Chiudo bene la bustina perché la caramella non si sciolga nello zaino.

Bustina is a diminutive of busta, so it means a small bag, little packet, sachet, or small pouch, depending on context. In this sentence, it suggests a small wrapper or packet that contains the candy.

The ending -ina often adds the idea of small size, and sometimes a slightly familiar or affectionate tone.

Chiudo means I close. It is the first-person singular present indicative of chiudere.

Italian often leaves out subject pronouns when they are clear from the verb ending, so io is omitted here.
So:

• Chiudo = I close
• Io chiudo = also possible, but less natural unless you want emphasis

Bene means well, properly, or in this context something like securely / tightly enough.

Italian adverbs like bene often come after the verb:

• Chiudo bene la bustina = I close the little bag properly

So the idea is not just I close it, but I make sure it is closed well.

Here perché means so that or in order that.

That is a very common point of confusion, because perché can mean both:

• because
• so that

You tell the difference from the context and, very often, from the verb mood that follows.

In this sentence, it introduces a purpose:

• Chiudo bene la bustina perché la caramella non si sciolga...
• I close the little bag well so that the candy doesn’t melt...

If it meant because, the structure would usually be different and would normally use the indicative, not the subjunctive.

Because after perché meaning so that / in order that, Italian normally uses the subjunctive.

So:

• si sciolga = present subjunctive
• si scioglie = present indicative

The speaker is expressing a goal or intended result, not stating a simple fact. That is why the subjunctive is used.

A useful contrast:

• Chiudo bene la bustina perché la caramella non si sciolga.
  = I close the little bag well so that the candy won’t melt.

• Chiudo bene la bustina perché la caramella si scioglie facilmente.
  = I close the little bag well because the candy melts easily.

Same perché, but different meaning and different mood.

Non simply negates the verb:

• si sciolga = it melts / may melt
• non si sciolga = it doesn’t melt / may not melt

So the purpose is negative:

• so that the candy does not melt

There is nothing unusual about the non here; it is just regular negation inside the subordinate clause.

Because the verb here is sciogliersi, which means to melt or to dissolve.

Compare:

• sciogliere = to melt/dissolve something
• sciogliersi = to melt/dissolve

Examples:

• Il sole scioglie il cioccolato. = The sun melts the chocolate.
• Il cioccolato si scioglie. = The chocolate melts.

So in your sentence, la caramella non si sciolga means so that the candy doesn’t melt.

The si is not a random extra word; it is part of the verb form.

Because in + lo contracts to nello.

Italian often combines a preposition with a definite article:

• in + il = nel
• in + lo = nello
• in + la = nella
• in + i = nei
• in + gli = negli
• in + le = nelle

Here the noun is zaino, and masculine singular nouns beginning with z normally take lo, so:

• lo zaino
• in + lo zaino = nello zaino

So nello zaino means in the backpack.

Italian uses definite articles very regularly with specific nouns. Here, both items are understood as specific, identifiable things in the situation:

• la bustina = the specific little packet/bag
• la caramella = the specific candy

In many places where English may sound lighter or more flexible, Italian still prefers the article.

So this sounds natural and expected in Italian.

Yes. You could say:

• Chiudo bene la bustina affinché la caramella non si sciolga nello zaino.

This is grammatically correct, and affinché also means so that / in order that.

However, affinché is usually more formal or literary than perché. In everyday speech, perché is much more common.

Both require the subjunctive in this kind of sentence.

It can mean more than just to melt. Depending on context, sciogliersi can also mean:

• to dissolve
• to loosen
• sometimes even figuratively to soften emotionally

In this sentence, with la caramella and nello zaino, the natural meaning is to melt.

So the context tells you which English translation fits best.