I have spares of 700x25C tube for my old bike. I bought a new hybrid bike. Can I use them for my new bike with tyre size 700x35C?
What does the tube’s packaging say?
Usually they are specified to work for a relatively wide range of widths. It will says something like “18-622 to 25-622”.
A few percent above the spec usually works fine (e.g. using a 25mm width tube in a 28mm wide tyre). But if 25mm is the maximum for your tube then a 35mm tyre is probably way too wide and the tube is going to explode during the first full inflation.