Measurement as quantitative observation

The purpose of each laboratory exercise in this book is to observe and measure characteristics of a biomolecule or a biological system. The characteristic is often quantitative, a single number or a group of numbers. These measured characteristics may be the molecular weight of a protein, the pH of a buffer solution, the absorbance of a colored solution, the rate of an enzyme-catalyzed reaction, or the radioactivity associated with a molecule. If you measure a quantitative characteristic many times under identical conditions, a slightly different result will most likely be obtained each time. For... 

Making observations. Observations may be qualitative (the sky is blue water is a liquid) or quantitative (water boils at 100°C a certain chemistry book weighs 2 kilograms). A qualitative observation does not involve a number. A quantitative observation (called a measurement) involves both a number and a unit. 

Making observations is fundamental to all science. A quantitative observation, or measurement, always consists of two parts a number and a scale (a unit). Both parts must be present for the measurement to be meaningful. 

After a qualitative analysis, the next question that you might ask is how much of each of the elements is present. For sucrose, the answer to that question is that 100 g of sucrose contains 42.1 g of carbon, 51.4 g of oxygen, and 6.5 g of hydrogen. This is a quantitative expression of composition. A quantitative observation is one that uses measurement You make quantitative measurements every day when you answer such questions as Whafs the temperature How long was the pass How much do you weigh Figure 1.9 shows some quantitative measurements being made. 

A quantitative observation is called a measurement and consists of a number and a unit. 

A measurement is a quantitative observation involving both a number and a unit. What is a qualitative observation What are the SI units for mass, length, and volume What is the assumed uncertainty in a number (unless stated otherwise) The uncertainty of a measurement depends on the precision of the measuring device. Explain. 

An analyte is simply the chemical species whose properties we are trying to measure in an analytical chemistry experiment. Most often, analytical chemists are interested in measuring the concentration of an analyte present in a sample. The measurement of an analyte concentration is a quantitative observation. 

Molecular fluorescence and, to a lesser extent, phosphorescence have been used for the direct or indirect quantitative analysis of analytes in a variety of matrices. A direct quantitative analysis is feasible when the analyte s quantum yield for fluorescence or phosphorescence is favorable. When the analyte is not fluorescent or phosphorescent or when the quantum yield for fluorescence or phosphorescence is unfavorable, an indirect analysis may be feasible. One approach to an indirect analysis is to react the analyte with a reagent, forming a product with fluorescent properties. Another approach is to measure a decrease in fluorescence when the analyte is added to a solution containing a fluorescent molecule. A decrease in fluorescence is observed when the reaction between the analyte and the fluorescent species enhances radiationless deactivation, or produces a nonfluorescent product. The application of fluorescence and phosphorescence to inorganic and organic analytes is considered in this section. 

Phase transitions in overlayers or surfaces. The structure of surface layers may undergo a transition with temperature or coverage. Observation of changes in the diffraction pattern gives a qualitative analysis of a phase transition. Measurement of the intensity and the shape of the profile gives a quantitative analysis of phase boundaries and the influence of finite sizes on the transition. ... 

The simplest technique introduced by Young as early as 1805 [18] is the measurement of the contact angle as a measure of surface tension and surface energy [1,19, 20,21], In many cases this gives an indication of surface composition and can be used to observe changes in composition, structure and/or roughness at the surface during a particular surface treatment. A quantitative description or distinction between different parameters is hardly possible in most cases. 

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