<form action="#" id="form">
  <div class="span">
    <input type="text" name="text" id="text" placeholder="Поиск">
    <input type="button" value="Go">
  </div>
</form>

body {
  padding: 50px;
}

#form input[type='button'] {
  display: none;
}

#form input[type='text']:focus + input[type='button'] {
  display: inline-block;
}