A compound is a pure substance formed by the chemical combination of two or more different elements.
A compound may be splitted into simpler substances by chemical reactions, and has different properties to those of the elements that form it.
The composition of a compound is fixed: every piece of a compound has the same kind of atoms, bonded in the same way and proportion.
Some examples of compounds are H₂O, NaCl, H₂O₂, CH₃COOH. As you see, they have a chemical formula which states the kind and number of the atoms that form them.
They are different to mixtures, which are formed by two or more compounds, in a variable proportion, and can be separated by physical media. Some examples of mixtures are the solutions (e.g. NaCl dissolved in H₂O), and some solid mixtures (e.g. a mixture of marbles and sand).
Velocity is said to be constant if its magnitude as well direction at any instant is remains the same. In D, if you draw a line parallel to y-axis at any time t, you can see that velocity is same. Hence, D is the correct graph.
The kinetic energy of gaseous molecules is greater than that of liquid molecules. Therefore, in gas, kinetic energy overcomes the force of attraction between molecules. In short, in gas phase, particles move at high speed and hence they have less force of attraction. In case of liquid phase, particles are close enough as a result there is much more force of attraction compared to gaseous molecules. In liquid state, kinetic energy cannot overcome force of attraction therefore, liquid molecules slow down.
Therefore, B is the correct answer.
