Whether the rain is acidic or not is indicated by _ _. The PH value of acid rain is generally less than _ _.

Analysis: the knowledge point examined in this question is the PH value of normal rain and acid rain. When answering, you can cut into the pH value of acid rain and normal rain. Rain with pH less than 5.6 is considered as acid rain, and the acidity of acid rain is expressed by pH value. The pH value of normal rain is neutral or slightly less than 7, slightly acidic. Generally, normal rain fluctuates around 7 and can be greater than 7. Some people think that it is caused by alkaline gas, but it should actually contain alkaline particles, such as the influence of sand blowing weather. Because the air contains CO2, the normal rainfall range should be 5.6-7, but the area with serious sand blowing can be more than 7. So, the answer is: pH; 5.6. Comments: The key to answering such questions is to be familiar with the pH values of acid rain and normal rain.