Group I properties

Two trends are routinely reversed: melting point said to increase down the group (importing a "bigger atom, higher melting point" idea that does not apply here), and reactivity with water said to decrease. Fix the directions once and use them together: from lithium down to potassium melting point falls, density generally rises, and reactivity with water rises.

"Describe how the properties change down Group I" normally carries a mark per trend, but candidates give reactivity alone -- the memorable one -- and drop melting point and density. Match the number of directed statements to the tariff: melting point decreases, density generally increases, reactivity with water increases. One trend reworded three ways scores once.

Calling an alkali metal simply "a very reactive metal" describes a consequence, not the definition, and earns little credit. Two structural parts are needed: the element is a metal in the first column (Group I) of the Periodic Table, and each atom has one electron in its outer shell, readily lost to form a 1+ ion. That outer electron is why the group behaves alike.

The expected trend is a general increase in density down Group I, not a perfectly regular one: potassium ($\approx 0.86\ \text{g/cm}^3$) is very slightly less dense than sodium ($\approx 0.97\ \text{g/cm}^3$). Say "general increase" whenever data is supplied, or the dip looks like a contradiction. Melting point and reactivity, by contrast, are regular across Li, Na and K.

"Explain why potassium reacts more vigorously than sodium" is not answered by repeating the observation ("it reacts faster", "it ignites") or by vague energy talk. The marks sit in the electron-structure chain: one more electron shell, so the outer electron is further from the nucleus and shielded by more inner shells, so it is held less strongly and lost more easily.

A predicted Group I element still forms a 1+ ion, so its hydroxide pairs one M$^+$ with one OH$^-$ in a 1:1 ratio: RbOH, CsOH -- never Rb$_2$OH or RbOH$_2$. Do not import a 2+ charge by analogy with another group. The same 1:1 logic is what keeps the equation at $2\text{M} + 2\text{H}_2\text{O} \rightarrow 2\text{MOH} + \text{H}_2$.

Group I reactivity increases down the group -- bigger atoms lose their single outer electron more easily. Group VII reactivity decreases down the group -- bigger atoms gain an extra electron less easily. Opposite directions for opposite reasons (losing versus gaining an electron). Borrowing the halogen trend is the usual source of a reversed Group I answer.