Answer:
The two observations we made from viewing the solar system model are as follows as:
1). When we look at the sky we observe that the motions of all the planets and the stars are perfect circular movements with a good and high speed, but not in velocity because in velocity direction doesn't charges.
2). The sun is at the centre of our solar system and all the planets are equidistant to each other planets and also revolve around the sun.
Answer:
Both molarity and formality express concentration as moles of solute per liter of solution. Formality is a substance's total concentration in solution without regard to its specific chemical form. ... The formality of a solution is defined as the number of formula mass of any solute dissolved in 1 litre of solution.
Answer : Yes, a precipitate form when a solution of calcium chloride and a solution of mercury(I) nitrate are mixed together.
The net ionic equation will be,

Explanation :
In the net ionic equations, we are not include the spectator ions in the equations.
Spectator ions : The ions present on reactant and product side which do not participate in a reactions. The same ions present on both the sides.
The given balanced ionic equation will be,

The ionic equation in separated aqueous solution will be,

In this equation,
are the spectator ions.
By removing the spectator ions from the balanced ionic equation, we get the net ionic equation.
The net ionic equation will be,

The answer is: [D]: a reactant.
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