Ggtkuk

Why this table goes outside the page:

    documentclass[11pt,twoside,a4paper]{report}

    usepackage{graphicx}



        usepackage{makeidx}

        usepackage{multirow}

        usepackage{tikz}

        usepackage{array,booktabs,siunitx}

        usepackage[italian]{babel}

        usepackage[T1]{fontenc}

        makeindex

        begin{document}

        tableofcontents

        printindex

               begin{table}[htbp]

begin{tabular}{|c|c|c|c|c|}

toprule

T $^circ$ C & 6 $^circ$ C & 20 $^circ$ C & 37 $^circ$ C \

toprule

Solvente & conc. (%p/V) & contenuto fase liq. (%) & contenuto fase liq. (%) & contenuto fase liq.(%)  \

midrule

multirow{4}{*}{Water}& 1 & 84.3 $pm$ 0.1 & 99.2 $pm$ 0.1 & 98.9 $pm$ 0.1\

& 2 & 84.3 $pm$ 0.1 & - & 98.9 $pm$ 0.2 \

& 3 & 84.3 $pm$ 0.1 & 99.2 $pm$ 0.1 & - \

& 4 & 84.5 $pm$ 0.1 & 90 $pm$ 5 & 83.3 $pm$ 0.2\

midrule

multirow{4}{*}{Saline} &1 &  99.1 $pm$ 0.1 & 98.9 $pm$ 0.1 & 99.3 $pm$ 0.1\

& 2 & 99.2 $pm$ 0.1 & 99.1 $pm$ 0.1 & 99.4 $pm$ 0.2 \

& 3 & 99.1 $pm$ 0.1 & 98.9 $pm$ 0.2 & 99.5 $pm$ 0.1 \

& 4 & 99.2 $pm$ 0.1 & 99.1 $pm$ 0.1 & 98.8 $pm$ 0.2 \

midrule

multirow{4}{*}{PBS } & 1 & 99.1 $pm$ 0.1 & - & 99.5 $pm$ 0.1 \

& 2 & 99.0 $pm$ 0.2 & - & 99.3 $pm$ 0.1\

& 3 & 99.4 $pm$ 0.1 & 99.7 $pm$ 0.1 & 99.5 $pm$ 0.1\

& 4 & 99.4 $pm$ 0.1 & 99.0 $pm$ 0.1 & 99.2 $pm$ 0.2\

bottomrule

end{tabular}

caption{emph{Rapporto liquido/solido per i campioni in acqua, in soluzione fisiologica e in tampone fosfato alle concentrazioni del 1, 2, 3, 4% p/V, alle temperature di 6, 20, 37$^circ$ C}}

end{table}

You asked,

Why [does] this table goes outside the page?

It's mostly because the header cells contains a lot of (repeated) information and no intra-cell line breaks are allowed.

The most appealing solution, in my opinion, consists of reorganizing and rearranging the header cells to get rid of the repetitive material.

I would also get rid of all vertical lines. They're not needed -- really!

documentclass[11pt,twoside,a4paper]{report}

    %%% (commented out all packages not needed for this example)

    %usepackage{graphicx}

    %usepackage{makeidx}

    %usepackage{multirow}

    %usepackage{tikz}

    usepackage{array,booktabs,siunitx}

    usepackage[italian]{babel}

    usepackage[T1]{fontenc}

    %makeindex

    begin{document}

    %tableofcontents

    %printindex



        begin{table}[htbp]

        centering

        begin{tabular}{@{} ccccc @{}}

        toprule

        conc. (%p/V)& multicolumn{3}{c}{Contenuto fase liq. (%)} & Solvente \

        cmidrule(lr){2-4}

        & SI{6}{celsius} & SI{20}{celsius} & SI{37}{celsius} \

        midrule

        1 & 84.3 $pm$ 0.1 & 99.2 $pm$ 0.1 & 98.9 $pm$ 0.1 & water\

        2 & 84.3 $pm$ 0.1 & -- & 98.9 $pm$ 0.2 & water \

        3 & 84.3 $pm$ 0.1 & 99.2 $pm$ 0.1 & -- & water \

        4 & 84.5 $pm$ 0.1 & 90 $pm$ 5 & 83.3 $pm$ 0.2 & water\ addlinespace

        1 & 99.1 $pm$ 0.1 & 98.9 $pm$ 0.1 & 99.3 $pm$ 0.1 & saline \

        2 & 99.2 $pm$ 0.1 & 99.1 $pm$ 0.1 & 99.4 $pm$ 0.2 & saline\

        3 & 99.1 $pm$ 0.1 & 98.9 $pm$ 0.2 & 99.5 $pm$ 0.1 & saline\

        4 & 99.2 $pm$ 0.1 & 99.1 $pm$ 0.1 & 98.8 $pm$ 0.2 & saline\ addlinespace

        1 & 99.1 $pm$ 0.1 & -- & 99.5 $pm$ 0.1 & PBS\

        2 & 99.0 $pm$ 0.2 & -- & 99.3 $pm$ 0.1 & PBS \

        3 & 99.4 $pm$ 0.1 & 99.7 $pm$ 0.1 & 99.5 $pm$ 0.1 & PBS\

        4 & 99.4 $pm$ 0.1 & 99.0 $pm$ 0.1 & 99.2 $pm$ 0.2 & PBS \

        bottomrule

        end{tabular}

        caption{Rapporto liquido/solido per i campioni in acqua, in soluzione fisiologica e in tampone fosfato alle concentrazioni del 1, 2, 3, 4% p/V, alle temperature di 6, 20, e SI{37}{celsius}.}

