Answer: When a substance is pure, it is composed of one type of molecule. For example, table salt is only composed of (more or less) salt molecules, while seawater has water and salt molecules. A more complicated example of a non - pure substance is soil. It has many different types of nutrients and compounds.
A doesn't change the model, that's what google's for, B can be easily used with the current model, and D is ridiculous.
The model of, for example, the atom, has changed over the years with multiple experiment sand multiple models. There was plum pudding, for example, with protons and electrons scattered throughout, until Rutherford's experiments (a new discovery) changed that because he found a dense nucleus.
The answer pops out at you
The rows in the top third - This group consists of elements like Sodium, Magnesium, Potassium and Calcium on the right and Chlorine, Carbon, Nitrogen and Oxygen on the left.
Sodium and Chlorine are components of salt, a very important compound of our blood, essential for transferring electrical signals from the brain to the rest of the body and vice versa. Calcium is the building block of our bones, while Magnesium and potassium ensure proper functioning of our organs.
