Gas sensor with an alarm

My first arduino project thanks to Tom Hudson for lending me his Uno

Very simple code, comments should explain it:

I’m using an MQ2 gas sensor.

#include "pitches.h"

int gasSensor = 0; // select input pin for gas Sensor
int val = 0; // variable to store the value coming from the sensor
int greenLed = 3; // the LED port
int redLed = 4; // the LED port
int speaker = 8; // Speaker

// notes in the melody:
int melody[] = {
  NOTE_C4, NOTE_G3,NOTE_G3, NOTE_A3, NOTE_G3,0, NOTE_B3, NOTE_C4
};

// note durations: 4 = quarter note, 8 = eighth note, etc.:
int noteDurations[] = {
  4, 8, 8, 4,4,4,4,4 
};

void setup() {
  Serial.begin(9600);
}

void loop() {
  val = analogRead(gasSensor); // read the value from the pot
  Serial.println( val );
  delay(100);
  if(val > 100){
    digitalWrite(greenLed, LOW); // turn the green LED off by making the voltage LOW
    digitalWrite(redLed, HIGH); // turn the red LED off
    
    // iterate over the notes of the melody:
    for (int thisNote = 0; thisNote < 8; thisNote++) {

      // to calculate the note duration, take one second 
      // divided by the note type.
      //e.g. quarter note = 1000 / 4, eighth note = 1000/8, etc.
      int noteDuration = 1000/noteDurations[thisNote];
      tone(speaker, melody[thisNote],noteDuration);
  
      // to distinguish the notes, set a minimum time between them.
      // the note's duration + 30% seems to work well:
      int pauseBetweenNotes = noteDuration * 1.30;
      delay(pauseBetweenNotes);
      // stop the tone playing:
      noTone(speaker);
    }
  }else{ // everyting okay.
    digitalWrite(greenLed, HIGH);  // The the green LED on
    digitalWrite(redLed, LOW);   // The the red LED off
  }
}