Newton’s 2nd law? Maybe I’m not completely sure
Answer:
Physical change because even though gas formation was observed, the water was undergoing a state change, which means that its original properties are preserved.
Explanation:
The gas formation seen is in fact the transition of the liquid water to a gas state, not the formation of a new gas.
Answer : The mass of reactant
remain would be, 0.20 grams.
Solution : Given,
Moles of
= 0.40 mol
Moles of
= 0.15 mol
Molar mass of
= 2 g/mole
First we have to calculate the limiting and excess reagent.
The balanced chemical reaction is,

From the balanced reaction we conclude that
As, 1 mole of
react with 2 mole of 
So, 0.15 moles of
react with
moles of 
From this we conclude that,
is an excess reagent because the given moles are greater than the required moles and
is a limiting reagent and it limits the formation of product.
The moles of reactant
remain = 0.40 - 0.30 = 0.10 mole
Now we have to calculate the mass of reactant
remain.


Therefore, the mass of reactant
remain would be, 0.20 grams.