        end{table}

end{document} 

To exemplify (I hope!) the above comments, I would try

documentclass[11pt,twoside,a4paper]{report}

    usepackage{array,booktabs,siunitx,rotating}

    usepackage[italian]{babel}

    usepackage[T1]{fontenc}

    begin{document}

begin{table}[p]

        begin{tabular}{ccccc}

        toprule

        T $^circ$ C & 6 $^circ$ C & 20 $^circ$ C & 37 $^circ$ C \

        conc. (%p/V) & multicolumn{1}{p{2cm}}{centering contenuto fase liq. (%)} & multicolumn{1}{p{2cm}}{centering 

        contenuto fase liq. (%)} & multicolumn{1}{p{2cm}}{centering contenuto fase liq.(%)} & solvente \

        midrule

        1 & 84.3 $pm$ 0.1 & 99.2 $pm$ 0.1 & 98.9 $pm$ 0.1 & water\

        2 & 84.3 $pm$ 0.1 & - & 98.9 $pm$ 0.2 & water \

        3 & 84.3 $pm$ 0.1 & 99.2 $pm$ 0.1 & - & water \

        4 & 84.5 $pm$ 0.1 & 90 $pm$ 5 & 83.3 $pm$ 0.2 & water\

        1 & 99.1 $pm$ 0.1 & 98.9 $pm$ 0.1 & 99.3 $pm$ 0.1 & saline \

        2 & 99.2 $pm$ 0.1 & 99.1 $pm$ 0.1 & 99.4 $pm$ 0.2 & saline\

        3 & 99.1 $pm$ 0.1 & 98.9 $pm$ 0.2 & 99.5 $pm$ 0.1 & saline\

        4 & 99.2 $pm$ 0.1 & 99.1 $pm$ 0.1 & 98.8 $pm$ 0.2 & saline\

        1 & 99.1 $pm$ 0.1 & - & 99.5 $pm$ 0.1 & PBS\

        2 & 99.0 $pm$ 0.2 & - & 99.3 $pm$ 0.1 & PBS \

        3 & 99.4 $pm$ 0.1 & 99.7 $pm$ 0.1 & 99.5 $pm$ 0.1 & PBS\

        4 & 99.4 $pm$ 0.1 & 99.0 $pm$ 0.1 & 99.2 $pm$ 0.2 v PBS \

        bottomrule

        end{tabular}

caption{Rapporto liquido/solido per i campioni in acqua, in soluzione fisiologica 

e in tampone fosfato alle concentrazioni del 1, 2, 3, 4% p/V, alle temperature di 6, 20, 37$^circ$ C}

end{table}

end{document}

mostly off-topic, as variation of nice @Mico answer. exploited is separate-uncertainty=true for shorter typing of numbers in S columns:

documentclass[11pt,twoside,a4paper]{report}

usepackage[italian]{babel}

usepackage[T1]{fontenc}

usepackage{array,booktabs,siunitx}



begin{document}

    begin{table}[htbp]

    centering

begin{tabular}{c

           *{3}{S[table-format=2.1(1),

                  separate-uncertainty=true,

                  output-decimal-marker={,}]}

                l}

toprule

T si{celsius}

    &   multicolumn{3}{c}{contenuto fase liq. (%)}

                                        &   solvente\

    cmidrule{2-5}

conc. (%p/V)

    & SI{6}{celsius}

                & SI{20}{celsius}

                            & SI{37}{celsius}     \

    midrule

1   & 84.3(1)  & 99.2(1)   & 98.9(1)   & water      \

2   & 84.3(1)    & {--}      & 98.9(2)   & water    \

3   & 84.3(1)    & 99.2(1)   & {--}      & water    \

4   & 84.5(1)    & 90(5)     & 83.3(2)   & water    \

1   & 99.1(1)    & 98.9(1)   & 99.3(1)   & saline   \

2   & 99.2(1)    & 99.1(1)   & 99.4(2)   & saline   \

3   & 99.1(1)    & 98.9(2)   & 99.5(1)   & saline   \

4   & 99.2(1)    & 99.1(1)   & 98.8(2)   & saline   \

1   & 99.1(1)    & {--}      & 99.5(1)   & PBS      \

2   & 99.0(2)    & {--}      & 99.3(1)   & PBS      \

3   & 99.4(1)    & 99.7(1)   & 99.5(1)   & PBS      \

4   & 99.4(1)    & 99.0(1)   & 99.2(2)   & PBS      \

    bottomrule

end{tabular}

caption{Rapporto liquido/solido per i campioni in acqua, in soluzione fisiologica e in tampone fosfato alle concentrazioni del 1, 2, 3, SI{4}{% p/V}, alle temperature di 6, 20, SI{37}{celsius}.}

end{table}

end{document}