C

1. Viscosity

The viscosity is suitable criterion of purity particularly with substances that polymerize easily. It is very difficult to determine the absolute value of the viscosity with a precision greater than 1 per cent

Paper chromatography: (To separate substances) a drop of the substance is placed at the centre of a piece of filter paper and allowed to dry. Three or four more drops are added to it. Water is dripped on, drip by drip, so the ink spreads creating different coloured circles.

                   Paper + rings = chromatogram.

Rings are created because different substances travel at different rates. (To identify substances) Spots of substances placed onto a pencilled line (as ink would separate) which is called the origin, and labelled. Paper goes in solvent, and solvent travels up paper, then paper is taken out. There are spots which have travelled different distances.

–Interpreting  simple chromatograms:

Number of rings/dots = number of substances

If two dots travel the same distance up the paper they are the same

You can calculate the Rf value to identify a substance, given by the formula:

Rf value = distance moved by substance / distance moved by solvent

To make colourless substances visible you use a locating agent: 1. Dry paper in oven 2. Spray it with locating agent 3. Heat it for 10 minutes in oven.

The stationary phase is the material on which the separation takes place (e.g. the paper). The mobile phase consists of the mixture you want to separate, dissolved in a solvent.

1. Analytical Methods for Testing Purity

The most accurate means of determining the purity of a substance is through the use of analytical methods. These methods, widely used in different industries, mostly involve chemical analysis, which can pinpoint the presence, identity and amount of impurities in the sample. The most simple chemical methods include gravimetry and titration. There are also the more advanced light-based or spectroscopic methods, such as UV-VIS spectroscopy, nuclear magnetic resonance and infrared spectroscopy. Chromatographic methods, such as gas chromatography and liquid chromatography, can also be used. Other methods used in testing the purity include mass spectroscopy, capillary electrophoresis, optical rotation and particle size analysis..