“Fire picture” experiment

How can you burn a pic­ture onto pa­per us­ing a chem­i­cal re­ac­tion? In this sim­ple ex­per­i­ment, we’ll show you how!

Safe­ty pre­cau­tions

Wear pro­tec­tive gloves and glass­es. Ob­serve safe­ty rules when work­ing with fire and flammable ob­jects.

Reagents and equip­ment:

• sat­u­rat­ed so­lu­tion of potas­si­um ni­trate;
• pa­per;
• brush;
• splin­ter.

Step-by-step in­struc­tions

To pre­pare the sat­u­rat­ed so­lu­tion of potas­si­um ni­trate, dis­solve 50 g of salt in 40 ml of hot wa­ter. Then ap­ply the so­lu­tion to the pa­per in a thick lay­er with a brush. Leave the “draw­ing” for 20 min­utes, so that it dries (dry­ing time de­pends on the tem­per­a­ture in the room). Then touch the “draw­ing”. Watch the pic­ture ap­pear!

Pro­cess­es de­scrip­tion

Potas­si­um ni­trate or salt­peter is a strong ox­i­diz­er in both sol­id and liq­uid state. It is found in gun­pow­der and fire­works, and used as fer­til­iz­er. When potas­si­um ni­trate is heat­ed above 400 ⁰C, it starts to break down and re­lease oxy­gen, which re­acts with the pa­per.

Pa­per (cel­lu­lose) burns more quick­ly than from the oxy­gen in the air, but it does not burn to the end, as oxy­gen and the re­ac­tion tem­per­a­ture are not suf­fi­cient for com­plete com­bus­tion. The re­ac­tion prod­ucts are car­bon diox­ide, wa­ter, char­coal and ash:

2КNO₃ = 2KNO₂ + O₂

nКNO₃ + (C₆H₁₀O₅)n = nKNO₂ +nН₂О + nС+ nСO₂

A sim­i­lar ex­per­i­ment is in­clud­ed in the “Ni­tro­gen” set — see Fire la­bel.