During the World Cup, the referees sprayed something on the ground. It looked like it was white paint, but it was foamy. It was right before a free-kick on goal.
What was the stuff they sprayed on the ground?
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It's called "Vanishing foam".
It is used by the referees to indicate the minimum distance the players must maintain from ball, in the event of a free kick, as well as the spot from where it is taken.
Some technical details: The can contains water (~80%), butane gas (~17%), surfactant (~1%), and other ingredients including vegetable oil (~2%).
The foam is now authorised for use in top flight football for the Bundesliga in Germany, however Germany's consumer protection agency has banned it due to greenhouse gas concerns, Serie A in Italy, Ligue 1 in France, La Liga in Spain, the Premier League in England, the Synot liga in Czech Republic, the A-League in Australia and Thai Premier League in Thailand.
Refer this wiki link.
Also another link.