Liquid assets

I assume u mean income(by way of interest)  from investment in liquid asset

Interest income from investment in liquid asset will usually come under income from Other Source since it is investment of surplus funds and not incidental to business.(In case of financial co. then PGBP)

On the other hand if u mean income from sale of liquid investment - then it will be classified under Capital Gains. Such income is classified under PGBP only when the company is engaged in financial services or having regard to relevant facts of the case the intention was to earn income over a short term, with large volume of transaction by way of purchase and sale etc... 

Hope this was useful