Why is acid always added to water and not the reverse?

A lot of warmth is discharged when solid acids are blended in with water. Including increasingly corrosive discharges more warmth. On the off chance that you add water to corrosive, you structure an incredibly thought arrangement of corrosive at first. So much warmth is discharged that the arrangement may bubble viciously, sprinkling concentrated corrosive out of the compartment! In the event that you add corrosive to water, the arrangement that structures is weaken and the limited quantity of warmth discharged isn't sufficient to disintegrate and splash it. So Always Add Acid to water, and never the switch.

the closest distance that she can see an object clearly when she wears her contacts is 22.2 cm

Explanation:

Given the data in the question,

near point = 20 cm

far point = 2 m = 200 cm

Now, for an object that is infinitely far away, the image is at is its far point.

so using the following expression, we can determine the focal length

1/f = 1/i + 1/o

where f is the focal length, i is the image distance and o is the object distance.

here, far point i = 2 m = 200 cm  and v is ∞

so we substitute

1/f = 1/(-200 cm)  +  1/∞

f = -200 cm

Also, for object at its closest point, the image appear at near point,

so

1/f = 1/i + 1/o

we make o the subject of formula

o = ( i × f ) / ( i - f )

given that near point i = 20 cm

we substitute

o = ( -20 × -200 ) / ( -20 - (-200) )

o = 4000 / 180

o = 22.2 cm

Therefore, the closest distance that she can see an object clearly when she wears her contacts is 22.2 cm