The physical explanation is that increasing temperature increases the kinetic energy of the gas molecules. Hence, their random motion breaks more intermolecular bonds and the gas is less dissolved in the solvent. In contrast, solid solutes in water have increased solubility with increased temperatures.
        
                    
             
        
        
        
It is important to use the same balance throughout the entire experiment since the calibration of each balance is not the same and changing balances could result in a systematic error. 
There are three types of errors that could affect the results of the experiment. The effect of random or indeterminate errors is hard to predict, its effect on the results of the experiment could be different every time. The second type of error is the systematic or determinate error, which causes a shift in results in a specific direction. The last type of error in an experiment is human error. 
The type of error that could be related to the use of different balances throughout the experiment is the systematic error. Instruments could be a source of error especially if they are poorly calibrated. Also, analytical balances are calibrated differently which may result in inaccuracy in the weighing of chemicals.
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Granite and pebbles are the answer
        
                    
             
        
        
        
Electrons in an atom can be classified as core electrons and valence electrons. Valence electrons are those electrons which are present in valence shell and participates in bond formation. While, Core electrons are all remaining electrons which are not present in valence shell, hence not take part in bonding.
Atomic number of Selenium (Se) is 34 hence it has 34 electrons with following electronic configuration;
                          1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p⁴
From electronic configuration it is found that the valence shell is 4, and the number of electrons present in valence shell are 6. So, 
             Core Electrons  =  Total Electrons - Valence Electrons
             Core Electrons  =  34 - 6
             Core Electrons  =  28
Result:
          There are 28 core electrons in Selenium.