Which of the following is an oxide of metal?

When soap is scrubbed on a stain of curry on a white cloth,why does it become reddish-brown,and why does it turn yellow again when the cloth is washed with plenty of water?

The $pH$ of the gastric juices released during digestion is

What is the nature of the aqueous solution of lemon juice?

Which of the following solutions is most basic?

When $Zn$ metal is treated with a dilute solution of a strong acid,a gas is evolved,which is utilised in the hydrogenation of oil. Name the gas evolved. Write the chemical equation of the reaction involved and also write a test to detect the gas formed.